Colour Science & Perception
what the eye settles, and what it cannot
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The thread: The instrument is the reader

This is the one subject where the page is displayed on the apparatus under discussion, and the reader's own eye is the measuring device. Several figures here are experiments rather than illustrations.
on a dark fieldon a light fieldboth patches are #818181asserted equal What the brain does

These two patches are identical

It is the most repeated and least checkable sentence in visual perception, because the whole point is that it does not look true. Every instance on this site is an assertion in code that stops the build if it ever stops being the case.

 one value on the left, the same value on the rightluminance profileboth plateaux are #898989asserted equal What the brain does

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.

 code 128 → 21% luminance50% luminance → code 188code valuerelative luminancegamma 2.2sRGBthe diagonal would be a linear displayIEC 61966-2-1 Difference and uniformity

The midpoint is not half

Code 128 sits halfway along the sRGB scale and carries about a fifth of white's luminance. Half the luminance is code 188. Almost every gradient, blur and resize on the web gets this wrong, and the errors are visible once known.

 one rectangle, or two?a visible seam means the display reaches past sRGBevaluated by the browser, not the buildCSS color(display-p3 …) Where the model breaks

The display is an unknown

This site is displayed on the very apparatus it is about, and it knows almost nothing about that apparatus. Two figures here stop assuming and ask instead — a probe for the transfer function and a probe for the gamut.

 400450500550600650700wavelength / nmunder D650.428, 0.474under A0.500, 0.467reflectance is a fraction, 0 to 1CIE 1931 2° observer What the brain does

Constancy is the default

A sheet of paper looks white in daylight and white under a tungsten lamp, although the light reaching the eye differs enormously. The visual system is solving one equation with two unknowns, and it solves it by assumption.

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