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URP Real-Time Global Illumination

A real-time global illumination system built on Unity URP 14.0.12, implementing two independent GI solutions on top of Deferred Rendering + DXR pipeline.

Showcase

Mode Comparison

SSGI (Screen Space)	RTGI (Ray Traced)	MixedDDGI	RTAO (Ray Traced)

GI On/Off Comparison

GI Off	GI On

DDGI Probe Visualization

Debug Visualization

IndirectDiffuse	HitDistance	SSGI Mask

System Architecture

URPSSGI (Assets/URPSSGI/)

Four GI modes supported:

• ScreenSpace — Hi-Z Ray Marching, pure screen-space tracing
• RayTraced — DXR hardware ray tracing
• Mixed — SSGI first, falls back to RTGI for missed pixels; miss shader samples skybox
• MixedDDGI — Same hybrid strategy as Mixed, but miss shader samples DDGI irradiance atlas instead

DDGILightProbe (Assets/DDGILightProbe/)

A dynamic diffuse probe system based on the NVIDIA RTXGI SDK. Probes are placed on a uniform 3D grid, with DXR rays dispatched each frame to update irradiance/distance atlases. Supports probe relocation, classification, variability-based adaptive update, and a full light leaking suppression pipeline (surface bias, wrap shading, Chebyshev visibility test, weight crushing).

System Integration

The two systems can run independently or work together via MixedDDGI mode: they share the same RTAS (built once per frame) and the same Closest Hit Shader. DDGI exposes atlas resources to URPSSGI through the DDGIResourceProvider static interface. When DDGI is disabled, the system automatically falls back to standard Mixed mode.

Rendering Pipeline

URPSSGI ScreenSpace:
  DepthPyramid → ColorPyramid → Hi-Z Trace → Reproject
  → Temporal Filter → Spatial Filter → [Bilateral Upsample] → Composite

URPSSGI Mixed / MixedDDGI:
  SSGI: DepthPyramid → Trace → Reproject(GBuffer) → Deferred Lighting → result + mask
  RTGI: DispatchRays(mask=0) → Deferred Lighting → Merge → Denoise → Composite

DDGILightProbe (per frame):
  Ray Trace → G-Buffer → [Relocation] → [Classification]
  → LightingCombined (single pass) → MC Integration → Variability Reduction
  → Border Update → Ping-Pong Swap

Key Technical Details

URPSSGI

• Mip levels packed into a single RWTexture2D (Mip Atlas) to work around URP's inability to bind mip chains as UAVs
• Spatial denoiser uses world-space disk sampling with adaptive kernel radius, combined with depth/normal/plane-distance trilateral bilateral weights
• In Mixed mode, both SSGI and RTGI are shaded through the GBuffer → Deferred Lighting path, eliminating tonal bias between ColorPyramid and Closest Hit Shader
• Temporal Filter inlines history buffer writes + Merge-On-Read strategy, saving 5 full-screen dispatches
• Sampling sequences based on Owen-scrambled Sobol + per-pixel ranking/scrambling (BND sequences)
• Full Camera Relative Rendering throughout
• 20 built-in debug visualization modes

DDGILightProbe

• Direct lighting, indirect lighting, and radiance compositing compressed into a single compute pass (LightingCombined)
• Atlas uses ping-pong double buffering for zero-copy swaps
• Variability reduction via multi-level parallel reduction + AsyncGPUReadback drives adaptive update frequency
• Probe visualization supports irradiance / distance / relocation offset / classification state / backface ratio modes

Directory Structure

Assets/
├── URPSSGI/
│   ├── Runtime/          SSGIRendererFeature, SSGIRenderPass, RTGIRenderPass,
│   │                     RTASManager, SSGICameraContext, SSGIHistoryManager ...
│   ├── Shaders/          SSGI.compute, SSGITemporalFilter.compute,
│   │                     SSGIDiffuseDenoiser.compute, RTGIIndirectDiffuse.raytrace,
│   │                     UnifiedClosestHit.hlsl ...
│   ├── Editor/           Inspector + Unit Tests
│   └── Textures/         Blue Noise Textures
│
├── DDGILightProbe/
│   ├── Runtime/Core/     DDGIVolume, DDGIRaytracingManager, DDGIProbeUpdater,
│   │                     DDGIAtlasManager, DDGIResourceProvider ...
│   ├── Runtime/Shaders/  DDGILightingCombined.compute, DDGIMonteCarloIntegration.compute,
│   │                     DDGISampling.hlsl, DDGIRaytracing/ ...
│   └── Editor/
│
├── SSRT3/                GTAO hemisphere slice sampling reference (HDRP, read-only)
└── com.unity.sponza-urp@ Sponza test scene assets

Requirements & Setup

• Unity 2022.3+, Windows
• GPU with DXR 1.0 support (falls back to ScreenSpace mode when unavailable)
• URP Deferred Rendering Path

Setup Steps

1. Add SSGIRendererFeature to the URP Renderer; bind required Compute Shaders and textures via Inspector
2. Add SSGIVolumeComponent to a scene Volume; select GI mode and adjust parameters
3. To enable DDGI, additionally add DDGIApplyGIRendererFeature and place a DDGIVolume in the scene
4. Switch GI mode to MixedDDGI to enable both systems working together

Notes

• This project modifies DeferredLights.cs in the local URP 14.0.12 package; manual merge is required when upgrading Unity
• The two systems reside under the URPSSGI and DDGI namespaces respectively
• Shader/texture references in SSGIRendererFeature are serialized via SerializeField; rebinding may be needed after switching scenes
• The project uses many multi_compile variant keywords; watch for keyword conflicts when porting to other projects

License

This project contains Sponza scene assets under the following licenses:

• Sponza model: CC BY 3.0 — © 2010 Frank Meinl, Crytek
• NoEmotion HDRs textures: CC BY-ND 4.0 — © 2022 Peter Sanitra

See Assets/com.unity.sponza-urp@5665fb87d0/copyright.txt for full copyright information.