Vision Processing with the Canny Edge Detection Reference Design

ID 683433
Date 2/14/2015
Document Table of Contents

1.4.4. Nonmaximal Suppression

Sobel edges tend to be quite thick because of the range of edge intensity values present. Thick edges are not desirable for Canny edge detection as it contains redundant information, slowing down processing for the edge-linking block as more strong-weak edge comparisons have to be made for the recursive grass-fire algorithm. Thin line edges are desirable. Nonmaximal suppression gives thin edges. This block traces the edges along its edge direction and sets any edge value that is non-maximal to 0.

For ease of implementation the hardware makes a localized search in a 3x3 window. As edge tracing occurs in an 8-point connected region, the design only uses only approximated edge angles of 45 degrees: 0, 45, 90 and 135. The 3x3 region is obtained using the same tapped delay line method used in the Gaussian and Sobel blocks.

Figure 5. Nonmaximal Suppression OutputShows thinned out edges.