No direct built in function that I know off. Use circshift to extract them in N Dimensions. If A is your data and I want to find the boundary as the first pixel inside the phase defined by 1s, do something like:
AX = circshift(A,[1 0 0]);
Xbound = AX-A;
If you do this in a dummy 1 or 2D case you can see what Xbound gives you and convince yourself about what to do with it.
