crow_feather

# crow_keratin.shader Everything the Old Crow is made of, on one surface. A bird is keratin throughout — the feather vane, the beak sheath, the scaled tarsus and the claw are one protein laid down three ways — so this shader carries all three patterns and `pattern_mode` (0 feather, 1 horn, 2 scale) picks the one standing on this material. It hands the engine a `PbrSurface`, so the moon, the first light and the cliff's shadows reach the crow the way they reach the rock, and `capture { pass = ... }` reads real albedo, roughness and emissive off it. Iridescence is in two halves. `iridescence*` drives the engine's thin-film lobe, broken up per vane so neighbouring feathers sit at different thicknesses and the hue crawls across the bird as it turns. `sheen_glow` adds the half a night scene needs — a view-dependent blue→violet→green wash on the grazing angles that does not wait for a light to be aimed at the bird. `rim_*` takes the same idea to the silhouette, because a black crow on black basalt has no edge otherwise. `wear` greys the pigment and lifts roughness in broad blotches; on `horn` it also polishes the tip, which is what an old beak looks like.