ddx(normal) approximates screen-space curvature. Use it to deposit “dirt” in concave creases or wear on convex edges. It’s physically inspired but runs at the cost of a few ALU ops.
: Because this happens in hardware at the SIMD (Single Instruction, Multiple Data) level, it provides a "free" way to understand how surface properties are changing without complex mathematical modeling. Primary Applications 1. Automatic Mipmap Selection derivative shaders