Colour Science & Perception
what the eye settles, and what it cannot
Concept

The max-RGB estimator — where it appears

An illuminant estimator taking the brightest value in each channel as the white. It works when something in the scene is a specular highlight and fails when the brightest thing is a coloured surface, which makes its errors scene-dependent rather than random.

Named by 2 essays across 2 fields — each of them below, with the objects they name alongside it.

five greens, no neutral, under D65 — four guesses at the illuminant. The scene's surfaces as they reach the eye, and each estimator's angular error in degrees against the illuminant that actually lit them. Grey-world 43.1°, Max-RGB 27.7°, Shades-of-grey (p = 6) 31.4°, Grey-edge 34.4°. The best here is Max-RGB, which is best because this scene happens to satisfy its assumption — something reflects fully in every band — and not because it is the better algorithm. Any hatched patch is a surface this display cannot show under this light.

The algorithms that guess the light

What reaches a sensor is an illuminant multiplied by a reflectance, and no arithmetic separates a product into its factors. Every white-balance algorithm therefore works by assuming something about the world — and the interesting content of each is not its formula but the assumption, because a scene can violate it.

brain · Appearance Recovering the lamp from a highlight rather than from an assumption. A green scene under illuminant A. The faint curve is the true lamp; the solid one is what the interface component of the glossy surfaces recovers, which is 0.30° from it. Grey-world on the same scene is 42.1° and max-RGB 17.5°, because both are assumptions about the surfaces and a Fresnel reflection is not.

The highlight is the white balance

Every estimator a camera uses is an assumption about the scene wearing the costume of a measurement. The specular component of a glossy surface is not — it carries the illuminant to within a third of a degree on a scene where averaging is fifty times worse, and it does so without knowing what colour the paint underneath is.

imaging · Capture

Named alongside it

The objects these essays reach for when they reach for this one.

The grey-world assumptionIlluminantWhite balanceAdaptationAngular errorCamera rawColour constancyConstancyDichromatic modelFresnelThe grey-edge assumptionIlluminant estimation

All concepts