The Generic Mapping Tools
C/C++ Application Programming Interface
Pål (Paul) Wessel
SOEST, University of Hawai’i at Manoa
Walter H. F. Smith
Laboratory for Satellite Altimetry, NOAA/NESDIS/STAR
Remko Scharroo
EUMETSAT, Darmstadt, Germany
Joaquim F. Luis
Universidade do Algarve, Faro, Portugal
Florian Wobbe
Alfred Wegener Institute, Germany
1. Introduction¶
1.1. Preamble¶
Prior to version 5, the bulk of GMT functionality was coded directly
in the standard GMT C program modules (e.g., surface.c
, psxy.c
, grdimage.c
, etc.). The
GMT library only offered access to low-level functions from which
those high-level GMT programs were built. The standard GMT programs
have been very successful, with tens of thousands of users world-wide.
However, the design of the main programs prevented developers from
leveraging GMT functionality from within other programming
environments since access to GMT tools could only be achieved via
system calls [1]. Consequently, all data i/o had to be done via
temporary files. The design also prevented the GMT developers
themselves from taking advantage of these modules directly. For
instance, the tool pslegend needed to
make extensive use of system calls to psxy and
pstext in order to plot the lines,
symbols and text that make up a map legend, making it a very awkward
program to maintain.
Starting with GMT version 5, all standard GMT programs have been
split into a short driver program (the “new” GMT programs) and a
function “module”. The drivers simply call the corresponding
GMT modules; it is these modules that do all the work. These new
functions have been placed in a new GMT high-level API library and can
be called from a variety of environments (C/C++, Fortran, Julia, Python,
MATLAB, Visual Basic, R, etc.) [2]. For example, the main
program blockmean.c
has been reconfigured as a high-level function
GMT_blockmean()
, which does the actual spatial averaging and can
pass the result back to the calling program (or write it to file). The
previous behavior of blockmean.c
is replicated by a short driver program that simply
collects user arguments and then calls GMT_blockmean()
. Indeed, the
driver programs for all the standard GMT programs are identical so
that the makefile generates them on-the-fly when it compiles and links
them with the GMT library into executables. Thus, blockmean.c
and others do in
fact no longer exist.
1.2. The i/o abstraction layer¶
In order for this interface to be as flexible as possible we have generalized the notion of input and output. Data that already reside in an application’s memory may serve as input to a GMT function. Other sources of input may be file pointers and file descriptors (as well as the already-supported mechanism for passing file names). For standard data table i/o, the GMT API takes care of the task of assembling any combination of files, pointers, and memory locations into a single virtual data set from which the GMT function may read (a) all records at once into memory, or (b) read one record at a time. Likewise, GMT functions may write their output to a virtual destination, which might be a memory location in the user’s application, a file pointer or descriptor, or an output file. The GMT functions are unaware of these details and simply read from a “source” and write to a “destination”.
1.3. Our audience¶
Here, we document the new functions in the GMT API library for application developers who wish to call these functions from their own custom programs. At this point, only the new high-level GMT API is fully documented and intended for public use. The structure and documentation of the under-lying lower-level GMT library is not finalized. Developers using these functions may risk disruption to their programs due to changes we may make in the library in support of the GMT API. However, developers who wish to make supplemental packages to be distributed as part of GMT will (other than talk to us) probably want to access the entire low-level GMT library as well. It is unlikely that the low-level library will ever be fully documented.
1.4. Definitions¶
For the purpose of this documentation a few definitions are needed:
- “Standard GMT program” refers to one of the traditional stand-alone command-line executables known to all GMT users, e.g., blockmean, psxy, grdimage, etc. Prior to version 5, these were the only GMT executables available.
- “GMT module” refers to the function in the GMT API library that
is responsible for all the action taken by the corresponding
GMT program. All such modules are given the same name as the
corresponding program but carry the prefix
GMT_
, e.g.,GMT_blockmean
. - “GMT application” refers to a new application written by any developer and may call one or more GMT functions to create a new GMT-compatible executable.
- In the API description that follows we will use the type
int
to mean a 4-byte integer. All integers used in the API are 4-byte integers with the exception of one function where a longer integer is used. Since different operating systems have their own way of defining 8-byte integers we use C99’sint64_t
for this purpose; it is guaranteed to yield the correct type that the GMT function expect.
In version 5, the standard GMT programs are themselves specific but overly simple examples of GMT applications that only call the single GMT function they are associated with. However, some programs such as pslegend, gmtconvert, grdblend, grdfilter and others call several modules.
1.5. Recognized resources¶
The GMT API knows how to read and write five types of data common to
GMT operations: CPT palette tables, data tables (ASCII or binary),
text tables, GMT grids and images (reading only). In addition, we
present two data types to facilitate the passing of simple user arrays
(one or more equal-length data columns of any data type, e.g., double,
char) and 2-D or 3-D user matrices (of any data type and column/row
organization [3]). We refer to these data types as GMT resources.
There are many attributes for each of these resources and therefore we
use a top-level structure for each type to keep them all in one
container. These containers are given or returned by the GMT API
functions using opaque pointers (void *
). Below we discuss these
containers in some detail; we will later present how they are used when
importing or exporting them to or from files, memory locations, or
streams. The first five are the standard GMT objects, while the latter
two are the special user data containers to facilitate passing user
data into and out of GMT modules. These resources are defined in the include
file gmt_resources.h
; please consult this file to ensure correctness
in case the documentation is not up-to-date.
1.5.1. Data tables¶
Much data processed in GMT come in the form of ASCII, netCDF, or
native binary data tables. These may have any number of header records
(ASCII files only) and perhaps segment headers. GMT programs will read
one or more such tables when importing data. However, to avoid memory
duplication or data limitations some programs may prefer to read records one
at the time. The GMT API has functions that let you read
record-by-record by presenting a virtual data set that combines all the
data tables specified as input. This simplifies record processing
considerably. A struct GMT_DATASET
may contain any number of tables,
each with any number of segments, each segment with any number of
records, and each record with any number of columns. Thus, the arguments
to GMT API functions that handle such data sets expect this type of
variable. All segments are expected to have the same number of columns.
struct GMT_DATASET { /* Single container for an array of GMT tables (files) */
/* Variables we document for the API: */
uint64_t n_tables; /* The total number of tables (files) contained */
uint64_t n_columns; /* The number of data columns */
uint64_t n_segments; /* The total number of segments across all tables */
uint64_t n_records; /* The total number of data records across all tables */
double *min; /* Minimum coordinate for each column */
double *max; /* Maximum coordinate for each column */
struct GMT_DATATABLE **table; /* Pointer to array of tables */
/* ---- Variables "hidden" from the API ---- */
uint64_t id; /* The internal number of the data set */
size_t n_alloc; /* The current allocation length of tables */
uint64_t dim[4]; /* Only used by GMT_Duplicate_Data to override dimensions */
unsigned int geometry; /* The geometry of this dataset */
unsigned int alloc_level; /* The level it was allocated at */
enum GMT_enum_dest io_mode; /* -1 means write OGR format (requires proper -a),
0 means write everything to one destination [Default],
1 means use table->file[GMT_OUT] to write separate table,
2 means use segment->file[GMT_OUT] to write separate segments.
3 is same as 2 but with no filenames we create filenames from
tbl and seg numbers */
enum GMT_enum_alloc alloc_mode; /* Allocation mode [GMT_ALLOCATED_BY_GMT] */
char *file[2]; /* Name of file or source [0 = in, 1 = out] */
};
struct GMT_DATATABLE { /* To hold an array of line segment structures and header information in one container */
/* Variables we document for the API: */
unsigned int n_headers; /* Number of file header records (0 if no header) */
uint64_t n_columns; /* Number of columns (fields) in each record */
uint64_t n_segments; /* Number of segments in the array */
uint64_t n_records; /* Total number of data records across all segments */
double *min; /* Minimum coordinate for each column */
double *max; /* Maximum coordinate for each column */
char **header; /* Array with all file header records, if any) */
struct GMT_DATASEGMENT **segment; /* Pointer to array of segments */
/* ---- Variables "hidden" from the API ---- */
uint64_t id; /* The internal number of the table */
size_t n_alloc; /* The current allocation length of segments */
double dist; /* Distance from a point to this feature */
enum GMT_enum_write mode; /* 0 = output table, 1 = output header only, 2 = skip table */
struct GMT_OGR *ogr; /* Pointer to struct with all things GMT/OGR (if MULTI-geometry and not MULTIPOINT) */
char *file[2]; /* Name of file or source [0 = in, 1 = out] */
};
struct GMT_DATASEGMENT { /* For holding segment lines in memory */
/* Variables we document for the API: */
uint64_t n_rows; /* Number of points in this segment */
uint64_t n_columns; /* Number of fields in each record (>= 2) */
double *min; /* Minimum coordinate for each column */
double *max; /* Maximum coordinate for each column */
double **coord; /* Coordinates x,y, and possibly other columns */
char *label; /* Label string (if applicable) */
char *header; /* Segment header (if applicable) */
/* ---- Variables "hidden" from the API ---- */
enum GMT_enum_write mode; /* 0 = output segment, 1 = output header only, 2 = skip segment */
enum GMT_enum_pol pol_mode;/* Either GMT_IS_PERIMETER [-Pp] or GMT_IS_HOLE [-Ph] (for polygons only) */
uint64_t id; /* The internal number of the segment */
size_t n_alloc; /* The current allocation length of each coord */
unsigned int range; /* Longitude reporting scheme, e.g. GMT_IS_GIVEN_RANGE [0] */
int pole; /* Spherical polygons only: If it encloses the S (-1) or N (+1) pole, or none (0) */
double dist; /* Distance from a point to this feature */
double lat_limit; /* For polar caps: the latitude of the point closest to the pole */
struct GMT_OGR_SEG *ogr; /* NULL unless OGR/GMT metadata exist for this segment */
struct GMT_DATASEGMENT *next; /* NULL unless polygon and has holes and pointing to next hole */
char *file[2]; /* Name of file or source [0 = in, 1 = out] */
};
struct GMT_OGR { /* Struct with all things GMT/OGR for a table */
/* The first parameters are usually set once per data set and do not change */
unsigned int geometry; /* @G: The geometry of this data set, if known [0 otherwise] */
unsigned int n_aspatial; /* @T: The number of aspatial fields */
char *region; /* @R: The region textstring [NULL if not set] */
char *proj[4]; /* @J: The 1-4 projection strings [NULL if not set] */
unsigned int *type; /* @T: The data types of the aspatial fields [NULL if not set] */
char **name; /* @N The names of the aspatial fields [NULL if not set] */
/* The following are for OGR data only. It is filled during parsing (current segment) but is then copied to the segment header so it can be accessed later */
enum GMT_enum_pol pol_mode;/* @P: Either GMT_IS_PERIMETER or GMT_IS_HOLE (for polygons only) */
char **tvalue; /* @D: The text values of the current aspatial fields */
double *dvalue; /* @D: Same but converted to double (assumed possible) */
};
struct GMT_OGR_SEG { /* Struct with GMT/OGR aspatial data for a segment */
enum GMT_enum_pol pol_mode;/* @P: Either GMT_IS_PERIMETER or GMT_IS_HOLE (for polygons only) */
unsigned int n_aspatial; /* @T: The number of aspatial fields */
char **tvalue; /* @D: The values of the current aspatial fields (uses GMT_OGR's n_aspatial as length) */
double *dvalue; /* @D: Same but converted to double (assumed possible) */
};
1.5.2. Text tables¶
Some data needed by GMT are simply free-form ASCII text tables. In many respects these
are handled similarly to data tables. E.g., they may have any number of
header records and even segment headers, and GMT programs can read one
or more tables or get text records one at the time. A
struct GMT_TEXTSET
may contain any number of tables, each with any
number of segments, and each segment with any number of records. Thus,
the arguments to GMT API functions that handle such data sets expect
this type of variable. The user’s program may then parse and process
such text records as required. This resources is particularly useful
when your data consist of a mix or data coordinates and ordinary text
since regular data tables will be parsed for floating-point columns
only.
struct GMT_TEXTSET { /* Single container for an array of GMT text tables (files) */
/* Variables we document for the API: */
uint64_t n_tables; /* The total number of tables (files) contained */
uint64_t n_segments; /* The total number of segments across all tables */
uint64_t n_records; /* The total number of data records across all tables */
struct GMT_TEXTTABLE **table; /* Pointer to array of tables */
/* ---- Variables "hidden" from the API ---- */
uint64_t id; /* The internal number of the data set */
size_t n_alloc; /* The current allocation length of tables */
unsigned int geometry; /* The geometry of this dataset */
unsigned int alloc_level; /* The level it was allocated at */
enum GMT_enum_dest io_mode; /*-1 means write OGR format (requires proper -a),
0 means write everything to one destination [Default],
1 means use table->file[GMT_OUT] to write separate table,
2 means use segment->file[GMT_OUT] to write separate segments.
