**Comments: **In two dimensions this
becomes a little bit more complicated. As demonstrated in one
dimension the derivatives of the wrapped and the unwrapped phase
should be identical. Therefore, the correct solution of the
unwrapping process is the one which mimimizes the differences between
the two derivatives. The minimum can be found by searching for the
second derivative to be zero.

This can be very easily realized with a least-square implementation using FFTs (IDL download). It is very fast, but the solution of this simple method is not too good, error propagation from low coherent areas and global distortions of the result limit its applicability. More advanced methods are described in literature.