Indexing the archive…
Your Universe of Digital Possibilities
A is a dark tile in the sun; B is a light tile in the shadow of the cylinder. They leave the screen as the same grey — the instrument reads both pixels back to prove it. Match each with a patch on a neutral grey, a few times each, and it computes how strongly your eye subtracts the shadow: 0 if you reported the light that reaches you, 1 if you reported the paint. Then turn the sun, soften the shadow, and join A to B.
What reaches the eye from a matte surface is its reflectance (the paint, R) times the light falling on it (E). One number, two causes: a dark paint in sun and a light paint in shade can send the same L, and the eye has to decide which product it is looking at.
M is the visitor’s match, S what a photometer would set (the luminance), R what full constancy would set (the paint). Zero is a luminance meter, one is perfect albedo recovery; the log form treats equal ratios as equal steps.
Walk from A to B summing the log luminance ratios between neighbours, but pass each through a threshold T that zeroes gentle steps: sharp steps are paint edges, gradual ones are illumination. A soft shadow edge is discounted; a hard one is read as paint.
Within each framework of illumination the highest luminance is anchored to white (reflectance 0.9) and everything else scales from it; the reported lightness blends the local framework (the shadow) with the global one (the whole scene).
A pixel value V is not a luminance. Every Iris stimulus is specified in linear light and encoded through this curve (IEC 61966-2-1), and ramps are dithered in linear light — so a ramp on the screen is a ramp of light, and two equal codes are two equal luminances.
This is the Iris Edition’s first percept and its whole grammar on one board: an honest trial (your matches, in a randomised order from a randomised start), your own number fitted from them, two published observers in silver predicting the same bias, and the world the bias was built for — where a surface keeps its paint as clouds and shadows pass, because the paint is what a surface is. Helmholtz called it unconscious inference: the light is estimated and discounted before you are asked. What a lateral-inhibition filter does to an edge is the next instrument, The Band; what a detector does with noise it cannot see past is The Noise.