3 is same as 2 but with no filenames we create filenames */
from tbl and seg numbers */
enum GMT_enum_alloc alloc_mode; /* Allocation mode [GMT_ALLOCATED_BY_GMT] */
char *file[2]; /* Name of file or source [0 = in, 1 = out] */
};
struct GMT_TEXTTABLE { /* To hold an array of text segment structures and header information in one container */
/* Variables we document for the API: */
unsigned int n_headers; /* Number of file header records (0 if no header) */
uint64_t n_segments; /* Number of segments in the array */
uint64_t n_records; /* Total number of data records across all segments */
char **header; /* Array with all file header records, if any) */
struct GMT_TEXTSEGMENT **segment; /* Pointer to array of segments */
/* ---- Variables "hidden" from the API ---- */
uint64_t id; /* The internal number of the table */
size_t n_alloc; /* The current allocation length of segments */
enum GMT_enum_write mode; /* 0 = output table, 1 = output header only, 2 = skip table */
char *file[2]; /* Name of file or source [0 = in, 1 = out] */
};
struct GMT_TEXTSEGMENT { /* For holding segment text records in memory */
/* Variables we document for the API: */
uint64_t n_rows; /* Number of rows in this segment */
char **record; /* Array of text records */
char *label; /* Label string (if applicable) */
char *header; /* Segment header (if applicable) */
/* ---- Variables "hidden" from the API ---- */
uint64_t id; /* The internal number of the table */
enum GMT_enum_write mode; /* 0 = output segment, 1 = output header only, 2 = skip segment */
size_t n_alloc; /* Number of rows allocated for this segment */
char *file[2]; /* Name of file or source [0 = in, 1 = out] */
char **tvalue; /* The values of the OGR/GMT aspatial fields */
};
1.5.3. GMT grids¶
GMT grids are used to represent equidistant and organized 2-D
surfaces. These can be plotted as contour maps, color images, or as
perspective surfaces. Because the native GMT grid is simply a 1-D
float array with all the metadata kept in a separate header, we pass
this information via a struct GMT_GRID
, which is a container that
holds both items. Thus, the arguments to GMT API functions that handle
GMT grids expect this type of variable.
struct GMT_GRID { /* To hold a GMT float grid and its header in one container */
struct GMT_GRID_HEADER *header; /* Pointer to full GMT header for the grid */
float *data; /* Pointer to the float grid */
/* ---- Variables "hidden" from the API ---- */
unsigned int id; /* The internal number of the grid */
unsigned int alloc_level; /* The level it was allocated at */
enum GMT_enum_alloc alloc_mode; /* Allocation mode [GMT_ALLOCATED_BY_GMT] */
void *extra; /* Row-by-row machinery information [NULL] */
};
struct GMT_GRID_HEADER {
/* Variables we document for the API:
* They are copied verbatim to the native grid header and must be 4-byte unsigned ints. */
uint32_t nx; /* Number of columns */
uint32_t ny; /* Number of rows */
uint32_t registration; /* GMT_GRID_NODE_REG (0) for node grids, GMT_GRID_PIXEL_REG (1) for pixel grids */
/* == The types of the following 12 elements must not be changed.
* == They are also copied verbatim to the native grid header. */
double wesn[4]; /* Min/max x and y coordinates */
double z_min; /* Minimum z value */
double z_max; /* Maximum z value */
double inc[2]; /* x and y increment */
double z_scale_factor; /* grd values must be multiplied by this */
double z_add_offset; /* After scaling, add this */
char x_units[GMT_GRID_UNIT_LEN80]; /* units in x-direction */
char y_units[GMT_GRID_UNIT_LEN80]; /* units in y-direction */
char z_units[GMT_GRID_UNIT_LEN80]; /* grid value units */
char title[GMT_GRID_TITLE_LEN80]; /* name of data set */
char command[GMT_GRID_COMMAND_LEN320];/* name of generating command */
char remark[GMT_GRID_REMARK_LEN160]; /* comments re this data set */
/* == End of "untouchable" header. */
/* ---- Variables "hidden" from the API ----
* This section is flexible. It is not copied to any grid header
* or stored in any file. It is considered private */
unsigned int type; /* Grid format */
unsigned int bits; /* Bits per data value (e.g., 32 for ints/floats; 8 for bytes) */
unsigned int complex_mode; /* 0 = normal, GMT_GRID_IS_COMPLEX_REAL = real part of complex grid, GMT_GRID_IS_COMPLEX_IMAG = imag part of complex grid */
unsigned int mx, my; /* Actual dimensions of the grid in memory, allowing for the padding */
size_t nm; /* Number of data items in this grid (nx * ny) [padding is excluded] */
size_t size; /* Actual number of items (not bytes) required to hold this grid (= mx * my) */
size_t n_alloc; /* Bytes allocated for this grid */
unsigned int trendmode; /* Holds status for detrending of grids. 0 if not detrended, 1 if mean, 2 if mid-value, and 3 if LS plane removed */
unsigned int arrangement; /* Holds status for complex grid as how the read/imag is placed in the grid (interleaved, R only, etc.) */
unsigned int n_bands; /* Number of bands [1]. Used with IMAGE containers and macros to get ij index from row,col, band */
unsigned int pad[4]; /* Padding on west, east, south, north sides [2,2,2,2] */
unsigned int BC[4]; /* Boundary condition applied on each side via pad [0 = not set, 1 = natural, 2 = periodic, 3 = data] */
unsigned int grdtype; /* 0 for Cartesian, > 0 for geographic and depends on 360 periodicity [see GMT_enum_grdtype above] */
char name[GMT_GRID_NAME_LEN256]; /* Actual name of the file after any ?<varname> and =<stuff> has been removed */
char varname[GMT_GRID_VARNAME_LEN80];/* NetCDF: variable name */
const char *ProjRefPROJ4; /* To store a referencing system string in PROJ.4 format */
const char *ProjRefWKT; /* To store a referencing system string in WKT format */
int row_order; /* NetCDF: k_nc_start_south if S->N, k_nc_start_north if N->S */
int z_id; /* NetCDF: id of z field */
int ncid; /* NetCDF: file ID */
int xy_dim[2]; /* NetCDF: dimension order of x and y; normally {1, 0} */
size_t t_index[3]; /* NetCDF: index of higher coordinates */
size_t data_offset; /* NetCDF: distance from the beginning of the in-memory grid */
unsigned int stride; /* NetCDF: distance between two rows in the in-memory grid */
float nan_value; /* Missing value as stored in grid file */
double xy_off; /* 0.0 (registration == GMT_GRID_NODE_REG) or 0.5 ( == GMT_GRID_PIXEL_REG) */
double r_inc[2]; /* Reciprocal incs, i.e. 1/inc */
char flags[4]; /* Flags used for ESRI grids */
char *pocket; /* GDAL: A working variable handy to transmit info between funcs e.g. +b<band_info> to gdalread */
double bcr_threshold; /* sum of cardinals must >= threshold in bilinear; else NaN */
unsigned int bcr_interpolant; /* Interpolation function used (0, 1, 2, 3) */
unsigned int bcr_n; /* Width of the interpolation function */
unsigned int nxp; /* if X periodic, nxp > 0 is the period in pixels */
unsigned int nyp; /* if Y periodic, nxp > 0 is the period in pixels */
unsigned int no_BC; /* If true we skip BC stuff entirely */
unsigned int gn; /* true if top edge will be set as N pole */
unsigned int gs; /* true if bottom edge will be set as S pole */
unsigned int is_netcdf4; /* true if netCDF-4/HDF5 format */
size_t z_chunksize[2]; /* chunk size (lat,lon) */
unsigned int z_shuffle; /* if shuffle filter is turned on */
unsigned int z_deflate_level; /* if deflate filter is in use */
unsigned int z_scale_autoadust; /* if z_scale_factor should be auto-detected */
unsigned int z_offset_autoadust; /* if z_add_offset should be auto-detected */
/* xy_*[] is separate settings for GMT_IN and GMT_OUT */
unsigned int xy_adjust[2]; /* 1 if +u<unit> was parsed and scale set, 3 if xy has been adjusted, 0 otherwise */
unsigned int xy_mode[2]; /* 1 if +U<unit> was parsed, 0 otherwise */
unsigned int xy_unit[2]; /* Unit enum specified via +u<unit> */
double xy_unit_to_meter[2];/* Scale, given xy_unit, to convert xy from <unit> to meters */
};
1.5.4. GMT images¶
GMT images are used to represent bit-mapped images typically obtained
via the GDAL bridge. These can be reprojected internally, such as when
used in grdimage. Since images and grids share the concept of a header,
we use the same header structure for grids as for images; however, some
additional metadata attributes are also needed. Finally, the image
itself may be of any data type and have more than one band (channel).
Both image and header information are passed via a struct GMT_IMAGE
,
which is a container that holds both items. Thus, the arguments to
GMT API functions that handle GMT images expect this type of
variable. Unlike the other objects, writing images has only partial
support via GMT_grdimage
[4].
struct GMT_IMAGE {
enum GMT_enum_type type; /* Data type, e.g. GMT_FLOAT */
int *ColorMap; /* Array with color lookup values */
int nIndexedColors; /* Number of colors in a paletted image */
struct GMT_GRID_HEADER *header; /* Pointer to full GMT header for the image */
unsigned char *data; /* Pointer to actual image */
/* ---- Variables "hidden" from the API ---- */
uint64_t id; /* The internal number of the data set */
unsigned int alloc_level; /* Level of initial allocation */
enum GMT_enum_alloc alloc_mode; /* Allocation info [0] */
const char *ColorInterp;
};
1.5.5. CPT palette tables¶
The color palette table files, or just CPT files, contain colors and
patterns used for plotting data such as surfaces (i.e., GMT grids) or
symbols, lines and polygons (i.e., GMT tables). GMT programs will
generally read in a CPT palette table, make it the current palette, do
the plotting, and destroy the table when done. The information is
referred to via a pointer to struct GMT_PALETTE
. Thus, the arguments
to GMT API functions that handle palettes expect this type of
variable. It is not expected that users will wish to manipulate a CPT
table directly, but rather use this mechanism to hold them in memory and
pass as arguments to GMT modules.
struct GMT_PALETTE { /* Holds all pen, color, and fill-related parameters */
/* Variables we document for the API: */
unsigned int n_headers; /* Number of CPT file header records (0 if no header) */
unsigned int n_colors; /* Number of colors in CPT lookup table */
unsigned int cpt_flags; /* Flags controlling use of BFN colors */
struct GMT_LUT *range; /* CPT lookup table read by GMT_read_cpt */
struct GMT_BFN_COLOR patch[3]; /* Structures with back/fore/nan colors */
char **header; /* Array with all CPT file header records, if any) */
/* ---- Variables "hidden" from the API ---- */
uint64_t id; /* The internal number of the data set */
enum GMT_enum_alloc alloc_mode; /* Allocation mode [GMT_ALLOCATED_BY_GMT] */
unsigned int alloc_level; /* The level it was allocated at */
unsigned int model; /* RGB, HSV, CMYK */
unsigned int is_gray; /* true if only grayshades are needed */
unsigned int is_bw; /* true if only black and white are needed */
unsigned int is_continuous; /* true if continuous color tables have been given */
unsigned int has_pattern; /* true if CPT file contains any patterns */
unsigned int skip; /* true if current z-slice is to be skipped */
unsigned int categorical; /* true if CPT applies to categorical data */
unsigned int z_adjust[2]; /* 1 if +u<unit> was parsed and scale set, 3 if z has been adjusted, 0 otherwise */
unsigned int z_mode[2]; /* 1 if +U<unit> was parsed, 0 otherwise */
unsigned int z_unit[2]; /* Unit enum specified via +u<unit> */
double z_unit_to_meter[2]; /* Scale, given z_unit, to convert z from <unit> to meters */
};
1.5.6. User data columns (GMT vectors)¶
Programs that wish to call GMT modules may hold data in their own
particular data structures. For instance, the user’s program may have
three column arrays of type float and wishes to use these as the input
source to the GMT_surface
module, which normally expects double
precision triplets via a struct GMT_DATASET
read from a file or
given by memory reference. Simply create a new struct GMT_VECTOR
(see section Create empty resources) and assign the union array pointers (see
univector) to your data columns and provide the required
information on length, data types, and optionally range (see
GMT_VECTOR). By letting the GMT module know you are passing a
data set via a struct GMT_VECTOR
it will know how to read the data correctly.
