Vision Processing with the Canny Edge Detection Reference Design

Download PDF
ID 683433
Date 2/14/2015
Public
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

1.4.3. 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
Level Two Title