Line drawings work because they represent the centers of symmetry for surfaces and volumes. They trigger the same center-neurons that the original shape would.
A hand could appear on you retina in different sizes and orientations. According to distance, the size will grow and shrink. But the center of symmetry will stay the same.
This goes further. There's also a center of symmetry between edges, and higher-level features as well. Our brain has no issue detecting these.