union GMT_UNIVECTOR {
uint8_t *uc1; /* Pointer for unsigned 1-byte array */
int8_t *sc1; /* Pointer for signed 1-byte array */
uint16_t *ui2; /* Pointer for unsigned 2-byte array */
int16_t *si2; /* Pointer for signed 2-byte array */
uint32_t *ui4; /* Pointer for unsigned 4-byte array */
int32_t *si4; /* Pointer for signed 4-byte array */
uint64_t *ui8; /* Pointer for unsigned 8-byte array */
int64_t *si8; /* Pointer for signed 8-byte array */
float *f4; /* Pointer for float array */
double *f8; /* Pointer for double array */
};
Table 1.1: Definition of the GMT_UNIVECTOR
union that holds a pointer to any array type.
struct GMT_VECTOR {
uint64_t n_columns; /* Number of vectors */
uint64_t n_rows; /* Number of rows in each vector */
enum GMT_enum_reg registration; /* 0 for gridline and 1 for pixel registration */
enum GMT_enum_type *type; /* Array with data type for each vector */
union GMT_UNIVECTOR *data; /* Array with unions for each column */
double range[2]; /* The min and max limits on t-range (or 0,0) */
char command[GMT_GRID_COMMAND_LEN320]; /* name of generating command */
char remark[GMT_GRID_REMARK_LEN160]; /* comments re this data set */
/* ---- Variables "hidden" from the API ---- */
uint64_t id; /* An identification number */
unsigned int alloc_level; /* Level of initial allocation */
enum GMT_enum_alloc alloc_mode; /* Determines if we may free the vectors or not */
};
1.5.7. User data matrices (GMT matrices)¶
Likewise, programs may have an integer 2-D matrix in memory and wish to
use that as the input grid to the GMT_grdfilter
module, which
normally expects a struct GMT_GRID
with floating point data via a
file or provided by memory reference. As for user vectors, we create a
struct GMT_MATRIX
(see Create empty resources), assign the appropriate
union pointer to your data matrix and provide information on dimensions
and data type. Let the GMT module know you
are passing a grid via a struct GMT_MATRIX
and it will know how to
read the matrix properly.
struct GMT_MATRIX {
uint64_t n_rows; /* Number of rows in the matrix */
uint64_t n_columns; /* Number of columns in the matrix */
uint64_t n_layers; /* Number of layers in a 3-D matrix */
enum GMT_enum_fmt shape; /* 0 = C (rows) and 1 = Fortran (cols) */
enum GMT_enum_reg registration; /* 0 for gridline and 1 for pixel registration */
size_t dim; /* Allocated length of longest C or Fortran dim */
size_t size; /* Byte length of data */
enum GMT_enum_type type; /* Data type, e.g. GMT_FLOAT */
double range[6]; /* Contains xmin/xmax/ymin/ymax[/zmin/zmax] */
union GMT_UNIVECTOR data; /* Union with pointer to actual matrix of the chosen type */
char command[GMT_GRID_COMMAND_LEN320]; /* name of generating command */
char remark[GMT_GRID_REMARK_LEN160]; /* comments re this data set */
/* ---- Variables "hidden" from the API ---- */
uint64_t id; /* The internal number of the data set */
unsigned int alloc_level; /* The level it was allocated at */
enum GMT_enum_alloc alloc_mode; /* Allocation mode [GMT_ALLOCATED_BY_GMT] */
};
The enum
types referenced in GMT_VECTOR and
Table GMT_MATRIX and summarized in Table types.
constant | description |
---|---|
GMT_CHAR | int8_t, 1-byte signed integer type |
GMT_UCHAR | int8_t, 1-byte unsigned integer type |
GMT_SHORT | int16_t, 2-byte signed integer type |
GMT_USHORT | uint16_t, 2-byte unsigned integer type |
GMT_INT | int32_t, 4-byte signed integer type |
GMT_UINT | uint32_t, 4-byte unsigned integer type |
GMT_LONG | int64_t, 8-byte signed integer type |
GMT_ULONG | uint64_t, 8-byte unsigned integer type |
GMT_FLOAT | 4-byte data float type |
GMT_DOUBLE | 8-byte data float type |
2. Overview of the GMT C Application Program Interface¶
Users who wish to create their own GMT application based on the API
must make sure their program goes through the steps below; details for
each step will be revealed in the following chapter. We have kept the
API simple: In addition to the GMT modules, there are only 53 public
functions to become familiar with, but most applications will only use a
small subset of this selection. Functions either return an integer error
code (when things go wrong; otherwise it is set to GMT_OK (0)
), or they
return a void pointer to a GMT resources (or NULL if things go wrong).
In either case the API will report what the error is. The layout here
assumes you wish to use data in memory as input sources; if the data are
simply command-line files then things simplify considerably.
- Initialize a new GMT session with GMT_Create_Session, which allocates a hidden GMT API control structure and returns an opaque pointer to it. This pointer is the first argument to all subsequent GMT API function calls within the session.
- For each intended call to a GMT module, several steps are involved:
- Register input sources and output destination with GMT_Register_IO.
- Each resource registration generates a unique ID number. For memory resources, we embed these numbers in unique filenames of the form “@GMTAPI@-######” with GMT_Encode_ID. When GMT i/o library functions encounter such filenames they extract the ID and make a connection to the corresponding resource. Multiple table data or text sources are combined into a single virtual source for GMT modules to operate on. In contrast, CPT, Grid, and Image resources are operated on individually.
- Enable data import once all registrations are complete (Resources init).
- Read data into memory. You may choose to read everything at once or read record-by-record.
- Prepare required arguments and call the GMT module you wish to use.
- For non-mapping modules, process any results returned to memory via pointers rather than written to files.
- Destroy the resources allocated by GMT modules to hold results, or let the garbage collector do this automatically at the end of the session.
- Repeat steps a–f as many times as your application requires.
- We terminate the GMT session by calling GMT_Destroy_Session.
The steps a–d collapse into a single step if data are simply read from files.
Advanced programs may be calling more than one GMT session and thus
run several sessions, perhaps concurrently as different threads on
multi-core machines. We will now discuss these steps in more detail.
Throughout, we will introduce upper-case GMT C enum constants in
lieu of simple integer constants. These are considered part of the API
and are available for developers via the gmt_resources.h
include file.
The C/C++ API is deliberately kept small to make it easy to use. Next table gives a list of all the functions and their purpose.
constant | description |
---|---|
GMT_Append_Option | Append new option structure to linked list |
GMT_Begin_IO | Enable record-by-record i/o |
GMT_Call_Module | Call any of the GMT modules |
GMT_Create_Args | Convert linked list of options to text array |
GMT_Create_Cmd | Convert linked list of options to command line |
GMT_Create_Data | Create an empty data resource |
GMT_Create_Options | Convert command line options to linked list |
GMT_Create_Session | Initialize a new GMT session |
GMT_Delete_Option | Delete an option structure from the linked list |
GMT_Destroy_Args | Delete text array of arguments |
GMT_Destroy_Cmd | Delete text command of arguments |
GMT_Destroy_Data | Delete a data resource |
GMT_Destroy_Options | Delete the linked list of option structures |
GMT_Destroy_Session | Terminate a GMT session |
GMT_Duplicate_Data | Make an identical copy of a data resources |
GMT_Encode_ID | Encode a resources ID as a special filename |
GMT_Encode_Options | Encode option arguments for external interfaces |
GMT_Expand_Option | Expand option with explicit memory references |
GMT_End_IO | Disable further record-by-record i/o |
GMT_FFT | Take the Fast Fourier Transform of data object |
GMT_FFT_1D | Take the Fast Fourier Transform of 1-D float data |
GMT_FFT_2D | Take the Fast Fourier Transform of 2-D float data |
GMT_FFT_Create | Initialize the FFT machinery |
GMT_FFT_Destroy | Terminate the FFT machinery |
GMT_FFT_Option | Explain the FFT options and modifiers |
GMT_FFT_Parse | Parse argument with FFT options and modifiers |
GMT_FFT_Wavenumber | Return wavenumber given data index |
GMT_Find_Option | Find an option in the linked list |
GMT_Get_Common | Determine if a GMT common option was set |
GMT_Get_Coord | Create a coordinate array |
GMT_Get_Data | Import a registered data resources |
GMT_Get_Default | Obtain one of the API or GMT default settings |
GMT_Get_ID | Obtain the ID of a given resource |
GMT_Get_Index | Convert row, col into a grid or image index |
GMT_Get_Record | Import a single data record |
GMT_Get_Row | Import a single grid row |
GMT_Get_Value | Convert string into coordinates or dimensions |
GMT_Init_IO | Initialize i/o given registered resources |
GMT_Make_Option | Create an option structure |
GMT_Message | Issue a message, optionally with time stamp |
GMT_Option | Explain one or more GMT common options |
GMT_Parse_Common | Parse the GMT common options |
GMT_Put_Data | Export to a registered data resource given by ID |
GMT_Put_Record | Export a data record |
GMT_Put_Row | Export a grid row |
GMT_Read_Data | Import a data resource or file |
GMT_Register_IO | Register a resources for i/o |
GMT_Report | Issue a message contingent upon verbosity level |
GMT_Retrieve_Data | Obtained link to data in memory via ID |
GMT_Set_Comment | Assign a comment to a data resource |
GMT_Set_Default | Set one of the API or GMT default settings |
GMT_Status_IO | Check status of record-by-record i/o |
GMT_Update_Option | Modify an option structure |
GMT_Write_Data | Export a data resource |
3. The GMT C Application Program Interface¶
3.1. Initialize a new GMT session¶
Most applications will need to initialize only a single GMT session. This is true of all the standard GMT programs since they only call one GMT module and then exit. Most user-developed GMT applications are likely to only initialize one session even though they may call many GMT modules. However, the GMT API supports any number of simultaneous sessions should the programmer wish to take advantage of it. This might be useful when you have access to several CPUs and want to spread the computing load [5]. In the following discussion we will simplify our treatment to the use of a single session only.
To initiate the new session we use
void *GMT_Create_Session (const char *tag, unsigned int pad, unsigned int mode, int (*print_func) (FILE *, const char *));
and you will typically call it thus:
void *API = NULL; API = GMT_Create_Session ("Session name", 2, 0, NULL);
where API
is an opaque pointer to the hidden GMT API control
structure. You will need to pass this pointer to all subsequent
GMT API functions; this is how essential internal information is
passed from module to module. The key task of this initialization is to
set up the GMT machinery and its internal variables used for map
projections, plotting, i/o, etc. The initialization also allocates space
for internal structures used to register resources. The pad
argument
sets how many rows and columns should be used as padding for grids and
images so that boundary conditions can be applied. GMT uses 2 so we
recommend that value. Note: if you choose 0 or 1 there may be certain
GMT modules that will be unable to do their work properly as they count on those
boundary rows and columns in the grids. The mode
argument is only used for external APIs that need
to communicate special needs during the session creation. This argument
is a sum of bit flags and the various bits control the following settings:
- Bit 1 (1): If set then GMT will not call the system exit function when a a serious problem has been detected but instead will simply return control to the calling environment. This is required by the GMT/MATLAB API since calling exit would also exit MATLAB itself. Unless your environment has this feature you should leave this bit alone.
- Bit 2 (2): If set then it means we are calling the GMT API from an external API such as MATLAB, Octave, or Python. Normal C/C++ programs should leave this bit alone. Its effect is to enable two additional modules for reading and writing GMT resources from these environments.
- Bit 3 (4): If 1 it means the external API uses a column-major format for matrices (e.g., MATLAB, Fortran). If not set we default to row-major format (C/C++, Python).
The print_func
argument is a pointer to a function that is used to print
messages from GMT via GMT_Message or GMT_Report from APIs that cannot use the
standard printf (this is the case for the MATLAB API, for instance).
For all other uses you should simply pass NULL for this argument.
Should something go wrong then API
will be returned as NULL
.
3.2. Register input or output resources¶
When using the standard GMT programs, you specify input files on the command line or via special program options (e.g., -Iintensity.nc). The output of the programs are either written to standard output (which you redirect to files or pipe to other programs) or to files specified by specific program options (e.g., -Goutput.nc). Alternatively, the GMT API allows you to specify input (and output) to be associated with open file handles or program variables. We will examine this more closely below. Registering a resource is a required step before attempting to import or export data that do not come from files or standard input/output.
3.2.1. Resource registration¶
Registration involves a direct or indirect call to
int GMT_Register_IO (void *API, unsigned int family, unsigned int method, unsigned int geometry, unsigned int direction, double wesn[], void *ptr);
where family specifies what kind of resource is to be registered,
method specifies
how we to access this resource (see Table methods for recognized
methods), geometry specifies the geometry of the data, ptr
is the address of the
pointer to the named resource. If direction
is GMT_OUT
and the
method
is not related to a file (filename, stream, or handle), then
ptr
must be NULL. Note there are some limitations on when you may pass a file pointer
as the method. Many grid file formats cannot be read via a stream (e.g., netCDF files) so in
those situations you cannot pass a file pointer [and GMT_Register_IO would have no way of knowing
this]. After the GMT module has written the data you
can use GMT_Retrieve_Data to assign a pointer to the memory location
(variable) where the output was allocated. For grid (and image)
resources you may request to obtain a subset via the wesn array; otherwise, pass NULL
(or an array with at least 4 items all set to 0) to obtain the
entire grid (or image). The direction
indicates input or output and
is either GMT_IN
or GMT_OUT
. Finally, the function returns a
unique resource ID, or GMT_NOTSET
if there was an error.
