User's Manual

Intel® 815 Chipset: Graphics Controller PRM, Rev 1.0
R
292
Bit Description
31 Source Constant Alpha Blend Enable.
1 = Enable Source Alpha Blending when the logical OR of the Source Key Mask Enables are asserted
within the Alpha Blend Window, and the comparison indicates that the overlay is to be displayed.
0 = Disable Source Alpha Blending.
30:27 Reserved.
26:24 Source Key Mask Enables. Each bit enables one channel. If the bit is a 1, the comparison result is
used; otherwise, it is ignored.
Bit 2 = Enables Comparison [23:16]
Bit 1 = Enables Comparison [15:8]
Bit 0 = Enables Comparison [7:0]
23:16 Constant Alpha Red [7:0]. This involves three 4 x 8 bit alpha multipliers to be inserted in the Overlay
and Primary Display merging logic. When the alpha blend is selected, the most significant 4 bits of
each alpha channel is used as the alpha term. If the original 8 bits of the alpha channel = FF, the alpha
channel value is treated as a 1. If alpha = 0, 1-0 must also be treated as a 1. This functionality only
works for a primary display of 16 and 24 bits per pixel.
pixel (R) = (alphaR * primary display (R)) + ((1-alphaR) * overlay (R))
15:8 Constant Alpha Green [7:0]. This involves three 4 x 8 bit alpha multipliers to be inserted in the
Overlay and Primary Display merging logic. When the alpha blend is selected, the most significant 4
bits of each alpha channel is used as the alpha term. If the original 8 bits of the alpha channel = FF,
the alpha channel value is treated as a 1. If alpha = 0, 1-0 must also be treated as a 1. This
functionality only works for a primary display of 16 and 24 bits per pixel.
pixel (G) = (alphaG * primary display (G)) + ((1-alphaG) * overlay (G))
7:0 Constant Alpha Blue [7:0]. This involves three 4 x 8 bit alpha multipliers to be inserted in the Overlay
and Primary Display merging logic. When the alpha blend is selected, the most significant 4 bits of
each alpha channel is used as the alpha term. If the original 8 bits of the alpha channel = FF, the alpha
channel value is treated as a 1. If alpha = 0, 1-0 must also be treated as a 1. This functionality only
works for a primary display of 16 and 24 bits per pixel.
pixel (B) = (alphaB * primary display (B)) + ((1-alphaB) * overlay (B))