A configuration of alternating dark and light gray shapes positioned in counterphase at the opposite side of a dividing line leads to distinct perceptual outcomes, depending on background luminance. With backgrounds of extreme luminance (black/white), the shapes appear independent and joining at the corner. Backgrounds of intermediate luminance unify the shapes into a surface appearing as “corrugated”, in which illuminance (shadows lines) and reflectance edges are distinguishable. A rotation of the shapes around the orthogonal axis produces either vertical or horizontal furrows on the corrugate surface, depending on which of their sides form, during rotation, the smoothest contour.