Two aspects of edge detection are analyzed, namely accuracy of localization and sensitivity to noise. The detection of corners and trihedral vertices is analyzed for gradient schemes and zero-crossing schemes. It is shown that neither scheme correctly detects corners of trihedral vertices, but that the gradient schemes are less sensitive to noise. A simple but important conclusion is that the noise present in digital images of typical indoor scenes is small and the signal-to-noise ratio is high. The noise present in digital images is so small as to make the performances of a variety of filters almost indistinguishable. As a consequence small filters can be used and the exact shape of the filter is not critical.>

A Frequency-to-Code Converter (FCC) is presented in which a counter clock automatically slows down its pace as the interval between the last pulse and the next one exceeds some prededetermined value. This technique ("frequency-grading") allows building a simple broad-band (10-65 000-Hz) FCC with a 1-percent accuracy for frequencies higher than 50 Hz.