The BMP driver (7972) is a special usage of both the GDI interface and the OpenGL library. GPHIGS creates a GDI memory context, then GPHIGS calls OpenGL for drawing into this context, finally GDI is called to store the memory buffer into a BMP file. See also the OpenGL driver for more details on the OpenGL specific features.
This driver draws by default a 512x512 8 bit image. The %rs directive can be used to change the default size and the %wd directive allows to increase the number of bit per color to 12 or 24.