Product specifications

VT8601A Apollo PLE133
Revision 1.82 October 22, 2001 -132 3D Graphics Engine Registers
Technologies, Inc.
Delivering Value
Delivering ValueDelivering Value
Delivering Value
GEbase + A4 – Texture Color...........................................RW
31-24 Alpha
Constant alpha value when there is no alpha in the
texture format
23-0 Texture Color Key
Texture transparency color (888 RGB)
GEbase + A8 – Texture Palette Data............................... WO
31-16 Texel n+1
15-0 Texel n
An internal counter is used in loading the texture palette.
Writing to the Texture register (GEbase+A0) resets the
counter to 0. Writing to the Texture Palette Data register
writes the data to the place pointed to by the counter then
increments the counter by 1. Each write writes two entries
into the palette.
GEbase + AC – Texture Boundary...................................RW
31-22 Maximum V
21-12 Minimum V
11-8 Reserved .........................................always reads 0
7 Reverse Texture Format
0 Disable
1 Enable
6 Texture Cache
0 Disable
1 Enable
5 Texture Map Shift
0 Disable
1 Enable
4-3 Compressed Texture Format
00 No compression
01 DXT1 format
10 DXT2 format
11 -reserved-
2-0 Dither Shift
000 Disable LOD dithering
001 100% LOD dithering
010 80% LOD dithering
011 60% LOD dithering
100 40% LOD dithering
101 20% LOD dithering
11x -reserved-
Texture Filtering
Texture data read back from the Memory Interface first goes
through palette translation if the texture is palettized. The
texture is then converted into common internal 8888 ARGB
format. If the texture doesn’t have alpha data, then a constant
alpha value is used. If the texture color key is enabled and the
texture color matches the key, set alpha to 0. Bi-linear or tri-
linear filtering is then performed on RGB and alpha. If the
color key is enabled and the result alpha is 0, the
corresponding pixel should be discarded. This is done by
attaching a validity bit with texture data passed from the
Texture Engine to the Data Path. It should be noted that
filtering depends on the LOD value. When LOD < 0, a
different filter may be applied. In bi-linear filtering, if the
texel nearest to the texture coordinate is masked by the color
key, then the texel is considered as masked. Otherwise, the
texel is considered not masked.