Vision Processing with the Canny Edge Detection Reference Design

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ID 683433
Date 2/14/2015
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Document Table of Contents
Document Table of Contents x
1. Vision Processing with the Canny Edge Detection Reference Design
1. Vision Processing with the Canny Edge Detection Reference Design x
1.1. About Canny Edge Detection 1.2. About the Canny Edge Detection Reference Design 1.3. Getting Started with the Canny Edge Reference Design 1.4. Canny Edge Detection Reference Design Block Description 1.5. Stream-to-Memory Conversion 1.6. Latency and Throughput 1.7. Canny Edge Reference Design Resource Usage
1.3. Getting Started with the Canny Edge Reference Design x
1.3.1. Hardware and Software Requirements 1.3.2. Connecting the Hardware to Use the Canny Edge Reference Design 1.3.3. Loading the Canny Edge Reference Design FPGA Image with the SD Card Image 1.3.4. Canny Edge Reference Design Initial Startup Problems 1.3.5. Controlling the FPGA Flow of the Canny Edge Reference Design 1.3.6. Capturing the Pixel Stream 1.3.7. Programming the FPGA with the Canny Edge Reference Design 1.3.8. Initializing the ARM Processor
1.4. Canny Edge Detection Reference Design Block Description x
1.4.1. Greyscale Conversion 1.4.2. Gaussian Blurring 1.4.3. Sobel Edge Intensity 1.4.4. Nonmaximal Suppression 1.4.5. Double Threshold 1.4.6. Edge Linking
1.5. Stream-to-Memory Conversion x
1.5.1. FPGA and ARM Processor Clock Domains 1.5.2. HPS SDRAM Partitions 1.5.3. Frame Buffering 1.5.4. Operating System and Video Driver 1.5.5. About the Edge-linking Algorithm
1. Vision Processing with the Canny Edge Detection Reference Design

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.
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