Indexing the archive…
Your Universe of Digital Possibilities
One picture, two garments: the colours you see depend on the light you assume is falling on them. Pick the two stripes’ colours as they look to you, and the instrument reads off which daylight your brain subtracted — a point on the colour diagram, between bluish shade and warm lamplight. Then give the light away with a grey card, and watch the disagreement close.
Discount a light by scaling each cone signal by the ratio of the target white to the assumed light’s white. Assume a different light and the same pixels become a different paint — the whole of the dress, in three ratios.
The chromaticities of typical daylight, from warm late sun to blue north sky, with xD a cubic in 1/T (4000–25 000 K). The visual system’s guesses about the light live along this curve; D65 sits on it at 6504 K.
Here Planck’s law draws the Planckian locus: each temperature’s spectrum integrated against the CIE 1931 colour-matching functions gives one point of the curve warm lamplight lives on.
The sixth percept is the colour of a thing as distinct from the colour of its light. A surface’s pixels carry both, and the eye divides out the light it believes in — von Kries’s independent gains, set by a guess about the illuminant. When the picture hides the light, people guess differently, and the same garment becomes two. What the same division does to lightness under a shadow is The Checker; what a hot body’s light looks like is The Line; what a spectrum is made of is The Spectrum.