Module ltc 

The linearly-transformed-cosine lookup tables the rectangular area-light shading path samples, generated at build time by the fitter in fit.rs.

Two tables, both LTC_LUT_SIZE square and indexed the same way: u = roughness in [0, 1] v = sqrt(1 - cos(theta_view)), which spends more of the axis on the grazing angles where the lobe changes fastest

matrix_texels holds 4 floats per cell: the non-trivial entries of the inverse transform, normalised so the middle entry is 1. The shader rebuilds [[x, 0, z], [0, 1, 0], [y, 0, w]], transforms the light quad’s corners by it, and evaluates the closed-form clamped-cosine polygon integral.

magnitude_texels holds 2 floats per cell: the lobe’s directional albedo and its Fresnel weight, recombined by the shader as f0 * albedo + (1 - f0) * fresnel.

Constants

LTC_LUT_SIZE

Edge of the square lookup table. 64 matches the resolution the parameterisation was chosen for: finer buys little once sqrt(1 - cos) has spread the grazing angles out, and the fit cost grows with the square.

Functions

magnitude_texels

RG32Float texels, LTC_LUT_SIZE square.

matrix_texels

RGBA32Float texels, LTC_LUT_SIZE square. The backend uploads these once.