One problem we do have to worry about, however, is that we are only able to measure the component of optical flow that is in the direction of the intensity gradient. We are unable to measure the component tangential to the intensity gradient. This problem is illustrated in figure 4, and further developed below.
Denote the intensity by I(x,y,t). This is a function of three variables as we now have spatiotemporal variation in our signal. To see how I changes in time, we differentiate with respect to t:
Now, we assume that the image intensity of each visible scene point is unchanging over time (for example, shadows and illuminations are not changing due to any motion), so we have
Ixu + Iyv + It = 0,where the partial derivatives of I are denoted by subscripts, and u and v are the x and y components of the optical flow vector.
This last equation is called the optical flow constraint equation since it expresses a constraint on the components u and v of the optical flow.
The optical flow constraint equation can be rewritten as
Thus, the component of the image velocity in the direction of the image intensity gradient at the image of a scene point is
We cannot, however, determine the component of the optical flow at right angles to this direction. This ambiguity is known as the aperture problem.
There are two main approaches to reconstructing three dimensional motion from motion in the image plane: