MATLAB: Canny Edge Detection Threshold Values Meaning

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I am using the Canny edge detection method on an image. I am setting the threshold values myself. I have a grayscale image with pixel values in the range 0 to 255. I have a current threshold range of [0.1 0.15] for the image.

My question: what do these two numbers actually mean? (i.e. do they represent a percentage value? so if the pixel value is 15% of 255 it turns it into an edge? This doesnt really make sense as 15% is very low compared to the pixel value that is getting considered an edge in the image)

Best Answer

The 'Canny' method uses two thresholds.

For example, if the threshold is [0.1 0.15] then the edge pixels above the upper limit(0.15) are considered and edge pixels below the threshold(0.1) are discarded.

Now, you may have a question "what about the pixels in between upper and lower threshold"?

They are considered only if they are connected to pixels in the upper threshold. Thus we get a clean edge.

For more details, refer to input arguments section of below link.

https://www.mathworks.com/help/images/ref/edge.html

Related Solutions

MATLAB: Adaptive Canny edge detection

Canny edge detection is accomplished using the edge function. Matlab will get "automatic/adaptive" thresholds if you don't specify thresholds as an argument. An easy way to implement this is to simply call Edge twice; first to get an initial capture of the thresholds, then to adjust thresholds as needed based on the automatic thresholds.

[~,threshOut = edge(I,'Canny');
threshold = threshOut*1.0;
BW1 = edge(I,'Canny',threshold);

Just adjust the multiplier to adjust thresholds. Matlab's adaptive technique is fairly straightforward, and I believe it is a variant of the Otsu method; they simply throw out 70% of the detected edge pixels for the high threshold, and the low threshold is some constant factor multiplied by the high threshold.

There are a number of other self-adaptive ways to choose Canny thresholds, a few are detailed here: http://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=5476095

MATLAB: How can i calculate the sum of intensity of the edges in MATLAB using automatic threshold sobel edge detector

Try this:

theSum = sum(yourSobelEdgeImage(:))

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Related Solutions

MATLAB: Adaptive Canny edge detection

MATLAB: How can i calculate the sum of intensity of the edges in MATLAB using automatic threshold sobel edge detector

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