of a Sobel edge detector on a medical image. Note the thickness of the boundary of
the heart ventricle as well as the presence of “spurious edges” due to noise. Canny
[14] proposed to add a smoothing pre-processing step (to reduce the influence of
the noise) as well as a thinning post-processing phase (to ensure that the edges are
uniquely localized). See [26] for a survey and evaluation of edge detectors using
gradient techniques.