pub fn curve_at(x: f64, a: f64, b: f64, c: f64) -> f64
Monotone cubic (Fritsch–Carlson) through (0,0) (0.25,a) (0.5,b) (0.75,c) (1,1) — the exact curve shaders::CURVES evaluates on the GPU, so preview and export agree.
shaders::CURVES