Lines generally have two contours on surfaces, providing optic contrasts.
Contrasts can come from corners or stains or shadows.
Therefore, they are ambiguous about their origins. The shapes of contours include several types - familiar ones, perspective, topological, fractal etc. Figures with abrupt shadow borders are often identifiable. Most of their borders can be shadows on continuous surfaces, not just shadows at corners. Yet outlines based on these figures are very hard to recognize.