VT8601 datasheet(139/144 Pages) ETC1

139 / 144 page

VT8601 Apollo ProMedia

Revision 1.3 September 8, 1999

-133

3D Graphics Engine Registers

7HFKQRORJLHV ,QF

:H&

:H&RRQQHFW

QQHFW

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

Reverse Texture Format

Disable

1Enable

Texture Cache

Disable

1Enable

Texture Map Shift

Disable

1Enable

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.

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