3.2.2. Object ID encoding¶
To use registered resources as program input or output arguments you must pass them via a text string that acts as a special file name (Chapter Overview). The proper filename formatting is guaranteed by using the function
int GMT_Encode_ID (void *API, char *filename, int ID);
which accepts the unique ID
and writes the corresponding
filename
. The variable filename
must have enough space to hold
16 bytes. The function returns 1 if there is an error; otherwise
it returns 0.
family | source points to |
---|---|
GMT_IS_DATASET | A [multi-segment] table file |
GMT_IS_TEXTSET | A [multi-segment] text file |
GMT_IS_GRID | A GMT grid file |
GMT_IS_CPT | A CPT file |
GMT_IS_IMAGE | A GMT image |
method | value | how to read/write data |
---|---|---|
GMT_IS_FILE | 0 | Pointer to name of a file |
GMT_IS_STREAM | 1 | Pointer to open stream (or process) |
GMT_IS_FDESC | 2 | Pointer to integer file descriptor |
GMT_IS_DUPLICATE | 3 | Pointer to memory we may duplicate data from |
GMT_IS_REFERENCE | 4 | Pointer to memory we may reference data from |
GMT_IS_DUPLICATE_VIA_VECTOR | 103 | Pointer to memory we may duplicate data from via vectors |
GMT_IS_REFERENCE_VIA_VECTOR | 104 | Pointer to memory we may reference data from via vectors |
GMT_IS_DUPLICATE_VIA_MATRIX | 203 | Pointer to memory we may duplicate data from via a matrix |
GMT_IS_REFERENCE_VIA_MATRIX | 204 | Pointer to memory we may reference data from via a matrix |
approach | how method is modified |
---|---|
GMT_VIA_VECTOR | User’s data columns are accessed via a GMT_VECTOR structure |
GMT_VIA_MATRIX | User’s matrix is accessed via a GMT_MATRIX structure |
geometry | description |
---|---|
GMT_IS_TEXT | Not a geographic item |
GMT_IS_POINT | Multi-dimensional point data |
GMT_IS_LINE | Geographic or Cartesian line segments |
GMT_IS_POLYGON | Geographic or Cartesian closed polygons |
GMT_IS_SURFACE | 2-D gridded surface |
index | description |
---|---|
GMT_XLO | x_min (west) boundary of grid subset |
GMT_XHI | x_max (east) boundary of grid subset |
GMT_YLO | y_min (south) boundary of grid subset |
GMT_YHI | y_max (north) boundary of grid subset |
GMT_ZLO | z_min (bottom) boundary of 3-D matrix subset |
GMT_ZHI | z_max (top) boundary of 3-D matrix subset |
3.2.3. Resource initialization¶
All GMT programs dealing with input or output files given on the command line, and perhaps defaulting to the standard input or output streams if no files are given, must call the i/o initializer function GMT_Init_IO once for each direction required (i.e., input and output separately). For input it determines how many input sources have already been registered. If none has been registered then it scans the program arguments for any filenames given on the command line and register these input resources. Finally, if we still have found no input sources we assign the standard input stream as the single input source. For output it is similar: If no single destination has been registered we specify the standard output stream as the output destination. Only one main output destination is allowed to be active when a module writes data (some modules also write additional output via program-specific options). The prototype for this function is
int GMT_Init_IO (void *API, unsigned int family, unsigned int geometry, unsigned int direction, unsigned int mode, unsigned int n_args, void *args);
where family specifies what kind of resource is to be registered,
geometry specifies the geometry of the data, direction
is either
GMT_IN
or GMT_OUT
, and mode
is a bit flag that determines
what we do if no resources have been registered. The choices are
GMT_ADD_FILES_IF_NONE (1) means “add command line (option) files if none have been registered already”
GMT_ADD_FILES_ALWAYS (2) means “always add any command line files”
GMT_ADD_STDIO_IF_NONE (4) means “add std* if no other input/output have been specified”
GMT_ADD_STDIO_ALWAYS (8) means “always add std* even if resources have been registered”.
GMT_ADD_EXISTING (16) means “only use already registered resources”.
The standard behavior is GMT_REG_DEFAULT
(5). Next, n_args
is 0
if args
is the head of a linked list of options (further discussed
in Prepare modules opts); otherwise args
is an array of n_args
strings (i.e., the int argc, char *argv[] model)
Many programs will register an export location where results of a GMT function (say, a filtered grid) should be returned, but may then wish to use that variable as an input resource in a subsequent module call. This is accomplished by re-registering the resource as an input source, thereby changing the direction of the data set. The function returns 1 if there is an error; otherwise it returns 0.
3.2.4. Dimension parameters for user 1-D column vectors¶
We refer to Table vector. The type
array must hold the data
type of each data column in the user’s program. All types other than
GMT_DOUBLE will be converted internally in GMT to double
, thus
possibly increasing memory requirements. If the type is GMT_DOUBLE
then
GMT will be able to use the column directly by reference. The
n_columns
and n_rows
parameters indicate the number of vectors
and their common length. For output these may not yet be known so you should
pass 0 for these values.
3.2.5. Dimension parameters for user 2-D table arrays¶
We refer to Table matrix. The type
parameter specifies the
data type used for the array in the user’s program. All types other than
GMT_FLOAT will be converted internally in GMT to float
, thus
possibly increasing memory requirements. If the type is GMT_FLOAT
then
GMT may be able to use the matrix directly by reference. The
n_rows
and n_columns
parameters indicate the dimensions of the
matrix. If these are not yet known you may pass 0 for these values and
set alloc_mode
to GMT_ALLOCATED_BY_GMT
; this will make sure GMT will
allocate the necessary memory at the location you specify. Fortran users
will instead have to specify a size large enough to hold the anticipated
output data. The registration
and range
gives the grid
registration and domain. Finally, use dim
to indicate if the memory
matrix has a dimension that exceeds that of the leading row (or column)
dimension. Note: For GMT_IS_TEXTSET
the user matrix is expected to be
a 2-D character array with a fixed row length of dim
but we only
consider the first n_columns
characters. For data grids you will
also need to specify the registration
(see the GMT Cookbook and
Reference, GMT File Formats for description of the two forms of registration)
and data domain range
.
3.3. Create empty resources¶
If your application needs to build and populate GMT resources in ways that do not depend on external resources (files, memory locations, etc.), then you can obtain a “blank slate” by calling
void *GMT_Create_Data (void *API, unsigned int family, unsigned int geometry, unsigned int mode, uint64_t par[], double *wesn, double *inc, unsigned int registration, int pad, void *data)
which returns a pointer to the allocated resource. Pass family as
one of GMT_IS_GRID
, GMT_IS_IMAGE
, GMT_IS_DATASET
,
GMT_IS_TEXTSET
, or GMT_IS_CPT
, or via the modifiers GMT_IS_VECTOR
or GMT_IS_MATRIX
when handling user data. Also pass a compatible
geometry. Depending on the family and your particular way of
representing dimensions you may pass the additional parameters in one of
two ways:
- Actual integer dimensions of items needed.
- Physical distances and increments of each dimension.
For the first case pass wesn
, inc
as NULL (or arrays with elements all set to 0),
and pass the par
array as indicated below:
- GMT_IS_GRID
- An empty GMT_GRID structure with a header is allocated; the data array is NULL. The
par
argument is not used. Herewesn
andinc
can be NULL but than -R and -I must have been set because they are inquired to get the necessary info. If they were not set, thanwesn
andinc
must in fact be transmitted.- GMT_IS_IMAGE
- Same as GMT_IS_GRID above but return an empty GMT_IMAGE
- GMT_IS_DATASET
- An empty GMT_DATASET structure consisting of
par[0]
tables, each withpar[1]
segments, each withpar[2]
rows, all withpar[3]
columns, is allocated. Thewesn
,inc
, andregistration
argument are ignored. Thedata
argument should be NULL.- GMT_IS_TEXTSET
- An empty GMT_TEXTSET structure consisting of
par[0]
tables, each withpar[1]
segments, all withpar[2]
text records (rows), is allocated. Thewesn
,inc
, andregistration
argument are ignored. Thedata
argument should be NULL.- GMT_IS_CPT
- An empty GMT_PALETTE structure with
par[0]
palette entries is allocated. Thewesn
,inc
, andregistration
argument are ignored. Thedata
argument should be NULL.- GMT_IS_VECTOR
- An empty GMT_VECTOR structure with
par[0]
column entries is allocated. Thewesn
,inc
, andregistration
argument are ignored. Thedata
argument should be NULL.- GMT_IS_MATRIX
- An empty GMT_MATRIX structure is allocated.
par[2]
indicates the number of layers for a 3-D matrix, or pass 0, 1, or NULL for a 2-D matrix. Here, par[0] is the number of columns while par[1] has the number of rows. Thedata
argument should be NULL.
For the second approach, you
instead pass wesn
, inc
, and registration
and leave par
as NULL
(or with all elements equal 0).
For grids and images you may pass pad
to set the padding, or -1 to
accept the GMT default. The mode
determines what is actually
allocated when you have chosen grids or images. As for GMT_Read_Data
you can pass GMT_GRID_ALL
to initialize the header and allocate
space for the array; here data
must be NULL. Alternatively, you can pass
GMT_GRID_HEADER_ONLY
to just initialize the grid or image header,
and call a second time, passing GMT_GRID_DATA_ONLY
, to allocate
space for the array. In that second call you pass the pointer returned
by the first call as data
and specify the family; all other
arguments should be NULL or 0. Normally, resources created by this
function are considered to be input (i.e., have a direction that is GMT_IN
).
The exception to this is for vectors and matrices which will have a direction
set to GMT_OUT
when the dimensions specified are not complete (i.e., the
row dimension of vectors is 0 and both dimensions are zero for matrices).
The function returns a pointer to the
data container. In case of an error we return a NULL pointer and pass an
error code via API->error
.
3.4. Duplicate resources¶
Often you have read or created a data resource and then need an identical copy, presumably to make modifications to. Or, you want a copy with the same dimensions and allocated memory, except data values should not be duplicated. Alternatively, perhaps you just want to duplicate the header and skip the allocation and duplication of the data. These tasks are addressed by
void *GMT_Duplicate_Data (void *API, unsigned int family, unsigned int mode, void *data);
which returns a pointer to the allocated resource. Specify which
family and select mode
from GMT_DUPLICATE_DATA
,
GMT_DUPLICATE_ALLOC
, and GMT_DUPLICATE_NONE
, as discussed above
(also see mode
discussion above). For datasets and textsets you can
add modifiers GMT_ALLOC_VERTICAL
or GMT_ALLOC_HORIZONTAL
if you
wish to put all data in a single long table or to paste all tables
side-by-side, respectively (thus getting one wide table instead).
Additional note for datasets: Normally we allocate the output given the
corresponding input dimensions. You can override these by specifying your
alternative dimensions in the input dataset variable dim[].
The data
is a pointer to the resource you wish to duplicate. In case
of an error we return a NULL pointer and pass an error code via
API->error
.
3.5. Get resource ID¶
Resources created by these two methods can be used as in various ways. Sometimes you want to pass them as input to other modules, in which case you need to registration ID of that resource. This task are performed by
void *GMT_Get_ID (void *API, unsigned int family, unsigned int direction, void *data);
which returns the ID number of the allocated resource. Specify which
family and select direction
from GMT_IN
or GMT_OUT
.
The data
is a pointer to the resource you whose ID you need. In case
of an error we return GMT_NOTSET
and pass an error code via
API->error
.
3.6. Import Data¶
If your main program needs to read any of the five recognized data types (CPT files, data tables, text tables, GMT grids, or images) you will use the GMT_Get_Data or GMT_Read_Data functions, which both return entire data sets. In the case of data and text tables you may also select record-by-record reading using the GMT_Get_Record function. As a general rule, your program development simplifies if you can read entire resources into memory with GMT_Get_Data or GMT_Read_Data. However, if this leads to unacceptable memory usage or if the program logic is particularly simple, you may obtain one data record at the time via GMT_Get_Record.
All input functions takes a parameter called mode
. The mode
parameter generally has different meanings for the different data types
and will be discussed below. However, one bit setting is common to all
types: By default, you are only allowed to read a data source once; the
source is then flagged as having been read and subsequent attempts to
read from the same source will result in a warning and no reading takes
place. In the unlikely event you need to re-read a source you can
override this default behavior by adding GMT_IO_RESET
to your mode
parameter. Note that this override does not apply to sources that are
streams or file handles, as it may not be possible to re-read their
contents.
