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. Curiously, ends of lines do give brightness effects. Outlines tracing shadow boundaries don't become dark in appearance on one side when we realize what they are copying. However, outlines tracing corners or edges do provide changes in the appearance of surface relief when we realize what they are copying.
Outline can function with very schematic pictures.
A. Outline and touch both use relief. B. Outline in touch could use ridges, in place of visual contours. C. Visual outline uses axes - so too could touch. D. Outline can be schematic - useful in touch. E. Outline works without training in vision.