D844 - A one-channel 32-bit float target is drawn and written back as it is held

Status: decided Date: 2026-10-14

A colour target whose CB_COLOR0_INFO names COLOR_32, NUMBER_FLOAT and SWAP_STD holds one 32-bit float a texel, red. It is drawn through a Vulkan R32_SFLOAT attachment. Its image is the same format, so seeding it from guest memory and writing it back move the guest's words untouched, as D773 does for 10_11_11. Any other swap on COLOR_32 stays refused.

Why: COLOR_32 is what Mesa targets a one-channel 32-bit format at (ac_formats.c 489), and SWAP_STD on one channel is red (892, the SX_RT_EXPORT_32_R case). Grand Theft Auto V's second draw, once its first ran (D843), writes a 5x5 COLOR_32 FLOAT target. That is a reduction target, and nothing an eight-bit attachment holds can be written back to it exactly.

Open: Vulkan does not require blending on R32_SFLOAT. The desktop hosts orbistoun runs on offer it, as they offer it on D773's packed format. A host without it would fail the pipeline rather than draw without blending.

Rejected:

• Drawing it through an eight-bit attachment and converting: a float written back from eight bits is not the value the shader wrote.
• R32_SFLOAT for the other swaps: SWAP_ALT_REV puts the channel in alpha (892), which a one-channel attachment does not hold.