XLIB Programming Manual, Rel. 5, Third Edition

7.7 The GrayScale Visual

On a gray-scale workstation or a GrayScale visual on a color workstation, a color application should still work correctly. The only problem might be that when colors are allocated, the closest physically possible colors (returned by XAllocColor()) will result in shades of gray that provide insufficient contrast. The best way to avoid this is to explicitly check for the StaticGray visual. For true bulletproof operation, it is a good idea to check any user-specified colors to make sure they contrast.

The color names “gray0” through “gray100”, spelled with an “e” or an “a”, can be used with XParseColor() to get RGB values for various grays.

You should set the red, green, and blue values to be equal. Some servers only use one of the values, and others combine all three according to the NTSC standard that makes color television signals work on black-and-white televisions:

intensity = (.30 * red) + (.59 * green) + (.11 * blue)

MIT’s implementations use a least-squares algorithm that determines the closest RGB values in the (gray) colormap to the RGB values specified. Exactly what algorithm is used is up to the server implementor.