Spektrafilm-style WebGL grain shader

Port of the Poisson-Binomial particle model from agx_emulsion/grain.py to a WebGL2 fragment shader. Drag the divider to compare original vs grain. The grain pass uses a single Poisson draw per channel per sublayer (Poisson-thinning shortcut), Gaussian-approximated for large λ, with a PCG3D hash for per-pixel seeding.

↔
Original
With grain
25 μm
0.20 μm²
0.970
2.0×
0.02
0.65 px · 16 μm

Algorithm. For each channel C ∈ {C, M, Y} and sublayer s, the shader computes n = pixel_area / (particle_area · scale_C · scale_s), develops probability p = density / density_max, and draws k ~ Poisson(n·p / saturation) via Gaussian approximation k ≈ n·p/sat + sqrt(n·p/sat) · z where z = Box-Muller(hash(x,y,c,s)). The contribution is k · (density_max/n) · saturation. Lognormal microstructure is multiplied per pixel; final separable Gaussian blur smooths the result.

Notes. Input sRGB → linear → density via -log₁₀(linear). This is a demo proxy for the proper spektrafilm spectral pipeline. The grain math itself is faithful to the Python reference. Gaussian Poisson is accurate for λ > ~10; below that you'd switch to Knuth's algorithm. Reproducible per-pixel seeding via PCG3D — change "New grain" to bump the seed.