394 AMD Geode™ LX Processors Data Book
Video Processor
33234H
6.7.5 Video Overlay
Video data is mixed with graphics data according to the
video window position. The video window position is pro-
grammable via the Video X Position (VP Memory Offset
010h) and Video Y Position (VP Memory Offset 018h) reg-
isters. A color-keying and alpha-blending mechanism is
employed to compare either the source (video) or destina-
tion (graphics) color to the color key programmed via the
Video Color Key register (VP Memory Offset 028h), and to
select the appropriate bits in the Video Color Mask register
(VP Memory Offset 030h). This mechanism greatly
reduces the software overhead for computing visible pix-
els, and ensures that the video display window can be par-
tially hidden by overlapping graphics data. See Figure 6-27
on page 395.
The Video Processor module accepts graphics data at the
graphics Dot clock rate. The Video Processor module can
display graphics resolutions up to 1920x1440 on CRT, at
color depths up to 24-bpp while simultaneously overlaying
a video window.
6.7.5.1 Alpha-Blending
Alpha-blending can be performed using RGB blending or
YUV blending:
•For RGB blending, graphic data in RGB format and
video data in RGB format (YUV to RGB conversion) are
blended.
•YUV blending eliminates video de-interlacing and YUV
to RGB conversion of video data. For YUV blending, the
graphic data is converted to YUV and blended with
video in YUV format.
Up to three alpha windows can be defined in the video win-
dow. Alpha values for blending are defined for each pixel in
the upper 8 bits of video data. If alpha windows overlap,
the alpha window with the highest priority (programmable)
is used (for the overlapped area).
Alpha-blending is performed using the following formula:
alpha * G + (1 - alpha) * V
Where G is the graphic value and V is the video value of
the current pixel.
Color Keys
A color key mechanism is used with alpha-blending. Color
key values are defined for a cursor color key and for a nor-
mal color key. The cursor color key is compared to each 24-
bit value of graphic input data. If a match is found, the
selected cursor color is displayed. Two possible cursor col-
ors can be defined. The COLOR_REG_OFFSET field (in
the Cursor Color Key register, VP Memory Offset
0A0h[28:24]) is used to select the bit in the input graphic
stream that determines the cursor color to use. Each cur-
sor color is stored in a separate cursor color register. Fig-
ure 6-28 on page 396 illustrates the logic used to
determine how to implement the color key and alpha-blend-
ing logic.