Best value for threshold in Canny

opencv edge-detection
Rubbic picture Rubbic · Aug 4, 2014 · Viewed 19.6k times · Source

I have an image which I want to detect edges on that. I found Canny has been used a lot ( I don't know whether I have a better option than that). I have set the values as follow:

  Imgproc.Canny(img, img, 10, 100, 3,true)

I've changed threshold values but don't see that much of a change in my image. Can anyone explain to me if there is a logical way to figure out numbers for threshold values (my image is gray scale)

Thank you...

Answer

Samer picture Samer · Aug 4, 2014

I think this should be taken case by case, if you post some sample images would be useful, but I will try to answer anyways. Here is from Opencv Documents 

Canny( detected_edges, detected_edges, lowThreshold, lowThreshold*ratio, kernel_size );
where the arguments are:

detected_edges: Source image, grayscale
detected_edges: Output of the detector (can be the same as the input)
lowThreshold: The value entered by the user moving the Trackbar
highThreshold: Set in the program as three times the lower threshold (following Canny’s recommendation)
kernel_size: We defined it to be 3 (the size of the Sobel kernel to be used internally)

What usually works for me is highThreshold = 255 and lowThreshold = 255/3

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