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# Objective - Faster meshlet rasterization path for small triangles - Avoid having to allocate and write out a triangle buffer - Refactor gpu_scene.rs ## Solution - Replace the 32bit visbuffer texture with a 64bit visbuffer buffer, where the left 32 bits encode depth, and the right 32 bits encode the existing cluster + triangle IDs. Can't use 64bit textures, wgpu/naga doesn't support atomic ops on textures yet. - Instead of writing out a buffer of packed cluster + triangle IDs (per triangle) to raster, the culling pass now writes out a buffer of just cluster IDs (per cluster, so less memory allocated, cheaper to write out). - Clusters for software raster are allocated from the left side - Clusters for hardware raster are allocated in the same buffer, from the right side - The buffer size is fixed at MeshletPlugin build time, and should be set to a reasonable value for your scene (no warning on overflow, and no good way to determine what value you need outside of renderdoc - I plan to fix this in a future PR adding a meshlet stats overlay) - Currently I don't have a heuristic for software vs hardware raster selection for each cluster. The existing code is just a placeholder. I need to profile on a release scene and come up with a heuristic, probably in a future PR. - The culling shader is getting pretty hard to follow at this point, but I don't want to spend time improving it as the entire shader/pass is getting rewritten/replaced in the near future. - Software raster is a compute workgroup per-cluster. Each workgroup loads and transforms the <=64 vertices of the cluster, and then rasterizes the <=64 triangles of the cluster. - Two variants are implemented: Scanline for clusters with any larger triangles (still smaller than hardware is good at), and brute-force for very very tiny triangles - Once the shader determines that a pixel should be filled in, it does an atomicMax() on the visbuffer to store the results, copying how Nanite works - On devices with a low max workgroups per dispatch limit, an extra compute pass is inserted before software raster to convert from a 1d to 2d dispatch (I don't think 3d would ever be necessary). - I haven't implemented the top-left rule or subpixel precision yet, I'm leaving that for a future PR since I get usable results without it for now - Resources used: https://kristoffer-dyrkorn.github.io/triangle-rasterizer and chapters 6-8 of https://fgiesen.wordpress.com/2013/02/17/optimizing-sw-occlusion-culling-index - Hardware raster now spawns 64*3 vertex invocations per meshlet, instead of the actual meshlet vertex count. Extra invocations just early-exit. - While this is slower than the existing system, hardware draws should be rare now that software raster is usable, and it saves a ton of memory using the unified cluster ID buffer. This would be fixed if wgpu had support for mesh shaders. - Instead of writing to a color+depth attachment, the hardware raster pass also does the same atomic visbuffer writes that software raster uses. - We have to bind a dummy render target anyways, as wgpu doesn't currently support render passes without any attachments - Material IDs are no longer written out during the main rasterization passes. - If we had async compute queues, we could overlap the software and hardware raster passes. - New material and depth resolve passes run at the end of the visbuffer node, and write out view depth and material ID depth textures ### Misc changes - Fixed cluster culling importing, but never actually using the previous view uniforms when doing occlusion culling - Fixed incorrectly adding the LOD error twice when building the meshlet mesh - Splitup gpu_scene module into meshlet_mesh_manager, instance_manager, and resource_manager - resource_manager is still too complex and inefficient (extract and prepare are way too expensive). I plan on improving this in a future PR, but for now ResourceManager is mostly a 1:1 port of the leftover MeshletGpuScene bits. - Material draw passes have been renamed to the more accurate material shade pass, as well as some other misc renaming (in the future, these will be compute shaders even, and not actual draw calls) --- ## Migration Guide - TBD (ask me at the end of the release for meshlet changes as a whole) --------- Co-authored-by: vero <email@atlasdostal.com> |
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Bevy PBR
