Access to GMT from Julia is accomplished via a main function (also called gmt), which offers full access to all of GMT’s ~140 modules as well as fundamental import, formatting, and export of GMT data objects. Internally, the GMT5 C API defines six high-level data structures (GMT6 will define only five) that handle input and output of data via GMT modules. These are data tables (representing one or more sets of points, lines, or polygons), grids (2-D equidistant data matrices), raster images (with 1–4 color bands), raw PostScript code, text tables (free-form text/data mixed records) and color palette tables (i.e., color maps). Correspondingly, we have defined five data structures that we use at the interface between GMT and Julia via the gmt function. The GMT.jl wrapper is responsible for translating between the GMT structures and native Julia structures, which are:
Grids: Many tools consider equidistant grids a particular data type and numerous file formats exist for saving such data. Because GMT relies on GDAL we are able to read and write almost all such formats in addition to a native netCDF4 format that complies with both the COARDS and CF netCDF conventions. We have designed a native Julia grid structure Grid type that holds header information from the GMT grid as well as the data matrix representing the gridded values. These structures may be passed to GMT modules that expect grids and are returned from GMT modules that produce such grids. In addition, we supply the function
mat2gridto convert a matrix and some metadata into a grid structure.
Images: The raster image shares many characteristics with the grid structure except the bytes representing each node reflect gray shade, color bands (1, 3, or 4 for indexed, RGB and RGBA, respectively), and possibly transparency values. We therefore represent images in another native structure Image type that among other items contains three components: The image matrix, a color map (present for indexed images only), and an alpha matrix (for images specifying transparency on a per-pixel level). As for grids, the wrapper function
mat2imgfor creating the correct structure is available.
Segments: GMT considers point, line, and polygon data to be organized in one or more segments in a data table. Modules that return segments uses a native Julia segment structure Dataset type that holds the segment data, which may be either numerical, text, or both; it also holds a segment header string which GMT uses to pass metadata. Thus, GMT modules returning segments will typically produce arrays of segments and you may pass these to any other module expecting points, lines, or polygons or use them directly in Julia. Since a matrix is one fundamental data type you can also pass a matrix directly to GMT modules as well. Consequently, it is very easy to pass data from Julia into GMT modules that process data tables as well as to receive data segments from GMT modules that process and produce data tables as output.
Color palettes: GMT uses its flexible Color Palette Table (CPT) format to describe how the color (or pattern) of symbols, lines, polygons or grids should vary as a function of a state variable. In Julia, this information is provided in another structure CPT type that holds the color map as well as an optional alpha array for transparency values. Like grids, these structures may be passed to GMT modules that expect CPTs and will be returned from GMT modules that normally would produce CPT files.
PostScript: While most users of the GMT.jl wrapper are unlikely to manipulate PostScript directly, it allows for the passing of PostScript via another data structure Postscript type.
Given this design the Julia wrapper is designed to work in two distinct ways.
The first way, referred as the monolithic, is the more feature reach and follows closely the GMT usage from shell(s) command line but still provide all the facilities of the Julia language. See the Monolithic for the Reference on how to use the Package.
The second way uses an upper level set of functions that abstract aspects that make the monolithic usage more complex. It provides an interface to some of the GMT modules using a option=val list type syntax. This makes it more appropriate for new commers but it won't release you from understanding the monolithic way. See the By Modules