3.6.1. Enable Data Import¶
Once all input resources have been registered, we signal the API that we are done with the registration phase and are ready to start the actual data import. This step is only required when reading one record at the time. We initialize record-by-record reading by calling GMT_Begin_IO This function enables dataset and textset record-by-record reading and prepares the registered sources for the upcoming import. The prototype is
int GMT_Begin_IO (void *API, unsigned int family, unsigned int direction, unsigned int mode, unsigned int header);
where family specifies the resource type to be read or written
(only GMT_IS_DATASET
and GMT_IS_TEXTSET
are
available for record-by-record handling). The direction
is either
GMT_IN
or GMT_OUT
, so for import we obviously use GMT_IN
. The
function determines the first input source and sets up procedures for
skipping to the next input source in a virtual data set. The
GMT_Get_Record function will not be able to read any data before
GMT_Begin_IO has been called. As you might guess, there is a
companion GMT_End_IO function that completes, then disables
record-by-record data access. You can use these several times to switch
modes between registering data resources, doing the importing/exporting,
and disabling further data access, perhaps to do more registration. We
will discuss GMT_End_IO once we are done with the data import. The
mode
option is used to allow output to write table header
information (GMT_HEADER_ON
) or not (GMT_HEADER_OFF
). This is
usually on unless you are writing messages and other non-data. The final
header
argument determines if the common header-block should be
written during initialization; choose between GMT_HEADER_ON
and
GMT_HEADER_OFF
. The function returns 1 if there is an
error; otherwise it returns 0.
3.6.2. Import a data set¶
If your program needs to import any of the five recognized data types
(CPT table, data table, text table, GMT grid, or image) you will use
either the GMT_Read_Data or GMT_Get_Data functions. The former
is typically used when reading from files, streams (e.g., stdin
), or
an open file handle, while the latter is only used with a registered
resource via its unique ID. Because of the similarities of these five
import functions we use an generic form that covers all of them.
3.6.2.1. Import from a file, stream, or handle¶
To read an entire resource from a file, stream, or file handle, use
void *GMT_Read_Data (void *API, unsigned int family, unsigned int method, unsigned int geometry, unsigned int mode, double wesn[], const char *input, void *ptr);
- API
- family
- method
- geometry
- mode – see below
- wesn
- input – a pointer to char holding the file name to read.
- ptr – NULL or the pointer returned by this function after a first call (when reading grids in two steps)
- Return: Pointer to data container, or NULL if there were errors (passed back via API->error)
where ptr
is NULL except when reading grids in two steps (i.e.,
first get a grid structure with a header, then read the data). Most of
these arguments have been discussed earlier. This function can be called
in three different situations:
- If you have a single source (filename, stream pointer, etc.) you can call GMT_Read_Data directly; there is no need to first register the source with GMT_Register_IO or gather the sources with GMT_Init_IO. However, if you did register a single source you can still pass it via an encoded filename (see GMT_Encode_ID) or you can instead use GMT_Get_Data using the integer ID directly (see next section).
- If you want to specify
stdin
as source then useinput
as NULL. - If you already registered all desired sources with GMT_Init_IO
then you indicate this by passing
geometry
= 0.
Space will be allocated to hold the results, if needed, and a pointer to
the object is returned. If there are errors we simply return NULL and
report the error. The mode
parameter has different meanings for
different data types.
- CPT table
mode
contains bit-flags that control how the CPT file’s back-, fore-, and NaN-colors should be initialized. Select 0 to use the CPT file’s back-, fore-, and NaN-colors, 2 to replace these with the GMT default values, or 4 to replace them with the color table’s entries for highest and lowest value.- Data table
mode
is currently not used.- Text table
mode
is currently not used.- GMT grid
- Here,
mode
determines how we read the grid: To read the entire grid and its header, passGMT_GRID_ALL
. However, if you need to extract a sub-region you must first read the header by passingGMT_GRID_HEADER_ONLY
, then examine the header structure range attributes and to specify a subset via the arraywesn
, and finally call GMT_Read_Data a second time, now withmode
=GMT_GRID_DATA_ONLY
and passing yourwesn
array and the grid structure returned from the first call asptr
. In the event your data array should be allocated to hold both the real and imaginary parts of a complex data set you must add eitherGMT_GRID_IS_COMPLEX_REAL
orGMT_GRID_IS_COMPLEX_IMAG
tomode
so as to allow for the extra memory needed and to stride the input values correctly. If your grid is huge and you must read it row-by-row, setmode
toGMT_GRID_HEADER_ONLY
|GMT_GRID_ROW_BY_ROW
. You can then access the grid row-by-row using GMT_Get_Row By default the rows will be automatically processed in order. To completely specify which row to be read, useGMT_GRID_ROW_BY_ROW_MANUAL
instead.
If you need to read the same resource more than once you should add the
bitflag GMT_IO_RESET to the given mode
.
3.6.2.2. Import from a memory location¶
If you are importing via variables or prefer to first register the source, then you should use GMT_Get_Data instead. This function requires fewer arguments since you simply pass the unique ID number of the resource. The function is described as follows:
void *GMT_Get_Data (void *API, int ID, unsigned int mode, void *ptr);
The ID
is the unique object ID you received when registering the
resource, mode
controls some aspects of the import (see
GMT_Read_Data above), while ptr
is NULL except when reading
grids in two steps (i.e., first get a grid structure with a header, then
read the data). Other arguments have been discussed earlier. Space will
be allocated to hold the results, if needed, and a pointer to the object
is returned. If there are errors we simply return NULL and report the error.
3.6.2.3. Retrieve an allocated result¶
Finally, if you need to access the result that a GMT module wrote to a
memory location, then you must register an output destination with
GMT_Register_IO first (passing ptr
== NULL). The GMT module will
then allocate space to hold the output and let the API know where this
memory resides. You can then use GMT_Retrieve_Data to get a pointer
to the container where the data set was stored. This function requires
fewer arguments since you simply pass the unique ID number of the
resource. The function is described as follows:
void *GMT_Retrieve_Data (void *API, int ID);
The ID
is the unique object ID you received when registering the
NULL resource earlier, Since this container has already been created, a
pointer to the object is returned. If there are errors we simply return
NULL and report the error.
3.6.3. Importing a data record¶
If your program will read data table records one-by-one you must first enable this input mechanism with GMT_Begin_IO and then read the records in a loop using
void *GMT_Get_Record (void *API, unsigned int mode, int *nfields);
where the returned value is either a pointer to a double array with the
current row values or to a character string with the current row,
depending on mode
. In either case these pointers point to memory
internal to GMT and should be considered read-only. When we reach
end-of-file, encounter conversion problems, read header comments, or
identify segment headers we return a NULL pointer. The nfields
pointer will return the number of fields returned; pass NULL if your
program should ignore this information.
Normally (mode
== GMT_READ_DOUBLE
), we return a pointer to
the double array. To read text records, supply instead mode
==
GMT_READ_TEXT
and we instead return a pointer to the text
record. However, if you have input records that mixes organized
floating-point columns with text items you could pass mode
==
GMT_READ_MIXED
. Then, GMT will attempt to extract the
floating-point values; you can still access the record string, as
discussed below. Finally, if your application needs to be notified when
GMT closes one file and opens the next, add GMT_FILE_BREAK
to
mode
and check for the status code GMT_IO_NEXT_FILE
(by default,
we treat the concatenation of many input files as a single virtual
file). Using GMT_Get_Record requires you to first initialize the
source(s) with GMT_Init_IO. For certain records, GMT_Get_Record
will return NULL and sets status codes that your program will need to
examine to take appropriate response. Table [tbl:iostatus] list the
various status codes you can check for, using GMT_Status_IO
(see
next section).
3.6.4. Examining record status¶
Programs that read record-by-record must be aware of what the current record represents. Given the presence of headers, data gaps, NaN-record, etc., the developer will want to check the status after reading the next record. The internal i/o status mode can be interrogated with the function
int GMT_Status_IO (void *API, unsigned int mode);
which returns 0 (false) or 1 (true) if the current status is reflected
by the specified mode
. There are 11 different modes available to
programmers; for a list see Table IO-status For an example of how
these may be used, see the test program testgmtio.c
. Developers who plan to import
data on a record-by-record basis may also consult the source code of,
say, blockmean.c
or pstext.c
, to see examples of working code.
mode | description and return value |
---|---|
GMT_IO_DATA_RECORD | 1 if we read a data record |
GMT_IO_TABLE_HEADER | 1 if we read a table header |
GMT_IO_SEGMENT_HEADER | 1 if we read a segment header |
GMT_IO_ANY_HEADER | 1 if we read either header record |
GMT_IO_MISMATCH | 1 if we read incorrect number of columns |
GMT_IO_EOF | 1 if we reached the end of the file (EOF) |
GMT_IO_NAN | 1 if we only read NaNs |
GMT_IO_GAP | 1 if this record implies a data gap |
GMT_IO_NEW_SEGMENT | 1 if we enter a new segment |
GMT_IO_LINE_BREAK | 1 if we encountered a segment header, EOF, NaNs or gap |
GMT_IO_NEXT_FILE | 1 if we finished one file but not the last |
3.6.5. Importing a grid row¶
If your program must read a grid file row-by-row you must first enable
row-by-row reading with GMT_Read_Data and then use the
GMT_Get_Row
function in a loop; the prototype is
int GMT_Get_Row (void *API, int row_no, struct GMT_GRID *G, float *row);
where row
is a pointer to a single-precision array to receive the
current row, G
is the grid in question, and row_no
is the number
of the current row to be read. Note this value is only considered if the
row-by-row mode was initialized with GMT_GRID_ROW_BY_ROW_MANUAL
.
The user must allocate enough space to hold the entire row in memory.
3.6.6. Disable Data Import¶
Once the record-by-record input processing has completed we disable
further input to prevent accidental reading from occurring (due to poor
program structure, bugs, etc.). We do so by calling GMT_End_IO
. This
function disables further record-by-record data import; its prototype is
int GMT_End_IO (void *API, unsigned int direction, unsigned int mode);
and we specify direction
= GMT_IN
. At the moment, mode
is not
used. This call will also reallocate any arrays obtained into their
proper lengths. The function returns 1 if there is an error
(which is passed back with API->error
), otherwise it returns 0.
3.7. Manipulate data¶
Once you have created and allocated empty resources, or read in resources from the outside, you will wish to manipulate their contents. This section discusses how to set up loops and access the important variables for the various data families. For grids and images it may be required to know what the coordinates are at each node point. This can be obtained via arrays of coordinates for each dimension, obtained by
double *GMT_Get_Coord (void *API, unsigned int family, unsigned int dim, void *data);
where family must be GMT_IS_GRID
or GMT_IS_DATASET
, dim
is either
GMT_IS_X
or GMT_IS_Y
, and data
is the grid or image pointer. This
function will be used below in our example on grid manipulation.
Another aspect of dealing with grids and images is to convert a row and column 2-D reference to our 1-D array index. Because of grid and image boundary padding the relationship is not straightforward, hence we supply
int64_t GMT_Get_Index (struct GMT_GRID_HEADER *header, int row, int col);
where the header
is the header of either a grid or image, and row
and
col
is the 2-D position in the grid or image. We return the 1-D array
position; again this function is used below in our example.
3.7.1. Manipulate grids¶
Most applications wishing to manipulate grids will want to loop over all the nodes, typically in a manner organized by rows and columns. In doing so, the coordinates at each node may also be required for a calculation. Below is a snippet of code that shows how to do visit all nodes in a grid and assign each node the product x * y:
int row, col, node; double *x_coord = NULL, *y_coord = NULL; < ... create a grid G or read one ... > x_coord = GMT_Get_Coord (API, GMT_IS_GRID, GMT_X, G); y_coord = GMT_Get_Coord (API, GMT_IS_GRID, GMT_Y, G); for (row = 0; row < G->header->ny) { for (col = 0; col < G->header->nx; col++) { node = GMT_Get_Index (G->header, row, col); G->data[node] = x_coord[col] * y_coord[row]; } }
Note the use of GMT_Get_Index to get the grid node number associated
with the row
and col
we are visiting. Because GMT grids have
padding (for boundary conditions) the relationship between rows,
columns, and node indices is more complicated and hence we hide that
complexity in GMT_Get_Index. Note that for trivial procedures such
setting all grid nodes to a constant (e.g., -9999.0) where the row and
column does not enter you can instead do a single loop:
int node; < ... create a grid G or read one ... > for (node = 0; node < G->header->size) G->data[node] = -9999.0;
Note we must use G->header->size
(size of allocated array) and not
G->header->nm
(number of nodes in grid) since the latter is smaller
due to the padding and a single loop like the above treats the pad as
part of the “inside” grid.
3.7.2. Manipulate data tables¶
Another common application is to process the records in a data table.
