ATI Technologies RADEON X1550 SERIES - Manual (Page 138)

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while “Pb” and “Pr” represent the blue and red channels respectively, both
of which have the luminance value subtracted from them. It is an equivalent
color space to the chrominance-based YCbCr, which is used for digital
video.
Z-buffer
The portion of video memory that keeps track of which onscreen elements
can be viewed and which are hidden behind other objects. In the case of a
3D image, it keeps track of which elements are occluded by the foreground
in relation to the user’s perspective, or by another 3D object.