samplelevel-with-zero-on-mipped-tex
Status: stub. The full-length analysis is queued for a v1.0.x patch release per ADR 0018, section 5, criterion #6. The companion rule page at docs/rules/samplelevel-with-zero-on-mipped-tex.md contains the canonical detection logic + GPU reasoning.
TL;DR
Mip chains exist for two reasons: reducing aliasing in minification, and enabling the texture cache to fetch smaller footprints from VRAM when the on-screen footprint is small. When a mipped resource is sampled exclusively at mip 0, every fetch hits the highest-resolution level regardless of the rasterised pixel footprint. This defeats both goals: cache lines are wasted on high-resolution texels that are averaged away by the hardware's aniso filter, and the lower mip levels occupy VRAM bandwidth during allocation and streaming for no return. A 2048x2048 RGBA8 texture carries 4 MB at mip 0 alone; pinning all samples to mip 0 means the additional 1.3 MB of mip chain is uploaded and retained in VRAM but never touched.
What the rule fires on
Calls to SampleLevel(sampler, uv, 0) — or SampleLevel(sampler, uv, 0.0) — where the third argument (the LOD parameter) is the literal zero, and where reflection data shows that the bound resource was declared with mip levels (a full mip chain or a partial chain with more than one level). The rule fires when a Texture2D, TextureCube, Texture2DArray, or similar mipped resource type is paired with an explicit mip-0 lock that is not guarded by a compile-time constant or a [mips(1)] resource annotation. It does not fire on resources explicitly declared as single-mip (Texture2D<float4> T : register(t0); // mips 1), on Buffer<> or RWTexture2D<> objects, or when the lod argument is a non-zero expression.
See the What it detects section of the rule page for the full pattern definition.
Why it matters
The full GPU-mechanism analysis lives in the Why it matters on a GPU section of the companion rule page.
Examples
The bad / good code snippets are kept canonical on the rule page; see samplelevel-with-zero-on-mipped-tex.md -> Examples.
See also
- Rule page -- canonical reference + change log.
- texture overview -- broader context.
- ADR 0018 -- v1.0 readiness plan.
This is a v1.0-ship stub. Full analysis pending; track issue link TBD.