D938 - A polygon offset in whole depth units pins depth to the range end

Status: decided Date: 2026-10-21 known_by: measured

With PA_SU_POLY_OFFSET_DB_FMT_CNTL never written (zero), the constant term counts whole depth units and the result is clamped to 0..1. On a Z_32_FLOAT target an offset of -10 reads back 0.0 and +10 reads back 1.0 (obSCEne REQ-20261020T2010Z-gt24, report-20261020-gt24-poly-offset.log rows 382 and 391). With 0x1e9, the RADV float form, the same offsets move a depth of 0.5 by ten float steps, as D32_SFLOAT depth bias does (rows 409 and 418).

An offset of magnitude one or more in those units, with no slope term and no clamp, moves every depth past an end of the range. The draw is built with its viewport's depth range collapsed onto that end: 0.0 for a negative offset, 1.0 for a positive one. Clipping runs in clip space before the range applies, so the draw keeps the primitives it had. Every other offset in whole units is refused by name.

Why: Grand Theft Auto V's loading-screen lighting draws with offsets of -10 and +10 and the format register left at zero. Vulkan's depth bias cannot give whole units on a float target without VK_EXT_depth_bias_control's floatRepresentation, which the development device lacks.

Rejected:

• Writing the offset depth from the pixel shader: it changes every translated pixel shader that meets an offset, for a case the viewport answers exactly.