Report Number: CS-TR-86-1130
Institution: Stanford University, Department of Computer Science
Title: On Detecting Edges
Author: Nalwa, Vishvjit S.
Author: Binford, Thomas O.
Date: March 1986
Abstract: An edge in an image corresponds to a discontinuity in the intensity surface of the underlying scene. It can be approximated by a piecewise straight curve composed of edgels, i.e., short, linear edge-elements, each characterized by a direction and a position. The approach to edgel-detection here, is to fit a series of one-dimensional surfaces to each window (kernel of the operator) and accept the surface-description which is adequate in the least squares sense and has the fewest parameters. (A one-dimensional surface is one which is constant along some direction.) The tanh is an adequate basis for the step-edge and its combinations are adequate for the roof-edge and the line-edge. The proposed method of step-edgel detection is robust with respect to noise; for (step-size/${\sigma}_{noise}$) >= 2.5, it has subpixel position localization (${\sigma}_{position}$ < 1/3) and an angular localization better than $10^\infty$; further, it is designed to be insensitive to smooth shading. These results are demonstrated by some simple analysis, statistical data and edgel-images. Also included is a comparison, of performance on a real image, with a typical operator (Difference-of-Gaussians). The results indicate that the proposed operator is superior with respect to detection, localization and resolution.