Fix CSM shadow casters incorrectly culled by camera distance; add cross-cascade blending - #1244
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…ade blending Directional-light shadow casters were culled by camera-depth distance against each cascade's own split, but an object outside a cascade's camera-depth interval can still project a shadow into it (e.g. a low sun casting a long shadow from off-screen). Cull casters against the fitted light-space cascade frustum instead, which is the correctness-preserving test; restore the lost draw-call optimization with a direction-agnostic distance reject measured from each cascade's own world-space bounding sphere instead of the camera. Also: - Cross-fade shadow visibility across the last 10% of each cascade interval instead of hard-switching, avoiding a visible pop as geometry crosses a split. The blend boundary is computed once per frame on the CPU (ShadowSystem.cascadeBlendStart) and uploaded to the shader, rather than independently re-derived on both sides. - Push the shadow-space sample point along the surface normal (normal-offset shadows) instead of only biasing depth, fixing acne from normal-mapped shading normals; guard against a zero-length normal so a degenerate G-buffer normal can't NaN the shadow lookup. - Convert shadow bias and PCF filter radius to world units and back so they stay physically consistent across cascades with different orthographic depth ranges and texel densities.
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Summary
a valid caster whenever it sat outside a cascade's camera-depth interval
but still projected a shadow into it (e.g. long shadows from a low sun).
Casters are now culled against the fitted light-space cascade frustum,
which is correctness-preserving. The lost draw-call optimization is
restored via a new distance reject measured from each cascade's own
world-space bounding sphere (direction-agnostic, so it can't reintroduce
the original bug).
now cross-fades between adjacent cascade shadow maps instead of hard-
switching, removing a visible pop at cascade splits. The blend boundary is
computed once per frame on the CPU and uploaded to the shader as data, so
the CPU frustum-widening and GPU cross-fade can't drift out of sync.
normals, with a guard against zero-length normals so a degenerate
G-buffer normal can't produce NaN in the shadow lookup.
back, keeping them physically consistent across cascades with different
orthographic depth ranges and texel densities.
This PR addressed issue #1221
Test plan
swift buildsucceedsbuildkernels.shrebuilds all.metallibtargets cleanlyswift test --filter "ShadowSystemCascade|ShadowSystemMakeUniformsTests|CsmCascadeCountTests|ShadowDistanceCullingTests"— 38/38 pass