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Testing for Graphics CapabilitiesTesting for Graphics Capabilities — Checking whether the library supports graphics |
Before calling Glk graphics functions, you should use the gestalt selector
gestalt_Graphics. To test for additional capabilities, you can also use the
gestalt_DrawImage and gestalt_GraphicsTransparency selectors.
#define GLK_MODULE_IMAGE
If you are writing a C program, there is an additional complication. A
library which does not support graphics may not implement the graphics
functions at all. Even if you put gestalt tests around your graphics calls,
you may get link-time errors.
If the glk.h file is so old that it does not declare the graphics functions
and constants, you may even get compile-time errors.
To avoid this, you can perform a preprocessor test for the existence of
GLK_MODULE_IMAGE. If this is defined, so are all the functions and constants
described in this section. If not, not.
To be extremely specific, there are two ways this can happen. If the
glk.h file that comes with the
library is too old to have the graphics declarations in it, it will of
course lack GLK_MODULE_IMAGE as well. If the glk.h file is recent, but the library is old,
the definition of GLK_MODULE_IMAGE should be removed from glk.h, to avoid link errors. This is not a
great solution. A better one is for the library to implement the graphics
functions as stubs that do nothing (or cause run-time errors). Since no
program will call the stubs without testing gestalt_Graphics, this is
sufficient.