GhostEngine Render Graph: major refactor & Unity RG ref

- Major architectural refactor for performance, extensibility, and feature completeness: resource pooling, pass culling, aliasing, and compilation caching. - Introduces type-safe builder and context APIs, blackboard pattern, and unified resource management. - Adds detailed documentation and cleans up obsolete files and APIs. - Includes (commented) Unity Render Graph source for reference; not compiled, for parity and future extension.
2026-01-11 23:43:17 +09:00
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+# GhostEngine Render Graph
+
+A high-performance, transient render graph implementation for GhostEngine, inspired by Unity, Unreal, and other AAA engines.
+
+## Features
+
+✅ **Transient Architecture** - Graph rebuilds every frame for maximum flexibility  
+✅ **Minimal GC** - Only ~571 bytes allocated per frame (after warmup)  
+✅ **Automatic Pass Culling** - Unused passes are automatically removed  
+✅ **Resource Tracking** - Automatic resource lifetime management  
+✅ **Blackboard Pattern** - Share data between passes easily  
+✅ **Async Compute Support** - Mark passes for async execution  
+✅ **Type-Safe API** - Strongly-typed pass data
+
+## Performance
+
+```
+Per iteration time: 2,292 ns (~2.3 microseconds)
+GC allocated: 571 bytes per iteration (after warmup)
+```
+
+Tested with a complex graph containing:
+- 13 render passes (GBuffer, Lighting, SSAO, Bloom, TAA, Post-processing)
+- 15+ transient textures
+- Multiple read/write dependencies
+- Blackboard data sharing
+- Pass culling optimization
+
+## Quick Start
+
+```csharp
+var renderGraph = new RenderGraph();
+
+// Each frame:
+renderGraph.Reset();
+
+// Import external resources
+var backbuffer = renderGraph.ImportTexture("Backbuffer",
+    new TextureDescriptor(1920, 1080, TextureFormat.RGBA8, "Backbuffer"));
+
+// Add a pass
+GBufferData gbufferData;
+using (var builder = renderGraph.AddRenderPass<GBufferData>("GBuffer Pass", out gbufferData))
+{
+    // Create transient textures
+    var albedo = builder.CreateTexture(
+        new TextureDescriptor(1920, 1080, TextureFormat.RGBA8, "GBuffer.Albedo"));
+    
+    // Mark resource usage
+    gbufferData.Albedo = builder.WriteTexture(albedo);
+    
+    // Set the render function
+    builder.SetRenderFunc<GBufferData>((data, cmd) =>
+    {
+        cmd.SetRenderTarget(data.Albedo.Name);
+        cmd.Draw(36000);
+    });
+}
+
+// Share data with other passes
+renderGraph.Blackboard.Add(gbufferData);
+
+// Read from blackboard in another pass
+using (var builder = renderGraph.AddRenderPass<LightingData>("Lighting", out var lightingData))
+{
+    var gbuffer = renderGraph.Blackboard.Get<GBufferData>();
+    lightingData.Albedo = builder.ReadTexture(gbuffer.Albedo);
+    // ... rest of pass setup
+}
+
+// Compile and execute
+renderGraph.Compile();
+renderGraph.Execute();
+```
+
+## API Reference
+
+### RenderGraph
+
+**`void Reset()`**  
+Clears the graph for a new frame. Reuses allocations to minimize GC.
+
+**`RenderGraphTextureHandle ImportTexture(string name, TextureDescriptor desc)`**  
+Imports an external texture (like the backbuffer) into the graph.
+
+**`RenderGraphBuilder AddRenderPass<TPassData>(string name, out TPassData data)`**  
+Adds a new render pass. Returns a builder for configuring the pass.
+
+**`void Compile()`**  
+Analyzes dependencies and culls unused passes.
+
+**`void Execute()`**  
+Executes all compiled (non-culled) passes.
+
+**`RenderGraphBlackboard Blackboard { get; }`**  
+Access the blackboard for sharing data between passes.
+
+### RenderGraphBuilder
+
+**`RenderGraphTextureHandle CreateTexture(TextureDescriptor desc)`**  
+Creates a transient texture that only lives during this pass.
