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Feature_GPU_Architecture.md

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Feature_GPU_Architecture.md

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GPU Architecture

Goal

Render dense grass fields with minimal CPU cost by generating, culling, and drawing entirely on the GPU.

Pipeline

  1. Generation (CSMain in GrassCompute.compute)
  • Layout: grid with jitter plus noise clumping.
  • Outputs per blade: position, height, facing vector, stiffness, wind phase, width scale.
  • Buffer: grassDataBuffer (StructuredBuffer, 32 bytes per blade).
  1. Culling and LOD (CSCull in GrassCompute.compute)
  • Inputs: source buffer, density map, frustum planes, HiZ pyramid, camera data.
  • Steps per blade: density mask check → frustum test → distance thinning + width compensation → HiZ occlusion → LOD routing to append buffers (LOD0/1/2).
  • Outputs: three AppendStructuredBuffers plus counts copied into indirect args.
  1. HiZ Generation (HiZGenerator.compute)
  • Pass 1: blit camera depth to mip 0 (point-sampled, reverse-Z aware).
  • Pass 2: reduce depth to mip chain storing farthest depth per 2x2 block.
  • Consumer: CSCull samples mip chosen from projected blade size.
  1. Rendering (GrassShader.shader)
  • Instancing: procedural setup reads GrassData per instance.
  • Deformation: Bezier wind, camera-facing tweak, normal rounding.
  • Lighting: gradient color, AO gradient, translucency, optional shadows.
  • Draws: three Graphics.DrawMeshInstancedIndirect calls (one per LOD).

Buffers

  • Source: sourceGrassBuffer (all blades).
  • LOD0/1/2: append buffers for survivors.
  • Args: three indirect argument buffers written via ComputeBuffer.CopyCount.
  • HiZ: RenderTexture with mips, RFloat, point filter.

Bounds and Transforms

  • Render bounds: centered on renderer transform, size = terrainSize * 2.
  • Procedural transform matrix built per instance from position, facing, width scale.

Key Settings (GrassRenderer)

  • Counts: grassCount, terrainSize.
  • LOD: lod0Distance, lod1Distance, maxDrawDistance, segment counts per LOD.
  • Culling: enableDensityScaling, minDensity, densityFalloffStart, widthCompensation, useOcclusionCulling, occlusionBias.
  • Wind: windDisableDistance, cameraFacing.

Verification Steps

  • Enable showDebugUI to view per-LOD counts and HiZ status.
  • In Frame Debugger, confirm three indirect draws and no per-instance CPU meshes.
  • In GPU Profiler, HiZ pass should be sub-millisecond at 1080p.

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