Colour with no pigment in it.

THE SYSTEM BEHIND THE SURFACE

Inside Anodized Fold.

Anodising does not pigment a metal. It grows a transparent oxide a few hundred nanometres thick on top of it, and the colour is interference between the light reflected off the top of that film and the light reflected off the metal underneath. So the colour here is computed, not sampled: the optical path is 2·n·d·cos(theta_t) with the refracted angle from Snell's law, and each palette colour stands in for one sampled wavelength. That only shows if the surface turns, so the geometry is built to turn. The sheet is a set of strips folded over one another, each rolling about its own centreline: a flat strip at roll theta shows |cos theta| of its width, so where it turns edge-on it creases and comes back showing its other face. Each strip is bent a little across its width, so its normal — and with it the colour — sweeps as you cross it. The metal is lit the way product photography lights metal, by what it reflects: a softbox key, a long strip light and a dark room, with Schlick Fresnel on a conductor's high F0 and the film tinting everything reflected. Strips are stacked back to front, each casting a contact shadow on the ones behind it.

Three wavelengths are not a spectrum, and this is a two-beam approximation: the multiple internal reflections of the full Airy summation are not solved, the oxide has one fixed index and no absorption, and the metal's complex refractive index is stood in for by a single Schlick F0. The strips are analytic sections, not a cloth or sheet-metal simulation, and the contact shadow is a screen-space occlusion rather than a traced one; there is no interreflection between folds. Nothing here is an electrochemistry model — the thickness drifts along the strips by an art-directed coupling, not by a solved current distribution.

1.966 ms GPU timestamp median; p95 2.556 ms. 1440x900 @ dpr 1.5; Metal driver on macOS Version 26.6.2 (Build 25G83); measured 2026-09-18. This is a recorded headless sample, not a browser FPS or mobile-performance guarantee.