Visuals are Prolog terms of one of the following forms:
gray(Writable, Depth, Size)
direct_color(Writable, Depth, Size,Bits_per_RGB, Rmask, Gmask, Bmask)
pseudo_color(Writable, Depth, Size, Bits_per_RGB)
Visual terms describe the color capability of a screen or window. A
gray/3 visual means that pixel values in a window specify an entry in
a colormap, but only one of the primary colors is actually used to
drive the grayscale (or monochrome) display 1. direct_color/7 means
that the window or screen support color hardware where separate
bits in a pixel value specify entries in separate colormaps for each
of the three primary colors. pseudo_color/4 means that each pixel
value specifies an entry in a colormap, which specifies all three
primary colors. Writable is either true or false,
indicating whether or not is is possible to allocate and change
cells in a colormap. Depth specifies the number of bitplanes that
are supported on this screen or window. Size is the number of
user-accessible colormap entries; this will always be less than or
equal to 2 to the power of Depth. Bits_per_RGB is the number of bits
used to specify each primary in a color specification. In some
sense, this is the precision of color specifications. And
finally, Rmask, Gmask, and Bmask are bit masks
(i.e. integers) that specify which bits of a direct_color visual
pixel specify the red, green, and blue primary, respectively.