mobile-shader-optimization

Mobile Shader Optimization for Unity

TRIGGER

Use this skill when the user is writing, reviewing, or optimizing Unity shaders for mobile platforms (Android, iOS) or any GPU-constrained environment (Meta Quest VR, low-end devices). Triggers include: mentions of "mobile", "Android", "iOS", "Mali", "Adreno", "PowerVR", "performance", "optimization", "fillrate", "overdraw", "bandwidth", "frame budget", "half precision", "LOD", "baked lighting", or any shader work targeting sub-60fps scenarios. Also trigger when the user is profiling shaders, asking about GPU architecture, or comparing Simple Lit vs Complex Lit in URP.

CORE RULES — HARD CONSTRAINTS

These rules must always be followed when writing mobile shaders:

1. Use half precision by default. Only use float for world-space positions, UV coordinates requiring high precision, and depth calculations. On Mali and Adreno GPUs, half triggers 16-bit fast paths that are 2x faster.

2. Never use discard (clip/alpha-test) on PowerVR or Apple GPUs. It breaks Early-Z/Hidden Surface Removal, forcing the GPU to process fragments it would otherwise skip. Use alpha blending instead, or move discard to a separate pass.

3. Keep Shader Graph node count under 100 for mobile. Each node compiles to shader instructions. Exceeding ~100 nodes risks exceeding mobile GPU instruction limits and causes performance cliffs.

4. Never use while or do-while loops in shaders targeting OpenGL ES 2.0 / WebGL 1.0. Only counting for loops are allowed. Even on ES 3.0+, avoid dynamic loop bounds — they prevent the compiler from unrolling.

5. Avoid dynamic branching on mobile. Mobile GPUs often execute both branches regardless. Use step(), lerp(), saturate() for branchless alternatives.

6. Multiply scalars before vectors. float3 result = scalar1 * scalar2 * vector is faster than float3 result = vector * scalar1 * scalar2 because the first scalar multiply is 1 ALU op instead of 3.

7. Limit texture samples. Each sample costs bandwidth. Target ≤4 texture samples per fragment on low-end mobile. Use channel packing to combine maps (see texture-packing-variant-stripping skill).