Because GMT consider the GMT_DATASET
resources to contain one or more
tables, each of which may contain one or more segments, all of which may
contain one or more columns, you will need to have multiple loops to
visit all entries. The following code snippet will visit all data
records and add 1 to all columns beyond the first two (x and y):
uint64_t tbl, seg, row, col; struct GMT_DATATABLE *T = NULL; struct GMT_DATASEGMENT *S = NULL; < ... create a dataset D or read one ... > for (tbl = 0; tbl < D->n_tables; tbl++) { /* For each table */ T = D->table[tbl]; /* Convenient shorthand for current table */ for (seg = 0; seg < T->n_segments; seg++) { /* For all segments */ S = T->segment[seg]; /* Convenient shorthand for current segment */ for (row = 0; row < S->n_rows; row++) { for (col = 2; col < T->n_columns; col++) { S->coord[col][row] += 1.0; } } } }
3.7.3. Manipulate text tables¶
When data file contain text mixed in with numbers you must open the file
as a GMT_TEXTSET
and do your own parsing of the data records. The
following code snippet will visit all text records and print them out:
uint64_t tbl, seg, row, col; struct GMT_TEXTTABLE *T = NULL; struct GMT_TEXTSEGMENT *S = NULL; < ... create a textset D or read one ... > for (tbl = 0; tbl < D->n_tables; tbl++) { /* For each table */ T = D->table[tbl]; /* Convenient shorthand for current table */ for (seg = 0; seg < T->n_segments; seg++) { /* For all segments */ S = T->segment[seg]; /* Convenient shorthand for current segment */ for (row = 0; row < S->n_rows; row++) { printf ("T=%d S=%d R=%d : %s\n", tbl, seg, row, S->record[row]); } } }
3.8. Message and Verbose Reporting¶
The API provides two functions for your program to present information to the user during the run of the program. One is used for messages that are always written while the other is used for reports that must exceed the verbosity settings specified via -V.
int GMT_Report (void *API, unsigned int level, const char *message, ...);
This function takes a verbosity level and a multi-part message (e.g., a
format statement and zero or more variables). The verbosity level
is
an integer in the 0–5 range; these are listed in Table [tbl:verbosity].
You assign an appropriate verbosity level to your message, and depending
on the chosen run-time verbosity level set via -V your message may
or may not be reported. Only messages whose stated verbosity level is
lower or equal to the -Vlevel will be printed.
constant | description |
---|---|
GMT_MSG_QUIET | No messages whatsoever |
GMT_MSG_NORMAL | Default output, e.g., warnings and errors only |
GMT_MSG_COMPAT | Compatibility warnings |
GMT_MSG_VERBOSE | Verbose level |
GMT_MSG_LONG_VERBOSE | Longer verbose |
GMT_MSG_DEBUG | Debug messages for developers mostly |
int GMT_Message (void *API, unsigned int mode, const char *format, ...);
This function always prints its message to the standard output. Use the
mode
value to control if a time stamp should preface the message.
and if selected how the time information should be formatted. See
Table timemodes for the various modes.
constant | description |
---|---|
GMT_TIME_NONE | Display no time information |
GMT_TIME_CLOCK | Display current local time |
GMT_TIME_ELAPSED | Display elapsed time since last reset |
GMT_TIME_RESET | Reset the elapsed time to 0 |
3.9. Presenting and accessing GMT options¶
As you develop a program you may need to rely on some of
the GMT common options. For instance, you may wish to have your
program present the -R
option to the user, let GMT handle the
parsing, and examine the values. You may also wish to encode your own
custom options that may require you to parse user text into the
corresponding floating point dimensions, constants, coordinates, time, etc.
The API provides several functions to simplify these tedious parsing
tasks. This section is intended to show how the programmer will obtain
information from the user that is necessary to do the task at hand
(e.g., special options to provide values and settings for the program).
In the following section we will concern ourselves with preparing
arguments for calling any of the GMT modules.
3.9.1. Display usage syntax for GMT common options¶
You can have your program menu display the standard usage message for a GMT common option by calling the function
int GMT_Option (void *API, const char *options);
where options
is a comma-separated list of GMT common options
(e.g., “R,J,O,X”). You can repeat this function with different sets of
options in order to intersperse your own custom options with in an
overall alphabetical order; see any GMT module for examples of typical
layouts.
3.9.2. Parsing the GMT common options¶
The parsing of all GMT common option is done by
int GMT_Parse_Common (void *API, const char *args, struct GMT_OPTION *list);
where args
is a string of the common GMT options your program may
use. An error will be reported if any of the common GMT options fail
to parse, and if so we return TRUE; if not errors we return FALSE. All
other options, including file names, will be silently ignored. The
parsing will update the internal GMT information structure that
affects program operations.
3.9.3. Inquiring about the GMT common options¶
The API provide only a limited window into the full GMT machinery accessible to the modules. You can determine if a particular common option has been parsed and in some cases determine the values that was set with
int GMT_Get_Common (void *API, unsigned int option, double *par);
where option
is a single option character (e.g., ‘R’) and par
is
a double array with at least a length of 6. If the particular option has
been parsed then the function returns the number of parameters passed
back via par
; otherwise we return -1. For instance, to determine if
the -R
was set and what the resulting region was set to you may call
if (GMT_Get_Common (API, 'R', wesn)) != -1) { /* wesn now contains the boundary information */ }
The wesn
array could now be passed to the various read and create
functions for GMT resources.
3.9.4. Parsing text values¶
Your program may need to request values from the user, such as distances, plot dimensions, coordinates, and other data. The conversion from such text to actual distances, taking units into account, is tedious to program. You can simplify this by using
int GMT_Get_Value (void *API, const char *arg, double par[]);
where arg
is the text item with one or more values that are
separated by commas, spaces, tabs, semi-colons, or slashes, and par
is an array long
enough to hold all the items you are parsing. The function returns the
number of items parsed, or -1 if there is an error. For instance, assume
the character string origin
was given by the user as two geographic
coordinates separated by a slash (e.g., "35:45W/19:30:55.3S"
). We
obtain the two coordinates as decimal degrees by calling
n = GMT_Get_Value (API, origin, pair);
Your program can now check that n
equals 2 and then use the values
in pairs
. Note: Dimensions given with units of inches, cm, or points
are converted to the current default unit set via PROJ_LENGTH_UNIT,
while distances given in km, nautical miles, miles, feet, or
survey feet are returned in meters. Arc lengths in minutes and seconds
are returned in decimal degrees, and date/time values are returned in
seconds since the epoch (1970).
3.9.5. Get or set an API or GMT default parameter¶
If your program needs to determine one or more of the current API or GMT default settings you can do so via
int GMT_Get_Default (void *API, const char *keyword, char *value);
where keyword
is one such keyword (e.g., PROJ_LENGTH_UNIT) and
value
must be a character array long enough to hold the answer. In
addition to the long list of GMT defaults you can also inquire about the
few API parameters API_PAD (the current pad setting), API_IMAGE_LAYOUT (the
order and structure of image memory storage), and API_GRID_LAYOUT (order of
grid memory storage).
Depending on what parameter you selected you could further convert it to
a numerical value with GMT_Get_Value
or just use it in a text comparison.
To change any of the API or GMT default settings you would use
int GMT_Set_Default (void *API, const char *keyword, const char *value);
where as before keyword
is one such keyword (e.g., PROJ_LENGTH_UNIT) and
value
must be a character string with the new setting.
Note that all settings are passed as text strings even if man are
inherently integers or floats.
3.10. Prepare module options¶
One of the advantages of programming with the API is that you
have access to the high-level GMT modules. For example, if your
program must compute the distance from a point to all other points on
the node you can simply set up options and call GMT_grdmath
to do it
for you and accept the result back as an input grid. All the module
interfaces are identical are looks like
int GMT_Call_Module (void *API, const char *module, int mode, void *args);
Here, module
can be any of the GMT modules, such as
psxy
or grdvolume
. All GMT modules may be called with one of
three sets of args
depending on mode
. The three modes differ in
how the options are passed to the module:
- mode ==
GMT_MODULE_EXIST
- Return GMT_NOERROR (0) if module exists, nonzero otherwise.
- mode ==
GMT_MODULE_PURPOSE
- Just prints the purpose of the module; args must be NULL.
- mode ==
GMT_MODULE_LIST
- Just prints a list of all modules; args must be NULL.
- mode ==
GMT_MODULE_OPT
- Expects
args
to be a pointer to a doubly-linked list of objects with individual options for the current program. We will see how API functions can help prepare such lists.- mode ==
GMT_MODULE_CMD
- Expects
args
to be a single text string with all required options.- mode > 0
- Expects
args
to be an array of text options andmode
to be a count of how many options are passed (i.e., theargc, argv[]
model used by the GMT programs themselves).
If module
equals NULL and mode == GMT_MODULE_PURPOSE
then we list summaries for all the modules
If no module by the given name is found we return -1.
3.10.1. Set program options via text array arguments¶
When mode > 0
we expect an array args
of character
strings that each holds a single command line options (e.g.,
“-R120:30/134:45/8S/3N”) and interpret mode
to be the count of how
many options are passed. This, of course, is almost exactly how the
stand-alone GMT programs are called (and reflects how they themselves
are activated internally). We call this the “argc-argv” mode. Depending
on how your program obtains the necessary options you may find that this
interface offers all you need.
3.10.2. Set program options via text command¶
If mode ==
0 then args
will be examined to see if it contains
several options within a single command string. If so we will break
these into separate options. This is useful if you wish to pass a single
string such as “-R120:30/134:45/8S/3N -JM6i mydata.txt -Sc0.2c”. We call
this the “command” mode.
3.10.3. Set program options via linked structures¶
The third, linked-list interface allows developers using higher-level
programming languages to pass all command options via a pointer to a
NULL-terminated, doubly-linked list of option structures, each
containing information about a single option. Here, instead of text
arguments we pass the pointer to the linked list of options mentioned
above, and mode
must be passed as -1 (or any negative value). Using
this interface can be more involved since you need to generate the
linked list of program options; however, utility functions exist to
simplify its use. This interface is intended for programs whose internal
workings are better suited to generate such arguments – we call this the
“options” mode. The order in the list is not important as GMT will
sort it internally according to need. The option structure is defined below.
struct GMT_OPTION { char option; /* Single character of the option (e.g., 'G' for -G) */ char *arg; /* String pointer with arguments (NULL if not used) */ struct GMT_OPTION *next; /* Pointer to next option (NULL for last option) */ struct GMT_OPTION *prev; /* Pointer to previous option (NULL for first option) */ };
3.10.4. Convert between text and linked structures¶
To assist programmers there are also two convenience functions that allow you to convert between the two argument formats. They are
struct GMT_OPTION *GMT_Create_Options (void *API, int argc, void *args);
This function accepts your array of text arguments (cast via a void
pointer), allocates the necessary space, performs the conversion, and
returns a pointer to the head of the linked list of program options.
However, in case of an error we return a NULL pointer and set
API->error
to indicate the nature of the problem. Otherwise, the
pointer may now be passed to the relevant GMT_module
. Note that if
your list of text arguments were obtained from a C main()
function
then argv[0]
will contain the name of the calling program. To avoid
passing this as a file name option, call GMT_Create_Options with
argc-1
and argv+1
. If you wish to pass a single text string with
multiple options (in lieu of an array of text strings), then pass
argc
= 0. When no longer needed you can remove the entire list by calling
int GMT_Destroy_Options (void *API, struct GMT_OPTION **list);
The function returns 1 if there is an error (which is passed back
with API->error
), otherwise it returns 0.
The inverse function prototype is
char **GMT_Create_Args (void *API, int *argc, struct GMT_OPTION *list);
which allocates space for the text strings and performs the conversion;
it passes back the count of the arguments via argc
and returns a
pointer to the text array. In the case of an error we return a NULL
pointer and set API->error
to reflect the error type. Note that
argv[0]
will not contain the name of the program as is the case the
arguments presented by a C main()
function. When you no longer have
any use for the text array, call
int GMT_Destroy_Args (void *API, int argc, char **argv[]);
to deallocate the space used. This function returns 1 if there is
an error (which is passed back with API->error
), otherwise it returns 0.
Finally, to convert the linked list of option structures to a single text string command, use
char *GMT_Create_Cmd (void *API, struct GMT_OPTION *list);
Developers who plan to import and export GMT shell scripts might find
it convenient to use these functions. In case of an error we return a
NULL pointer and set API->error
, otherwise a pointer to an allocated
string is returned. When you no longer have
any use for the text string, call
int GMT_Destroy_Cmd (void *API, char **argv);
to deallocate the space used. This function returns 1 if there is
an error (which is passed back with API->error
), otherwise it
returns 0.
3.10.5. Manage the linked list of options¶
Several additional utility functions are available for programmers who wish to manipulate program option structures within their own programs. These allow you to create new option structures, append them to the linked list, replace existing options with new values, find a particular option, and remove options from the list. Note: The order in which the options appear in the linked list is of no consequence to GMT. Internally, GMT will sort and process the options in the manner required. Externally, you are free to maintain your own order.
