# Objective
- Using multiple raster passes to generate the depth pyramid is
extremely slow
- Pulling data from the source image is the largest bottleneck, it's
important to sample in a cache-aware pattern
- Barriers and pipeline drain between the raster passes is the second
largest bottleneck
- Each separate RenderPass on the CPU is _really_ expensive
## Solution
- Port [FidelityFX SPD](https://gpuopen.com/fidelityfx-spd) to WGSL,
replacing meshlet's existing multiple raster passes with a ~~single~~
two compute dispatches. Lack of coherent buffers means we have to do the
the last 64x64 tile from mip 7+ in a separate dispatch to ensure the mip
6 writes were flushed :(
- Workgroup shared memory version only at the moment, as the subgroup
operation is blocked by our upgrade to wgpu 0.20 #13186
- Don't enforce a power-of-2 depth pyramid texture size, simply scaling
by 0.5 is fine
# 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.
