refactor(ssgi): tidy GI compute shader internals

[alandtse]

Summary

• Replace 9-tap box history clamp with 5-tap SVGF cross pattern (Schied 2017); skip on saturated history (lerpFactor < 0.15)
• Express GI inner-loop math identities at source level: per-slice logLenOmega for the mip selection, rsqrt(dot(v,v)) for cosNorm (also guards a degenerate-normal NaN edge case via max(..., 1e-20)), and folding sampleBackPos into sampleDelta - viewVec * Thickness
• Remove dead srcPrevAo SRV declaration from gi.cs.hlsl (declared at t5 but never read); drop the matching C++ bind and collapse srcPrevY/CoCg/GISpecular/Normal from t6..t9 down to t5..t8

All math changes are bit-equivalent for non-degenerate inputs. No new settings.

Test plan

• hlslkit-compile validates SSGI variants (HALF_RES / QUARTER_RES / full, GI on/off, GI_SPECULAR on/off, TEMPORAL_DENOISER on/off, VR + flat)
• Visual: SSGI looks unchanged on stationary scenes (interior cell, conversation)
• Visual: no new motion-induced artefacts in VR (head turn, fast camera pan)
• Tracy: SSGI - GI per-dispatch cost stays within scene-noise of upstream baseline

🤖 Generated with Claude Code

Summary by CodeRabbit

• Improvements
  • More stable and cheaper Screen Space Global Illumination (GI) sampling via safer projection math and guarded mip-level selection.
  • Refined temporal denoising: history clamping now uses a 5-tap cross neighborhood and is applied conditionally for stronger blending stability.
  • Added numeric safety for degenerate vectors to prevent instability in GI calculations.

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ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro

Run ID: 4bfb62fb-a59b-4dca-8da9-ae1f77858bb8

📥 Commits

Reviewing files that changed from the base of the PR and between 964bc16 and 4eb7c7d.

📒 Files selected for processing (3)

• features/Screen Space GI/Shaders/ScreenSpaceGI/gi.cs.hlsl
• package/Shaders/Common/Math.hlsli
• src/Features/ScreenSpaceGI.cpp

✅ Files skipped from review due to trivial changes (1)

• package/Shaders/Common/Math.hlsli

🚧 Files skipped from review as they are similar to previous changes (1)

• features/Screen Space GI/Shaders/ScreenSpaceGI/gi.cs.hlsl

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📝 Walkthrough

Walkthrough

Reworks Screen Space GI shader sampling and temporal history handling (guarded rsqrt, revised mip selection, direct back-side vectors, 5-tap cross clamping with lerp threshold) and updates C++ SRV bindings to match the HLSL parameter reordering. No public API changes.

Changes

Cohort / File(s)	Summary
Shader: SSGI sampling & temporal clamping features/Screen Space GI/Shaders/ScreenSpaceGI/gi.cs.hlsl	Removed srcPrevAo, shifted temporal input registers; replaced division with guarded rsqrt(max(..., 1e-20)); changed mip-level calc to split-form using log2(s) + per-slice logLenOmega (guarded); compute back-side horizon vectors directly from sampleDelta; replace 3x3 clamp with 5-tap cross neighborhood and apply clamping only when lerpFactor >= 0.15.
Common math constants package/Shaders/Common/Math.hlsli	Added EPSILON_LENGTH_SQ constant to guard squared-length prior to rsqrt.
C++ SRV binding adjustments src/Features/ScreenSpaceGI.cpp	Reordered SRV bindings for the GI compute pass: removed AO SRV slot and shifted texIlY, texIlCoCg, texGiSpecular, texNormal down one slot to match updated HLSL parameter ordering.

Sequence Diagram(s)

sequenceDiagram
  participant CPU as DrawSSGI (CPU)
  participant GPU as Compute Shader (GI)
  participant SRV as Texture SRVs
  participant UAV as GI UAVs

  CPU->>SRV: Bind SRVs (`texIlY`,`texIlCoCg`,`texGiSpecular`,`texNormal`,...)
  CPU->>GPU: Dispatch giCompute (with updated SRV slots)
  GPU->>SRV: Sample temporal histories, normals, indirect maps
  GPU->>GPU: Compute GI using guarded rsqrt, split mip selection, back-side vectors
  GPU-->>GPU: Apply temporal denoiser (5-tap cross clamp when lerpFactor >= 0.15)
  GPU->>UAV: Write GI outputs (radiance, normal mips, specular)

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Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~20 minutes

Possibly related PRs

• perf(SSGI): eliminate radiance copy via direct UAV write #2127: Reorders SRV/UAV bindings in ScreenSpaceGI compute pass (binding-level overlap).
• fix: add checks to prevent NaN for BRDFs #1226: Adds/standardizes epsilon guards in Common/Math.hlsli to avoid rsqrt/divide NaNs.
• perf: optimise ssgi normal #2189: Introduces dedicated srcNormal handling and adjusts SSGI shader/C++ bindings.

Suggested reviewers

• davo0411
• doodlum
• jiayev

Poem

🐰 In pixel meadows where light hops free,
I guard each length with gentle glee—rsqrt and me.
Five taps in a cross, I nibble the noise away,
Mips and horizons stitched at play.
Hop, patch, render—brightened day! ✨

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title 'refactor(ssgi): tidy GI compute shader internals' accurately summarizes the main objective of the changeset—internal refactoring of the GI compute shader with improved math stability, history clamping logic, and removal of dead code.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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