Vision Processing with the Canny Edge Detection Reference Design

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

1.5.5. About the Edge-linking Algorithm

The Canny edge reference design eliminates a second pass of the video frame, which increases the frame rate by two times.

The algorithm has to make a second pass of the processed video frame to suppress any rogue weak edge pixels. The design off loads this sequential process to the FPGA by suppressing the rogue weak edge pixels as they stream out of the output frame buffer. The design uses the FPGA as both as a pre-processing and post-processing hardware accelerator.

The design reduces the number of eight-point neighborhood comparisons. If the design encounters a strong edge pixel during the first pass of the video frame, the design may have burned it by a previous recursive step with its eight-point neighborhood checked. Checking the eight-point connected neighborhood is redundant. To eliminate this unnecessary step, the design gives pixels to be burned in a recursive step a special marker. The design skips any previously burned area. However, the design must convert this marker to the actual high value of 255.