Shader Review

Shader Review

Perform a thorough review of GLSL shader code in the Lunatic engine (and any game-specific shaders), evaluating correctness, performance, visual quality, and adherence to modern real-time rendering practices.

When to Use

• The user requests a shader review, audit, or optimization pass
• New shaders have been added or existing shaders significantly modified
• The user asks about shader best practices or SOTA techniques

Shader Locations

• Scene shaders: engine/shaders/scene/ (default.vert, default.frag)
• Shadow shaders: engine/shaders/shadow/ (shadow.vert, shadow.frag)
• Post-processing shaders: engine/shaders/postprocess/ (bloom, DoF, composite, lens flare, etc.)
• Game shaders: Any .vert/.frag files outside engine/ added by the game project

Review Process

1. Gather Context

Read all shader files using Glob (engine/shaders/**/*.{vert,frag}) and any game-specific shaders. Also read relevant engine-side setup in renderer.zig and postprocess.zig to understand uniform layouts, texture bindings, and pipeline configuration.

2. Research SOTA Practices

Use WebSearch to look up current best practices for each technique found in the shaders. Suggested queries (adapt based on what the shaders actually implement):

• "PBR GLSL best practices 2025" for physically-based rendering
• "GGX specular anti-aliasing real-time" for specular highlights
• "cascaded shadow maps filtering techniques" for shadow quality
• "physically based bloom real-time rendering" for bloom
• "bokeh depth of field GPU" for DoF
• "ACES tonemap improvements filmic" for tonemapping
• "clustered forward shading GPU optimization" for light culling
• "normal map blending techniques" for tangent-space normal mapping
• "PCF shadow filtering quality" for shadow softness
• "chromatic aberration film grain shader" for post-FX realism
• "GLSL shader performance mobile desktop" for general optimization

Focus searches on techniques actually present in the codebase. Skip irrelevant topics.

3. Evaluate Each Shader

For every shader file, assess the following dimensions:

Correctness

• Precision qualifiers (mediump/highp usage where it matters)
• Correct coordinate spaces (world vs view vs clip)
• Proper normalization of vectors after interpolation
• Shadow bias and acne prevention
• Depth linearization accuracy
• sRGB vs linear color space handling

Performance

• Unnecessary dependent texture reads
• Redundant calculations that could move to the vertex shader
• Branch coherence (avoid divergent branches in fragment shaders)
• Texture sampling efficiency (LOD hints, gather operations)
• Register pressure from excessive temporaries
• Opportunities for MAD (multiply-add) fusion
• Per-fragment work that could be per-vertex or per-tile

Visual Quality

• PBR energy conservation
• Specular aliasing mitigation
• Shadow filtering quality (PCF kernel, Poisson disk, PCSS)
• Bloom thresholding (soft knee vs hard threshold)
• Tonemapping curve accuracy and highlight rolloff
• DoF bokeh shape and edge quality
• Fog integration with lighting

Robustness

• Division by zero guards
• NaN/Inf propagation prevention
• Clamping of values that could go out of range
• Handling of edge cases (zero-length normals, degenerate tangents)

Cross-Platform

• Metal Y-flip correctness for render-to-texture sampling
• SPIRV-Cross compatibility concerns
• Vulkan vs Metal behavioral differences

4. Produce the Report

Structure findings as:

## Shader Review Report

### Summary
Brief overall assessment — what's solid, what needs attention.

### Critical Issues
Issues that cause visual artifacts or incorrect rendering.

### Performance Opportunities
Optimizations ranked by expected impact.

### Quality Improvements
SOTA techniques that could improve visual fidelity.

### Minor / Style
Naming, comments, consistency.

### Per-Shader Notes
Detailed notes for each shader file reviewed.

For each finding, include:

• File and line reference
• What the issue is
• Why it matters (with SOTA reference where applicable)
• Suggested fix (concrete code snippet when possible)

5. Prioritize Recommendations

Rank all recommendations by impact-to-effort ratio. Group into:

• Quick wins: High impact, low effort (e.g., adding a missing clamp, fixing a bias)
• Medium effort: Meaningful improvement, moderate refactoring (e.g., upgrading PCF to PCSS)
• Major work: Significant technique upgrades (e.g., switching tonemapping curves, adding screen-space reflections)

Engine-Specific Gotchas

Refer to the project's CLAUDE.md for critical details:

• Metal Y-flip: Shadow atlas and any render-to-texture must flip Y on Metal
• HDR alpha = linear depth: Fragment shaders must write linear depth to alpha for DoF
• sRGB round-trip: Clear/fog colors are inverse-ACES-transformed in renderer.zig
• Specular clamp: Output clamped to 64.0 to prevent fireflies
• PostProcess ping-pong: DoF writes to hdr_texture_b then swaps; be aware of read/write hazards
• SPIRV-Cross cache: Shader cache may be stale after edits — rm -rf .zig-cache if behavior is unexpected