Whether consisting of one or two bars, the middle shape is painted uniformly throughout. However its perception is affected by the gradient scale distribution of the color in the background. The illusion is noticeable even for small ranges of the background gray and almost independently of the relative shape of the bar(s).

The bar could be made to fill the entire drawing area. Even then, in a brightness aftereffect, it seems to be gradient filled, at least for a few seconds. The impression disappears on close inspection.