06. Brightness Constancy Assumption

Second Assumption/Constraint

You’ll note that the brightness constancy assumption gives us one equation with two unknowns (u and v), and so we also have to have another constraint; another equation or assumption that we can use to solve this problem.

Recall that in addition to assuming brightness constancy, we also assume that neighboring pixels have similar motion. Mathematically this means that pixels in a local patch have very similar motion vectors. For example, think of a moving person, if you choose to track a collection of points on that person’s face, all of those points should be moving at roughly the same speed. Your nose can’t be moving the opposite way of your chin.

This means that I shouldn’t get big changes in the flow vectors (u, v), and optical flow uses this idea of motion smoothness to estimate u and v for any point.