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# Objective - Per-cluster (instance of a meshlet) data upload is ridiculously expensive in both CPU and GPU time (8 bytes per cluster, millions of clusters, you very quickly run into PCIE bandwidth maximums, and lots of CPU-side copies and malloc). - We need to be uploading only per-instance/entity data. Anything else needs to be done on the GPU. ## Solution - Per instance, upload: - `meshlet_instance_meshlet_counts_prefix_sum` - An exclusive prefix sum over the count of how many clusters each instance has. - `meshlet_instance_meshlet_slice_starts` - The starting index of the meshlets for each instance within the `meshlets` buffer. - A new `fill_cluster_buffers` pass once at the start of the frame has a thread per cluster, and finds its instance ID and meshlet ID via a binary search of `meshlet_instance_meshlet_counts_prefix_sum` to find what instance it belongs to, and then uses that plus `meshlet_instance_meshlet_slice_starts` to find what number meshlet within the instance it is. The shader then writes out the per-cluster instance/meshlet ID buffers for later passes to quickly read from. - I've gone from 45 -> 180 FPS in my stress test scene, and saved ~30ms/frame of overall CPU/GPU time. |
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Bevy PBR