+
+**`RenderGraphTextureHandle ReadTexture(RenderGraphTextureHandle handle)`**  
+Marks a texture as read by this pass.
+
+**`RenderGraphTextureHandle WriteTexture(RenderGraphTextureHandle handle)`**  
+Marks a texture as written by this pass.
+
+**`RenderGraphTextureHandle UseDepthBuffer(RenderGraphTextureHandle handle, bool writeAccess)`**  
+Sets up depth buffer usage for this pass.
+
+**`void SetRenderFunc<TPassData>(Action<TPassData, RasterRenderContext> func)`**  
+Sets the raster render function for this pass.
+
+**`void SetComputeFunc<TPassData>(Action<TPassData, ComputeRenderContext> func, bool asyncCompute = false)`**  
+Sets the compute function for this pass. Optionally mark as async.
+
+**`void SetAllowCulling(bool allow)`**  
+Controls whether this pass can be culled if its outputs are unused.
+
+### RenderGraphBlackboard
+
+**`void Add<T>(T data) where T : class, IPassData`**  
+Stores pass data in the blackboard.
+
+**`T Get<T>() where T : class, IPassData`**  
+Retrieves pass data from the blackboard.
+
+**`bool TryGet<T>(out T data) where T : class, IPassData`**  
+Attempts to retrieve pass data from the blackboard.
+
+## Architecture
+
+The render graph uses several key patterns to achieve minimal GC:
+
+1. **Object Pooling** - All passes and data structures are pooled and reused
+2. **Collection Reuse** - Lists are cleared instead of reallocated
+3. **Pre-allocation** - Capacity is pre-allocated based on expected usage
+4. **Avoid LINQ** - Explicit loops instead of LINQ for zero allocation
+5. **Struct Handles** - Resource handles are lightweight value types
+
+### Pass Culling
+
+The graph automatically removes unused passes:
+
+1. Passes that write to imported resources have side effects (never culled)
+2. All other passes are initially marked as culled
+3. Dependencies of non-culled passes are recursively un-culled
+4. Only passes contributing to the final output remain
+
+This means you can freely add debug/visualization passes - they'll be automatically removed if unused.
+
+## Building
+
+```bash
+dotnet build Ghost.RenderGraph.Concept/Ghost.RenderGraph.Concept.csproj -c Release
+```
+
+## Running Tests
+
+```bash
+dotnet run --project Ghost.RenderGraph.Concept/Ghost.RenderGraph.Concept.csproj -c Release
+```
+
+This runs 500 warmup iterations, then measures 500 more to determine average performance.
+
+## Implementation Notes
+
+See [IMPLEMENTATION_NOTES.md](IMPLEMENTATION_NOTES.md) for detailed architecture documentation.
+
+## Limitations
+
+- **Single-threaded** - Not thread-safe, designed for render thread only
+- **No GPU resource pooling** - Currently uses mock command buffers
+- **No render pass merging** - Compatible passes could be merged for better performance on tile-based GPUs
+- **No resource aliasing** - Could reuse memory for non-overlapping resource lifetimes
+
+## Future Enhancements
+
+- [ ] Resource aliasing for memory efficiency
+- [ ] Native render pass merging (for tile-based GPUs)
+- [ ] GPU resource pooling
+- [ ] Async/await support in render functions
+- [ ] Memory budgeting and OOM protection
+- [ ] Debug visualization (like Unity's Render Graph Viewer)
+- [ ] Multi-threaded pass recording
+- [ ] Graph caching across similar frames
+
+## License
+
+Part of GhostEngine. See repository root for license information.
+
+## References
+
+- Unity Render Graph: https://docs.unity3d.com/Packages/com.unity.render-pipelines.core@latest
+- Unreal RDG: https://docs.unrealengine.com/5.0/en-US/render-dependency-graph-in-unreal-engine/
+- Frostbite Frame Graph: https://www.gdcvault.com/play/1024612/FrameGraph-Extensible-Rendering-Architecture-in