Well, they were actually some kind of "lightmap" :) they are used to modulate the computed light value. The way the real color is output is quite weird. I'll try to explain:

The rasterizer generates the "3d" layer to the framebuffer the following way:
The polygon "base" colour stored in the polygon data is a 10 bit palette index, and it's output directly to the framebuffer.
The light value computed by the Geo DSP is multiplied by the texel value (4-bit) and then reduced to 6 bit through a table (Luma conversion table, selectable per polygon IIRC). This 6 bit value is output to the framebuffer along with the 10 bit palette value.

Then for each screen pixel, the video mixer (color generator or something like that) gets the tilemap values and framebuffer value and arranges them in the right priority (tilemaps can be over 3d plane or over 3d plane, selectable per tile). If the visible one is the 3d plane, then it computes a RGB555 value corresponding to the polygon "base" color by indexing the 10bit color in a 1024 entries palette table.
Then separates each component and looks in 3 separate tables (one for each component). The lookup address is computed combining the pixel color (5 bit) with the luminance value (6 bit) and each entry stores a 8 bit value. By doing it for each component it converts from RGB555+6bit luma to a RGB888 value, that is sent to display.





> my mate mark was saying Model 2 has monochromic textures and the way it made
> colour was by stacking several together? Got any more info on this, it sounds
> really unusual!
>
> Also he said it was dervied from military flight simulator chipsets, hence why
> it's so hard to get docs on it!
>
> But Model 3 was a real 3dfx-type chip yeah?
>
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