what is "primary" refer to in visual

[Zhang, Xing Z]

Hi experts:
         From xlib manual, here are sentences for color:

GrayScale is treated the same way as PseudoColor except that the *primary* that drives the
screen is undefined. Thus, the client should always store the same value for red, green, and
blue in the colormaps.

TrueColor is treated the same way as DirectColor except that the colormap has predefined,
read-only RGB values. These RGB values are server dependent but provide linear or
near-linear ramps in each *primary*.

       I apologize for my bad English, I am confused by the word "primary". I wonder what does
"primary" mean here? Thank you.


Zhang Xin(Wing)
Intel(SSG/OTC) ShangHai China

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