Vision Processing with the Canny Edge Detection Reference Design

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

1.4.3. Sobel Edge Intensity

The design applies a Sobel operator to extract the image gradient intensity. Canny edge detection often uses a Sobel operator as its kernel weights have a degree of noise suppression.

The Sobel output is a 2D vector matrix in which its magnitude represents edge intensity strength and its direction indicates the orientation of the edge direction.

For more information: Rafael C. Gonzalez, Richard E. Woods Digital Image Processing Second Edition, 2nd ed., Prentice Hall pp.577-578 andl pp.579

The edge magnitude may exceed the maximum 8 bit pixel value of 255 so the design saturates the edge output pixel to 255 rather than normalizing, which does not give a very bright image output. Thus the Sobel output flushed to the monitor screen is saturated. The subsequent Canny blocks use the full Sobel output bit range for maximum precision.
Figure 4. Sobel Output