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

Edge integration — where it appears

The visual system's habit of inferring surface brightness from the differences at boundaries rather than from the light arriving at each point. It explains why a gradient reads as illumination and a step reads as a change of material, and it is what makes a soft edge informative.

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

The Cornsweet edge, with its luminance profile. The two plateaux are both #898989 and are flat to exactly — the profile below shows that everything which differs lies within a narrow band at the boundary. Cover the centre line and the two halves become obviously identical.

Brightness is inferred from edges

The Cornsweet effect makes two identical regions look different by altering nothing except a narrow band at the boundary between them. Cover the boundary and the difference vanishes, which says the visual system is reconstructing surfaces rather than reading off intensities.

brain · Appearance What is read at each distance from the edge of a lit region. Three materials under a half-plane of light, with the boundary at the centre of the horizontal axis and the lit side on the right. The vertical axis is the radiance leaving the surface as a share of what it leaves far inside the lit region. On the unlit side the sample is emitting light while receiving none, so the ratio the model calls a reflectance has a zero denominator there. The distance over which the curve runs from a tenth to nine tenths is 0.21 millimetres on coated paper and 5.1 on pale marble — which is the width of the neighbourhood a point's colour is decided by.

Two blurs before the eye's own

A translucent object arrives at the eye already blurred, by a kernel that is a property of the material rather than of the optics. On every material in this collection's table that blur is coarser at ordinary reading distance than the finest detail the eye can resolve — so the softness of marble or skin or wax is not a failure of vision, it is the object.

eye · Cones

Named alongside it

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

AcuityAnchoringContrast sensitivityThe Cornsweet effectDiffusion lengthLightnessLightness constancyLuminanceMarginalisationPoint spread functionReflectanceScene interpretation

All concepts