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I am doing my own implementation of Canny and Sobel in another language (not in Mathematica). I was testing the EdgeDetect method (Sobel and Canny), and my own, and I am getting different results.

What kind of preprocessing does EdgeDetect do to the image before detecting the edges, and what kind of threshold does it use?

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    $\begingroup$ While I don't know much about these methods, looking at what Mathematica and MATLAB do differently: it seems thresholding is not done the same way. See under Details in the EdgeDetect docs. $\endgroup$ – Szabolcs Apr 5 '16 at 7:47
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If you drill down the documents' details and options you may find some useful information. Here is my take:

EdgeDetect has 2 main parameters pixel range r and t for threshold. The document says the r's default value is 2. For threshold t uses hysteresis thresholding with a global threshold t. For additional forms of the threshold parameter, see the reference page for MorphologicalBinarize is pointed in the document. MorphologicalBinarize automatically choosing a suitable value for the lower threshold. FindThreshold uses default "Cluster" method which is cluster variance maximization (Otsu's algorithm).

In addition the method may contain StraightEdges; Method->{"method","StraightEdges"->s}. The default behavior is equivalent to "StraightEdges"->0. A setting "StraightEdges"->1 selects only the longest and strongest lines.

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