D775 - An anisotropic sampler samples at the host's anisotropy, at the ratio radv encodes

Status: decided Date: 2026-10-08 known_by: assumed (radv's sampler encoding, radv_sampler.c:64-116, read in reverse)

A sampler descriptor whose XY_MAG_FILTER or XY_MIN_FILTER is anisotropic (ANISO_POINT 2, ANISO_BILINEAR 3, gfx6.json SQ_TEX_XY_FILTER) is sampled with the host's anisotropic filter. The base filter is point or bilinear, as the value names it. The maximum anisotropy is 1 << MAX_ANISO_RATIO, from SQ_IMG_SAMP_WORD0 bits 11:9.

radv writes exactly these fields from a VkSamplerCreateInfo:

• the XY filter is the anisotropic form of the Vulkan filter when maxAnisotropy > 1;
• MAX_ANISO_RATIO is min(log2(maxAnisotropy), 4).

So the host sampler built here is the Vulkan sampler from which radv would build this descriptor. ANISO_THRESHOLD, ANISO_BIAS and ANISO_OVERRIDE tune the hardware's own footprint; they are not read. A ratio of zero under an anisotropic filter is one sample, as radv never encodes and the host does not distinguish. A host device without samplerAnisotropy samples with the base filter alone.

Why: PPSA02664 samples its 12-level BC3 texture through a 16x anisotropic sampler. The decoder refused every anisotropic filter as not reproduced, so the draw ran with no texture bound, was refused, and nothing after it in the submission retired. No host reproduces a GPU's anisotropic footprint exactly, so refusing it would block every title that asks for it. The choice is assumed until a framebuffer comparison against hardware measures it.