3.10.5.1. Make a new option structure¶
GMT_Make_Option
will allocate a new option structure, assign it
values given the option
and arg
parameter (pass NULL if there is
no argument for this option), and returns a pointer to the allocated
structure. The prototype is
struct GMT_OPTION *GMT_Make_Option (void *API, char option, const char *arg);
Should memory allocation fail the function will print an error message
set an error code via API->error
, and return NULL.
3.10.5.2. Append an option to the linked list¶
GMT_Append_Option
will append the specified option
to the end of
the doubly-linked list
. The prototype is
struct GMT_OPTION *GMT_Append_Option (void *API, struct GMT_OPTION *option, struct GMT_OPTION *list);
We return the list back, and if list
is given as NULL we return
option
as the start of the new list. Any errors results in a NULL
pointer with API->error
holding the error type.
3.10.5.3. Find an option in the linked list¶
GMT_Find_Option
will return a pointer ptr
to the first option in
the linked list starting at list
whose option character equals
option
. If not found we return NULL. While this is not necessarily
an error we still set API->error
accordingly. The prototype is
struct GMT_OPTION *GMT_Find_Option (void *API, char option, struct GMT_OPTION *list);
If you need to look for multiple occurrences of a certain option you
will need to call GMT_Find_Option again, passing the option
following the previously found option as the list
entry, i.e.,
list = *ptr->next;
3.10.5.4. Update an existing option in the list¶
GMT_Update_Option will replace the argument of current
with the
new argument arg
and otherwise leave the option at its place in the
list. The prototype is
int GMT_Update_Option (void *API, struct GMT_OPTION *current, const char *arg);
An error will be reported if (a) current
is NULL or (b) arg
is
NULL. The function returns 1 if there is an error, otherwise it returns 0.
3.10.5.5. Delete an existing option in the linked list¶
You may use GMT_Delete_Option
to remove option
from the linked
list
. The prototype is
int GMT_Delete_Option (void *API, struct GMT_OPTION *current);
We return TRUE if the option is not found in the list and set
API->error
accordingly. Note: Only the first occurrence of the
specified option will be deleted. If you need to delete all such options
you will need to call this function in a loop until it returns a
non-zero status.
3.10.5.6. Specify a file via an linked option¶
To specify an input file name via an option, simply use < as the option (this is what GMT_Create_Options does when it finds filenames on the command line). Likewise, > can be used to explicitly indicate an output file. In order to append to an existing file, use >>. For example the following command would read from file.A and append to file.B:
gmt convert -<file.A ->>file.B
These options also work on the command line but usually one would have to escape the special characters < and > as they are used for file redirection.
3.10.5.7. Encode option arguments for external interfaces¶
Developers writing interfaces between GMT and external platforms such as other languages (Python, Java, Julia, etc.) or tools (MATLAB, Octave, etc.) need to manipulate linked options in a special way. For instance, a GMT call in the MATLAB or Octave application might look like
table = gmt ('blockmean -R30W/30E/10S/10N -I2m', [x y z]);
grid = gmt ('surface -R -I2m -Lu$', high_limit_grid, table);
and in such environments we need the ability to (1) specify references
to memory items (via a marker, here “$”) and (2) supply implicit
module arguments (here a command-line “$” will be added to both commands
to represent the input 3-column table, a “> $” to indicate the output
of blockmean should go to a memory reference (eventually end up in the
variable table, and a “-G$” to indicate the output grid from surface
should be written to a memory reference, ending up in the variable grid).
Such explicit and implicit references to data sources requires processing
and even the addition of extra options to the linked list of options.
API developers may use GMT_Encode_Options
to do so.
The prototype is
struct GMT_RESOURCE *GMT_Encode_Options (void *API, const char *module, char marker, int n_in, struct GMT_OPTION **head, int *n_items);
where module
is the name of the module whose linked options are
pointed to by *head
, the marker
is the special character that
identifies a data resource (usually $), n_in
contains the number of input
objects we have to connect (or -1 if not known) and we return an array
that contains specific information for those options that
(after processing) contain explicit memory references. The number of
items in the array is returned via the n_items
variable. The function
returns NULL if there are errors and sets API->error
to the corresponding
error number. The GMT_RESOURCE structure is defined below:
struct GMT_RESOURCE { /* Information for passing resources */
enum GMT_enum_family family; /* GMT data family */
enum GMT_enum_geometry geometry; /* One of the recognized GMT geometries */
enum GMT_enum_std direction; /* Either GMT_IN or GMT_OUT */
struct GMT_OPTION *option; /* Pointer to the corresponding module option */
int object_ID; /* Object ID returned by GMT_Register_IO */
int pos; /* Corresponding index into external object in|out arrays */
void *object; /* Pointer to the registered GMT object */
};
API developers will need to provide specific code to handle the registration of native structures in their language or application and to translate between the GMT resources and the corresponding native items. Developers should look at an existing and working interface such as the MATLAB mex-gmt to see the required steps.
3.10.5.8. Expand option with explicit memory references¶
When the external tool or application knows the name of special file name
used for memory references the developer should replace the marker
character
in any option string with the actual reference name. This is accomplished by
calling GMT_Expand_Option
, with prototype
int GMT_Expand_Option (void *API, struct GMT_OPTION *option, char marker, const char *name);
where option
is the current option, marker
is the character
identifier representing an explicit memory reference, and name
is the special file name for the memory reference.
3.11. Calling a GMT module¶
Given your linked list of program options (or text array) and possibly some registered resources, you can now call the required GMT module using one of the two flavors discussed in section All modules return an error or status code that your program should consider before processing the results.
3.12. Adjusting headers and comments¶
All header records in incoming datasets are stored in memory. You may wish to replace these records with new information, or append new information to the existing headers. This is achieved with
int GMT_Set_Comment (void *API, unsigned int family, unsigned int mode void *arg, void *data)
Again, family selects which kind of resource is passed via data
.
The mode
determines what kind of comment is being considered, how it
should be included, and in what form the comment passed via arg
is.
Table comments lists the available options, which may be combined
by adding (bitwise “or”). The GMT_Set_Comment
does not actually
output anything but sets the relevant comment and header records in the
relevant structure. When a file is written out the information will be
output as well (Note: Users can always decide if they wish to turn
header output on or off via the common GMT option -h
. For
record-by-record writing you must enable the header block output when
you call GMT_Begin_IO
constant | description |
---|---|
GMT_COMMENT_IS_TEXT | Comment is a text string |
GMT_COMMENT_IS_OPTION | Comment is a linked list of GMT_OPTION structures |
GMT_COMMENT_IS_COMMAND | Comment is the command |
GMT_COMMENT_IS_REMARK | Comment is the remark |
GMT_COMMENT_IS_TITLE | Comment is the title |
GMT_COMMENT_IS_NAME_X | Comment is the x variable name (grids only) |
GMT_COMMENT_IS_NAME_Y | Comment is the y variable name (grids only) |
GMT_COMMENT_IS_NAME_Z | Comment is the z variable name (grids only) |
GMT_COMMENT_IS_COLNAMES | Comment is the column names header |
GMT_COMMENT_IS_RESET | Comment replaces existing information |
The named modes (command, remark, title, name_x,y,z and
colnames are used to distinguish regular text comments from specific
fields in the header structures of the data resources, such as
GMT_GRID
. For the various table resources (e.g., GMT_DATASET
)
these modifiers result in a specially formatted comments beginning with
“Command: ” or “Remark: ”, reflecting how this type of information is
encoded in the headers.
3.13. Exporting Data¶
If your program needs to write any of the four recognized data types (CPT files, data tables, text tables, or GMT grids) you can use the GMT_Put_Data. In the case of data and text tables, you may also consider the GMT_Put_Record function. As a general rule, your program organization may simplify if you can write and export the entire resource with GMT_Put_Data. However, if the program logic is simple or already involves using GMT_Get_Record, it may be better to export one data record at the time via GMT_Put_Record.
Both of these output functions takes a parameter called mode
. The
mode
parameter generally takes on different meanings for the
different data types and will be discussed below. However, one bit
setting is common to all types: By default, you are only allowed to
write a data resource once; the resource is then flagged to have been
written and subsequent attempts to write to the same resource will
quietly be ignored. In the unlikely event you need to re-write a
resource you can override this default behavior by adding GMT_IO_RESET
to your mode
parameter.
3.13.1. Enable Data Export¶
Similar to the data import procedures, once all output destinations have
been registered, we signal the API that we are done with the
registration phase and are ready to start the actual data export. As for
input, this step is only needed when dealing with record-by-record
writing. Again, we enable record-by-record writing by calling
GMT_Begin_IO, this time with direction
= GMT_OUT
. This function
enables data export and prepares the registered destinations for the
upcoming writing.
3.13.2. Exporting a data set¶
To have your program accept results from GMT modules and write them separately requires you to use the GMT_Write_Data or GMT_Put_Data functions. They are very similar to the GMT_Read_Data and GMT_Get_Data functions encountered earlier.
3.13.2.1. Exporting a data set to a file, stream, or handle¶
The prototype for writing to a file (via name, stream, or file handle) is
int GMT_Write_Data (void *API, unsigned int family, unsigned int method, unsigned int geometry, unsigned int mode, double wesn[], void *output, void *data);
- API
- family
- method
- geometry
- mode – specific to each data type (see below)
- wesn
- output –
- data – A pointer to any of the four structures GMT_VECTOR, GMT_MATRIX, GMT_GRID, GMT_PALETTE
- Return: 0 on success, otherwise return -1 and set API->error to reflect to cause.
where data
is a pointer to any of the four structures discussed previously.
- CPT table
mode
controls if the CPT table’s back-, fore-, and NaN-colors should be written (1) or not (0).- Data table
If
method
isGMT_IS_FILE
, then the value ofmode
affects how the data set is written:- GMT_WRITE_SET
- The entire data set will be written to the single file [0].
- GMT_WRITE_TABLE
- Each table in the data set is written to individual files [1].
You can either specify an output file name that must contain
one C-style format specifier for a int variable (e.g.,
“New_Table_%06d.txt”), which will be replaced with the table
number (a running number from 0) or you must assign to each
table i a unique output file name via the
D->table[i]->file[GMT_OUT]
variables prior to calling the function. - GMT_WRITE_SEGMENT
- Each segment in the data set is written to an individual file
[2]. Same setup as for
GMT_WRITE_TABLE
except we use sequential segment numbers to build the file names. - GMT_WRITE_TABLE_SEGMENT
- Each segment in the data set is written to an individual file
[3]. You can either specify an output file name that must
contain two C-style format specifiers for two int variables
(e.g., “New_Table_%06d_Segment_%03d.txt”), which will be
replaced with the table and segment numbers, or you must
assign to each segment j in each table i a unique output
file name via the
D->table[i]->segment[j]->file[GMT_OUT]
variables prior to calling the function. - GMT_WRITE_OGR
- Writes the dataset in OGR/GMT format in conjunction with the
-a
setting [4].
- Text table
- The
mode
is used the same way as for data tables. - GMT grid
- Here,
mode
may beGMT_GRID_HEADER_ONLY
to only update a file’s header structure, but normally it is simplyGMT_GRID_ALL
so the entire grid and its header will be exported (a subset is not allowed during export). However, in the event your data array holds both the real and imaginary parts of a complex data set you must add eitherGMT_GRID_IS_COMPLEX_REAL
orGMT_GRID_IS_COMPLEX_IMAG
tomode
so as to export the corresponding grid values correctly. Finally, for native binary grids you may skip writing the grid header by addingGMT_GRID_NO_HEADER
; this setting is ignored for other grid formats. If your output grid is huge and you are building it row-by-row, setmode
toGMT_GRID_HEADER_ONLY
|GMT_GRID_ROW_BY_ROW
. You can then write the grid row-by-row using GMT_Put_Row. By default the rows will be automatically processed in order. To completely specify which row to be written, useGMT_GRID_ROW_BY_ROW_MANUAL
instead.
Note: If method
is GMT_IS_FILE, family is GMT_IS_GRID
,
and the filename implies a change from NaN to another value then the grid is
modified accordingly. If you continue to use that grid after writing please be
aware that the changes you specified were applied to the grid.
3.13.2.2. Exporting a data set to memory¶
If writing to a memory destination you will want to first register that
destination and then use the returned ID with GMT_Put_Data
instead:
int GMT_Put_Data (void *API, int ID, unsigned int mode, void *data);
where ID
is the unique ID of the registered destination, mode
is
specific to each data type (and controls aspects of the output
structuring), and data
is a pointer to any of the four structures
discussed previously. For more detail, see GMT_Write_Data above. If
successful the function returns 0; otherwise we return 1
and set API->error
to reflect to cause.
