D798 - A quad list is seeded two triangles a quad

Status: decided Date: 2026-10-11

A draw whose input primitive is DI_PT_QUADLIST (19, gfx10.json:810) seeds its primitive shader two triangles for each whole quad of four vertices, split as Mesa splits a quad for its provoking convention (u_indices_gen.py:129-136): (v0, v1, v3) and (v1, v2, v3) when the last vertex provokes, (v0, v1, v2) and (v0, v2, v3) when the first does, the convention read from PA_SU_SC_MODE_CNTL as a strip's is (D730). Vertices past the last whole quad make nothing.

Assumed (known_by = assumed): that the geometry engine splits a quad along the same diagonal. Another split covers the same pixels and differs only in how attributes interpolate across the quad. radeonsi hands a GL quad list to the hardware as this primitive (si_state_draw.cpp's primitive table).

Why: mesa-demos draws with GL_QUADS, and its vertex shader was refused at the first such draw once its quad strips (D797) were seeded.

Rejected:

• Seeding a quad as a two-triangle strip: the strip's second triangle reverses its winding rule, which is right for a strip's shared edge and not for a quad's own vertices.