5.3.1. Edge detectors. Consider the ideal case of a bright object O on a dark background.
The physical object is represented by its projections on the image I. The
characteristic function 1O of the object is the ideal segmentation, and since the
object is contrasted on the background, the variations of the intensity I are large
on the boundary ∂O. It is therefore natural to characterize the boundary ∂O as
the locus of points where the norm of the gradient |∇I| is large. In fact, if ∂O is
piecewise smooth then |∇I| is a singular measure whose support is exactly ∂O.
This is the approach taken in the 60’s and 70’s by Roberts [81] and Sobel [91] who
proposed slightly different discrete convolution masks to approximate the gradient
of digital images. Disadvantages with these approaches are that edges are not
precisely localized, and may be corrupted by noise. See Figure 5.3(b) is the result