3.13.3. Exporting a data record¶
If your program must write data table records one-by-one you must first
enable record-by-record writing with GMT_Begin_IO and then use the
GMT_Put_Record
function in a loop; the prototype is
int GMT_Put_Record (void *API, unsigned int mode, void *rec);
where rec
is a pointer to either (a) a double-precision array with
the current row. Then, rec
is expected to hold at least as many
items as the current setting of n_col[GMT_OUT]
, which represents the
number of columns in the output destination. Alternatively (b), rec
points to a text string. The mode
parameter must be set to reflect
what is passed. Using GMT_Put_Record requires you to first
initialize the destination with GMT_Init_IO. Note that for families
GMT_IS_DATASET and GMT_IS_TEXTSET
the methods GMT_IS_DUPLICATE
and
GMT_IS_REFERENCE are not supported since you can simply populate the
GMT_DATASET structure directly. As mentioned, mode
affects what is
actually written:
- GMT_WRITE_DOUBLE
- Normal operation that builds the current output record from the values in
rec
. - GMT_WRITE_TEXT
- For ASCII output mode we write the text string
rec
. Ifrec
is NULL then we use the current (last imported) text record. If binary output mode we quietly skip writing this record. - GMT_WRITE_TABLE_HEADER
- For ASCII output mode we write the text string
rec
. Ifrec
is NULL then we write the last read header record (and ensures it starts with #). If binary output mode we quietly skip writing this record. - GMT_WRITE_SEGMENT_HEADER
- For ASCII output mode we use the text string
rec
as the segment header. Ifrec
is NULL then we use the current (last read) segment header record. If binary output mode instead we write a record composed of NaNs.
The function returns 1 if there was an error associated with the
writing (which is passed back with API->error
), otherwise it returns
0.
3.13.4. Exporting a grid row¶
If your program must write a grid file row-by-row you must first enable
row-by-row writing with GMT_Read_Data and then use the
GMT_Put_Row
function in a loop; the prototype is
int GMT_Put_Row (void *API, int row_no, struct GMT_GRID *G, float *row);
where row
is a pointer to a single-precision array with the current
row, G
is the grid in question, and row_no
is the number of the
current row to be written. Note this value is only considered if the
row-by-row mode was initialized with GMT_GRID_ROW_BY_ROW_MANUAL
.
3.13.5. Disable Data Export¶
Once the record-by-record output has completed we disable further output
to prevent accidental writing from occurring (due to poor program
structure, bugs, etc.). We do so by calling GMT_End_IO
. This
function disables further record-by-record data export; here, we
obviously pass direction
as GMT_OUT
.
3.14. Destroy allocated resources¶
If your session imported any data sets into memory then you may
explicitly free this memory once it is no longer needed and before
terminating the session. This is done with the GMT_Destroy_Data
function, whose prototype is
int GMT_Destroy_Data (void *API, void *data);
where data
is the address of the pointer to a data container. Note that
when each module completes it will automatically free memory created by
the API; similarly, when the session is destroyed we also automatically
free up memory. Thus, GMT_Destroy_Data
is therefore generally only
needed when you wish to directly free up memory to avoid running out of
it. The function returns 1 if there is an error when trying to
free the memory (the error code is passed back with API->error
),
otherwise it returns 0.
3.15. Terminate a GMT session¶
Before your program exits it should properly terminate the GMT session, which involves a call to
int GMT_Destroy_Session (void *API);
which simply takes the pointer to the GMT API control structure as its
only arguments. It terminates the GMT machinery and deallocates all
memory used by the GMT API book-keeping. It also unregisters any
remaining resources previously registered with the session. The
GMT API will only close files that it was responsible for opening in
the first place. Finally, the API structure itself is freed so your main
program does not need to do so. The function returns 1 if there
is an error when trying to free the memory (the error code is passed
back with API->error
), otherwise it returns 0.
4. The GMT FFT Interface¶
While the i/o options presented so far lets you easily read in a data table or grid and manipulated them, if you need to do so in the wavenumber domain then this chapter is for you. Here we outline how to take the Fourier transform of such data, perform calculations in the wavenumber domain, and take the inverse transform before writing the results. To assist programmers we also distribute fully functioning demonstration programs that takes you through the steps we are about to discuss; these demo programs may be used as your starting point for further development.
4.1. Presenting and Parsing the FFT options¶
Several GMT programs using FFTs present the same unified option and modifier sets to the user. The API makes these available as well. If your program needs to present the option usage you can call
unsigned int GMT_FFT_Option (void *API, char option, unsigned int dim, const char *string);
Here, option
is the unique character used for this particular
program option (most GMT programs have standardized on using ‘N’ but
you are free to choose whatever you want except existing GMT common
options). The dim
sets the dimension of the transform, currently you
must choose 1 or 2, while the string
is a one-line message that
states what the option does; you should tailor this to your program. If
NULL then a generic message is placed instead.
To parse the user’s selection you call
void *GMT_FFT_Parse (void *API, char option, unsigned int dim, const char *args);
which accepts the user’s string option via args
; the other arguments
are the same as those above. The function returns an opaque pointer to a
structure with the chosen parameters.
4.2. Initializing the FFT machinery¶
Before your can take any transforms you must initialize the FFT machinery. This process involves a series of preparatory steps that are conveniently performed for you by
void *GMT_FFT_Create (void *API, void *X, unsigned int dim, unsigned int mode, void *F);
Here, X
is either your dataset or grid pointer, dim
is the
dimension of the transform (1 or 2 only), mode
passes various flags to the setup, such as whether
the data is real, imaginary, or complex, and F
is the opaque pointer
returned by GMT_FFT_Parse. Depending on the options you chose to
pass to GMT_FFT_Parse, the data may have a constant or a trend
removed, reflected and extended by various symmetries, padded and
tapered to desired transform dimensions, and possibly there are
temporary files written out before the transform takes place. See the grdfft
man page for a full explanation of the options presented by GMT_FFT_Option.
4.3. Taking the FFT¶
Now that everything has been set up you can perform the transform with
void *GMT_FFT (void *API, void *X, int direction, unsigned int mode, void *K);
which takes as direction
either GMT_FFT_FWD
or GMT_FFT_INV
. The
mode is used to specify if we pass a real (GMT_FFT_REAL) or complex
(GMT_FFT_COMPLEX) data set, and K
is the opaque pointer returned
by GMT_FFT_Create. The transform is performed in place and returned
via X
. When done with your manipulations (below) you can call it
again with the inverse flag to recover the corresponding space-domain
version of your data. The FFT is fully normalized so that calling
forward followed by inverse yields the original data set. The information
passed via K
determines if a 1-D or 2-D transform takes place; the
key work is done via GMT_FFT_1D
or GMT_FFT_2D
below.
4.4. Taking the 1-D FFT¶
A lower-level 1-D FFT is also available via
int GMT_FFT_1D (void *API, float *data, uint64_t n, int direction, unsigned int mode);
which takes as direction
either GMT_FFT_FWD
or GMT_FFT_INV
. The
mode is used to specify if we pass a real (GMT_FFT_REAL) or complex
(GMT_FFT_COMPLEX) data set, and data
is the 1-D data array of length
n
that we wish
to transform. The transform is performed in place and returned
via data
. When done with your manipulations (below) you can call it
again with the inverse flag to recover the corresponding space-domain
version of your data. The 1-D FFT is fully normalized so that calling
forward followed by inverse yields the original data set. Note that unlike
GMT_FFT
, this function does not do any data extension, mirroring, detrending,
etc. but operates directly on the data array given.
4.5. Taking the 2-D FFT¶
A lower-level 2-D FFT is also available via
int GMT_FFT_2D (void *API, float *data, unsigned int nx, unsigned int ny, int direction, unsigned int mode);
which takes as direction
either GMT_FFT_FWD
or GMT_FFT_INV
. The
mode is used to specify if we pass a real (GMT_FFT_REAL) or complex
(GMT_FFT_COMPLEX) data set, and data
is the 2-D data array in
row-major format, with row length nx
and column length ny
.
The transform is performed in place and returned
via data
. When done with your manipulations (below) you can call it
again with the inverse flag to recover the corresponding space-domain
version of your data. The 2-D FFT is fully normalized so that calling
forward followed by inverse yields the original data set. Note that unlike
GMT_FFT
, this function does not do any data extension, mirroring, detrending,
etc. but operates directly on the data array given.
4.6. Wavenumber calculations¶
As your data have been transformed to the wavenumber domain you may wish to operate on the various values as a function of wavenumber. We will show how this is done for datasets and grids separately. First, we present the function that returns an individual wavenumber:
double GMT_FFT_Wavenumber (void *API, uint64_t k, unsigned int mode, void *K);
where k
is the index into the array or grid, mode
specifies
which wavenumber we want (it is not used for 1-D transform but for the
2-D transform we can select either the x-wavenumber (0), the
y-wavenumber (1), or the radial wavenumber (2)), and finally the opaque
vector used earlier.
4.6.1. 1-D FFT manipulation¶
To be added after gmtfft has been added as new module.
4.6.2. 2-D FFT manipulation¶
The number of complex pairs in the grid is given by the header’s nm
variable, while size
will be twice that value as it holds the number
of components. To visit all the complex values and obtain the
corresponding wavenumber we simply need to loop over size
and call
GMT_FFT_Wavenumber
. This code snippet multiples the complex grid by
the radial wavenumber:
uint64_t k; for (k = 0; k < Grid->header->size; k++) { wave = GMT_FFT_Wavenumber (API, k, 2, K); Grid->data[k] *= wave; }
Alternatively, you may choose to be more specific about which components are real and imaginary (especially if they are to be treated differently), and set up the loop this way:
uint64_t re, im; for (re = 0, im = 1; re < Grid->header->size; re += 2, im += 2) { wave = GMT_FFT_Wavenumber (API, re, 2, K); Grid->data[re] *= wave; Grid->data[im] *= 2.0 * wave; }
4.7. Destroying the FFT machinery¶
When done you terminate the FFT machinery with
double GMT_FFT_Destroy (void *API, void *K);
which simply frees up the memory allocated by the FFT machinery with GMT_FFT_Create
.
5. FORTRAN interfaces¶
FORTRAN 90 developers who wish to use the GMT API may use the same
API functions as discussed in Chapter 2. As we do not have much (i.e., any) experience
with modern Fortran we are not sure to what extent you are able to access
the members of the various structures, such as the GMT_GRID
structure. Thus,
this part will depend on feedback and for the time being is to be considered
preliminary and subject to change. We encourage you to take contact should you
wish to use the API with your Fortran 90 programs.
5.1. FORTRAN 77 Grid i/o¶
Because of a lack of structure pointers we can only provide a low level of support for Fortran 77. This API is limited to help you inquire, read and write GMT grids directly from Fortran 77. To inquire about the range of information in a grid, use
int GMT_F77_readgrdinfo (unsigned int dim[], double limits[], double inc[], char *title, char *remark, const char *file)
where dim
returns the grid width, height, and registration, limits
returns the min and max values for x, y, and z
as three consecutive pairs, inc
returns the x and y increment, the title
and remark
returns the values of these strings. The file
argument is the name of the file we wish to inquire about. The function returns 0 unless there is an error.
Note that you must declare your variables so that limits
has at least 6 elements and inc
and dime
have at least 2 each.
To actually read the grid, we use
int GMT_F77_readgrd (float *array, unsigned int dim[], double wesn[], double inc[], char *title, char *remark, const char *file)
where array
is the 1-D grid data array, dim
returns the grid width, height, and registration,
limits
returns the min and max values for x, y, and z, inc
returns the x and y increments,
the title
and remark
returns the values of these strings. The file
argument is the name of the file we wish to read from. The function returns 0 unless there is an error.
Note on input, dim[2]
can be set to 1 which means we will allocate the array for you; otherwise
we assume space has already been secured. Also, if dim[3]
is set to 1 we will in-place transpose
the array from C-style row-major array order to Fortran column-major array order.
Finally, to write a grid to file you can use
int GMT_F77_writegrd_(float *array, unsigned int dim[], double wesn[], double inc[], const char *title, const char *remark, const char *file)
where array
is the 1-D grid data array, dim
specifies the grid width, height, and registration,
limits
may be used to specify a subset (normally, just pass zeros), inc
specifies the x and y increments,
the title
and remark
supplies the values of these strings. The file
argument is the name of the file we wish to write to. The function returns 0 unless there is an error.
If dim[3]
is set to 1 we will in-place transpose
the array from Fortran column-major array order to C-style row-major array order before writing. Note
this means array will have been transposed when the function returns.
[1] | or via a very confusing and ever-changing myriad of low-level library functions for bold programmers. |
[2] | Currently, C/C++, FORTRAN, MATLAB and Julia are being tested. |
[3] | At the moment, GMT does not have native support for 3-D grids. |
[4] | This may change in later releases. |
[5] | However, there is no thread-support yet. |