Dynamic shading, motion parallax and qualitative shape

The authors address the problem of qualitative shape recovery from moving surfaces. The analysis is unique in that they consider specular interreflections and explore the effects of both motion parallax and changes in shading. To study this situation they define an image flow field called the reflection flow field, which describes the motion of reflection points and the motion of the surface. From a kinematic analysis, they show that the reflection flow is qualitatively different from the motion parallax because it is discontinuous at or near parabolic curves. They also show that when the gradient of the reflected image is strong, gradient-based flow measurement techniques approximate the reflection flow field and not the motion parallax. They conclude that reliable qualitative shape information is generally available only at discontinuities in the image flow field