AMD Geode LX 800@0.9W - Manual (Page 251)

AMD Geode™ LX Processors Data Book 251
Graphics Processor 33234H
6.3.8.3 Source Expansion
The Graphics Processor contains hardware support for
color expansion of monochrome source data. Those pixels
corresponding to a clear bit in the source data are rendered
using the color specified in the GP_SRC_COLOR_BG reg-
ister (GP Memory Offset 14h), and the pixels that are set in
the source data are rendered using the color specified in
the GP_SRC_COLOR_FG register (GP Memory Offset
10h).
6.3.8.4 Source Transparency
If the source transparency bit is set in the
GP_RASTER_MODE register (GP Memory Offset
38h[11]), not all source pixels result in a write to the frame
buffer.
In monochrome mode, source pixels that are clear are
inhibited from writing to the frame buffer, so only fore-
ground colored pixels are written.
In color mode, the source pixel is compared to the value
stored in the GP_SRC_COLOR_FG register (GP Memory
Offset 10h). The resulting compare is masked by the value
in the GP_SRC_COLOR_BG register (GP Memory Offset
14h), allowing color keying on specific channels within a
pixel. If all the bits that are not masked compare with their
corresponding bits in the GP_SRC_COLOR_FG register,
then the pixel write is inhibited. For example, to make all
blue pixels transparent in 8-bpp mode,
GP_SRC_COLOR_FG is loaded with 03h (hardware
expands this into four blue pixels) and
GP_SRC_COLOR_BG (GP Memory Offset 14h) is loaded
with FFh (perform compare on all bits). To make all pixels
transparent that have more than 50% in their alpha chan-
nel for 32-bpp data, load GP_SRC_COLOR_FG with
80000000h and GP_SRC_COLOR_BG with 80000000h.
6.3.9 Destination Data
When required by the raster operation or alpha blender,
destination data is fetched from the frame buffer memory.
This data is required to be in color at the depth specified (8,
16, or 32-bpp). Source or pattern transparent mode does
not necessarily require destination data to be fetched,
since transparent pixels are inhibited from being written to
the frame buffer rather than re-written with the destination
data. Transparency is never keyed off of destination data.
6.3.10 Raster Operations (ROP)
The GP_RASTER_MODE register (GP Memory Offset
38h) specifies how the pattern data, source data, and des-
tination data are combined to produce the output from the
Graphics Processor. The definition of the ROP value
matches that of the Microsoft
®
API. This allows Microsoft
Windows
®
display drivers to load the raster operation
directly into hardware. See Table 6-25 and Table 6-26 for
the definition of the ROP value.
Table 6-25. GP_RASTER_MODE Bit Patterns
Pattern (bit) Source (bit) Destination (bit) Output (bit)
0 0 0 ROP[0]
0 0 1 ROP[1]
0 1 0 ROP[2]
0 1 1 ROP[3]
1 0 0 ROP[4]
1 0 1 ROP[5]
1 1 0 ROP[6]
1 1 1 ROP[7]
Table 6-26. Common Raster Operations
ROP Description
F0h Output = Pattern
CCh Output = Source
5Ah Output = Pattern xor destination
66h Output = Source xor destination
55h Output = ~Destination
33h Output = ~Source