D847 - A two-channel half-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_16_16, NUMBER_FLOAT and SWAP_STD holds two half floats a texel, red then green, four bytes. It lies by the 32-bpp layout. It is drawn through a Vulkan R16G16_SFLOAT attachment whose image is the same format, so seeding and write-back move the guest's words untouched, as D773, D844 and D845 do. A draw into it writes red and green of its target mask. Any other swap stays refused.

Why: COLOR_16_16 is what Mesa targets a two-channel 16-bit format at (ac_formats.c 500), and SWAP_STD on two channels is XY__ (592). Grand Theft Auto V's fourth draw writes a 1920 x 1080 target of it, once its first three run (D843, D844, D845). Blending on R16G16_SFLOAT is required of every Vulkan implementation.

Rejected:

• Drawing it through an eight-bit attachment: a half float is not recoverable from eight bits.
• Keeping all four channels of the target mask: the attachment holds two, and a draw would be refused for channels its pixel shader need not export.