smin_grad(float4,float4,float) carries Inigo Quilez's cubic-polynomial smooth
minimum to invariant 8's value-and-gradient form: the gradient blend lerp(b,a,h)
is the exact analytic derivative, not an approximation, since the h-derivative
terms cancel algebraically inside the smoothing band.
cellular(float3) is Worley's cellular texture basis function (SIGGRAPH 1996),
searched over the jittered 3x3x3 neighbourhood of pcg3d-selected feature
points. It returns the new CultCellular struct (nearest, edge, id), following
the repo's existing degenerate-normal convention
(GameCult.Geometry.CultGeometryIsoSurface.EmitOrientedTriangle) to return a
finite zero gradient instead of NaN at a feature point.
Both are fresh implementations from the cited literature, not ports, so they
get a design.md/doc-comment citation instead of a THIRD-PARTY-NOTICES entry.
design.md gains an "Invariant 8: Value-and-Gradient Primitives" section
documenting both primitives' semantics and conventions.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
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