Cornell Box

The Cornell Box is computer graphics' most famous room. Cornell's graphics group built the real thing in 1984 — a box with one red wall and one green wall, a light in the ceiling, two white blocks on the floor — measured it, photographed it, and rendered the same scene, so simulation could be held up against a photograph of the room it was meant to predict. It became the standard test for whether a renderer gets indirect light right: not the light that arrives straight from the source, but the light that bounces, taking on the colour of every surface it touches along the way.

This one is solved with radiosity — the method, introduced with that very box, that treats every surface as both lit and lighting. Each wall, each block, and the light itself is broken into small patches; the brightest patch that hasn't yet shot its light sends it out to every patch it can see, that light reflects and becomes brightness to shoot in turn, and the room fills in bounce by bounce. You watch it happen — the first shots are the bare ceiling light casting hard shadows, then the shadows soften, the ceiling catches glow thrown up off the floor, and, the whole point, the red and green walls bleed their colour onto the white. That colour is computed, not painted: it is the indirect light made visible.

It is also as neat an argument as I know for keeping this site's own frame in black and white. The house rule is that the work supplies the colour and the frame stays out of the way — and there's no better case of light being the work than a Cornell Box.

Drag to orbit — the box opens toward you from whatever side you turn to it — scroll to zoom, shift-drag to pan, double-click to reset. Set the patch resolution and watch it re-solve, resize the ceiling light to trade crisp shadows for soft, lift the exposure, hide the blocks for the empty room, or switch on show patches to see the mesh the solver actually reasons over.

Plain HTML5 canvas + JS, no libraries — a hand-rolled perspective-orbit camera, a from-scratch patch/form-factor radiosity solver with ray-traced visibility for the shadows, and a painter's-algorithm renderer. Light or dark to match the page. Source on GitHub.