splat_cull

Stage 1 of the Gaussian splat per-frame depth sort. One thread per splat.

Transforms the splat's packed position to world space, frustum-culls it at three sigma (the Gaussian's visible support), computes its view depth from the clip-space w, and files the survivor into one of bucket_count depth buckets ordered far to near. The bucket id lands in keys[i]; a culled splat gets bucket_count, one past the last real bucket, so it sorts behind every survivor and the scatter can recognise it.

Buckets are spaced logarithmically between near and far, so each spans a constant ratio of depth: near geometry gets the same relative ordering precision as far geometry.

indices is seeded to the identity permutation the radix passes start from.

Bindings

Slot	Access	Contents
records	read	Packed splat records, 6 u32 words each.
view	read	CullUniforms — view-projection, model, frustum planes, params.
keys	read_write	Per-splat depth bucket, or bucket_count when culled.
indices	read_write	The sort's input permutation, seeded to the identity.
stats	read_write	[0] = surviving splat count (atomic).

A splat whose whole footprint projects to less than screen.z pixels of radius is dropped as well. Distance shrinks that projection, so a cloud far from the camera resolves to progressively fewer splats while a near one keeps every splat it has. The radius is estimated by projecting the three world axes from the splat's centre and keeping the widest, which holds under a sheared or non-uniformly scaled cloud.

params is (splat count, bucket count, near depth, far depth) and screen is (viewport width, viewport height, min screen radius in pixels, 0).

Dispatch one workgroup of 64 per 64 splats.