S3 ViRGE builds draw 15-bit pixels: their 16-bit surfaces are 5-5-5

Symptoms

In a stand-in for the S3D Toolkit, the chip's triangles come out in the wrong colours against the game's software-drawn pixels, or the reverse.

Cause

The ViRGE's 3D engine draws 8-, 15- or 24-bit pixels, never 5-6-5: the S3D Toolkit's surface formats are S3DTK_VIDEORGB8 (0), RGB15 (4) and RGB24 (8). POD's S3 build describes its 16-bit draw surfaces as RGB15 (measured: sfFormat 4 in the S3DTK_SURFACE it passes), so on the real card its DirectDraw surfaces were 5-5-5 (inferred).

Evidence

A probe S3DTKW.dll dumping the S3DTK_SURFACE set as S3DTK_DRAWSURFACE: {linear address, 640, 480, 4}.

What to do

Draw the chip's pixels in the format the game names (sfFormat), and make the DirectDraw surfaces report the same masks (0x7c00, 0x3e0, 0x1f). POD's software renderer reads the surface's pixel format and follows it, so both halves agree (wrappers/ddraw: DDRAW_PIXELS=555).