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

Contour — where it appears

A place where a boundary system reports an edge, as distinct from the step in luminance or colour that produced it. In a filling-in model it is where a spreading signal cannot cross, so it decides the shape of every region a surface signal settles into.

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

Also named here as filling in, local adaptation, troxler fading — the same set of essays touches all of them, so they are one junction rather than several.

The same border signal, filled in with a boundary and without one. Two fields, each 6 degrees across. The signal is injected along a ring just inside a contour and varies around it, brightest on one side and dimmest on the other. On the left the signal diffuses and the contour is impermeable: the interior settles to 1.000 against a border mean of 1.000, which is the mean-value property of a harmonic function arriving as a prediction about appearance. On the right the same signal is handed to a Gaussian pool of 0.5°, which has no notion of inside: it reaches 0.040 at the centre, because a kernel weights the near rim more than the far one and a filled region does not.

A pool with an edge

A Gaussian pool says how much of a stabilised image survives and can say nothing about what the remainder looks like, because a Gaussian has no edge. Give the pool a boundary and the interior of a faded region takes the average of its own border — exactly, by the mean value theorem, arriving as a prediction about appearance.

scene · Scene A gap in the wall and a gap in the drive are not the same gap. What the centre of a region settles to under three conditions. With the contour closed it reaches its border's value exactly. Open a 16% hole in the barrier and leave the border signal unbroken and it still reaches it, to 1e-8 — a ring of driven cells encloses the centre whatever the wall outside it is doing, so nothing can escape. Break the signal too and it falls to 0.960. All of what a gap costs is the piece of border that stopped driving, and none of it is the hole.

A gap in the drive, not in the wall

Break the contour around a region and leave its border signal unbroken, and the interior does not move by one part in a million — a ring of driven cells encloses a centre whatever the wall outside it is doing. Break the signal too and the shortfall goes as the square of what is missing. All of what a gap costs is the piece of border that stopped driving.

scene · Scene

Named alongside it

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

AssertionDiffusionFilling inLocal adaptationTroxler fadingColour appearanceEccentricityIllusory contourPsychophysicsReceptive fieldSimultaneous contrastSpatial frequency

All concepts