The boundary of an object provides a cue to the basic shape of an object not only through the position of the boundary but by the fact that the normal to the surface at the boundary is typically at 90 degees to the viewing direction.

Here are some manually defined occluding boundaries on Mona. |

Here is part of a needle diagram showing the surface normals set so that they point outwards from the occluding contours. At all other points it is assumed the surface normal points directly towards the viewer. |

This very crude approximation of what the surface normals are for the whole scene is enough to construct a simple 2½D representation of the shape of the scene. For this I use my Shape from Shapelets reconstruction method. Note that, strictly speaking, the vector field is far from integrable (at line terminations and T junctions) but the Shapelet reconstruction approach has no difficulties with this. Some results for Mona are shown below. (Note I chose not to define a contour along her chin. I found that when I did this the reconstruction produced a very pointy chin, giving her a mean look!)

Here is another simple example