The Laplacian filter is a standard Laplacian of Gaussian
convolution. This is a second derivative function designed to
measure changes in intensity without being overly sensitive to
noise. The function produces a peak at the start of the change in
intensity and then at the end of the change.
Because the Laplacian of Gaussian produces a fairly wide
convolution for a small radius this filter can become quite
computationally expensive as radius is increased.