Canny Edge Detection

The Canny edge detector finds the edges in an image or video frame. It is one of the most popular algorithms for edge detection. Canny algorithm aims to satisfy three main criteria:

  1. Low error rate: A good detection of only existent edges.
  2. Good localization: The distance between edge pixels detected and real edge pixels have to be minimized.
  3. Minimal response: Only one detector response per edge.

In this algorithm, the noise in the image is reduced first by applying a Gaussian mask.The Gaussian mask used here is the average mask of size 3x3.Thereafter, gradients along x and y directions are computed using the Sobel gradient function. The gradients are used to compute the magnitude and phase of the pixels. The phase is quantized and the pixels are binned accordingly. Non-maximal suppression is applied on the pixels to remove the weaker edges.

Edge tracing is applied on the remaining pixels to draw the edges on the image. In this algorithm, the canny up to non-maxima suppression is in one kernel and the edge linking module is in another kernel. After non-maxima suppression, the output is represented as 2-bit per pixel, Where:

  • 00 - represents the background
  • 01 - represents the weaker edge
  • 11 - represents the strong edge

The output is packed as 8-bit (four 2-bit pixels) in 1 pixel per cycle operation and packed as 16-bit (eight 2-bit pixels) in 8 pixel per cycle operation. For the edge linking module, the input is 64-bit, such 32 pixels of 2-bit are packed into a 64-bit. The edge tracing is applied on the pixels and returns the edges in the image.

API Syntax

The API syntax for xFcanny is: 
template<int FILTER_TYPE,int NORM_TYPE,int SRC_T,int DST_T, int ROWS, int COLS,int NPC,int NPC1>
void xFcanny(xF::Mat<SRC_T, ROWS, COLS, NPC> & _src_mat,xF::Mat<DST_T, ROWS, COLS, NPC1> & _dst_mat,unsigned char _lowthreshold,unsigned char _highthreshold)
The API syntax for xFEdgeTracing is: 
template<int SRC_T, int DST_T, int ROWS, int COLS,int NPC_SRC,int NPC_DST>
void xFEdgeTracing(xF::Mat<SRC_T, ROWS, COLS, NPC_SRC> & _src,xF::Mat<DST_T, ROWS, COLS, NPC_DST> & _dst)

Parameter Descriptions

The following table describes the xfcanny template and function parameters:

Table 1. xfcanny Function Parameter Descriptions
Parameter Description
FILTER_TYPE The filter window dimensions.The options are 3 and 5.
NORM_TYPE The type of norm used. The options for norm type are L1NORM and L2NORM.
SRC_T Input pixel type. Only 8-bit, unsigned, 1 channel is supported (XF_8UC1)
DST_T Output pixel type .The output in 1pixel case is 8-bit and packing 4-bit, 2-bit pixel values into 8-bit. The Output in 8pixel case is 16-bit, 8-bit, 2-bit pixel values are packing into 16-bit.
ROWS Maximum height of input and output image (must be a multiple of 8)
COLS Maximum width of input and output image (must be a multiple of 8)
NPC Number of pixels to be processed per cycle; possible options are XF_NPPC1 and XF_NPPC8 for 1 pixel and 8 pixel operations respectively.
_src_mat Input image
_dst_mat Output image
_lowthreshold The lower value of threshold for binary thresholding.
_highthreshold The higher value of threshold for binary thresholding.

The following table describes the xFEdgeTracing template and function parameters:

Table 2. xFEdgeTracing Function Parameter Descriptions
Parameter Description
SRC_T Input pixel type
DST_T Output pixel type
ROWS Maximum height of input and output image (must be a multiple of 8)
COLS Maximum width of input and output image (must be a multiple of 8)
NPC_SRC Number of pixels to be processed per cycle. Fixed to XF_NPPC32.
NPC_DST Number of pixels to be written to destination. Fixed to XF_NPPC8.
_src Input image
_dst Output image

Resource Utilization

The following table summarizes the resource utilization of xFcanny and xFEdgeTracing in different configurations, generated using Vivado HLS 2017.1 tool for the Xilinx Xczu9eg-ffvb1156-1-i-es1 FPGA, to process a grayscale HD (1080x1920) image for Filter size is 3.

Table 3. xFcanny and xFEdgeTracing Function Resource Utilization Summary
Name Resource Utilization
1 pixel 1 pixel 8 pixel 8 pixel Edge Linking Edge Linking
L1NORM,FS:3 L2NORM,FS:3 L1NORM,FS:3 L2NORM,FS:3
300 MHz 300 MHz 150 MHz 150 MHz 300 MHz 150 MHz
BRAM_18K 22 18 36 32 84 84
DSP48E 2 4 16 32 3 3
FF 3027 3507 4899 6208 17600 14356
LUT 2626 3170 6518 9560 15764 14274
CLB 606 708 1264 1871 2955 3241

Performance Estimate

The following table summarizes the performance of the kernel in different configurations, as generated using Vivado HLS 2017.1 tool for the Xilinx Xczu9eg-ffvb1156-1-i-es1, to process a grayscale HD (1080x1920) image for L1NORM, filter size is 3 and including the edge linking module.

Table 4. xFcanny and xFEdgeTracing Function Performance Estimate Summary
Operating Mode Latency Estimate

Operating Frequency (MHz)

 Latency (in ms)
1 pixel 300 10.2
8 pixel 150 8

Deviation from OpenCV

Gaussian filter is not available in OpenCV. Edge linking module is not available in the xfOpenCV library.