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You are right, it doesn't blur the image beforehand (you should do it, see also first link).

Since you are interested in Canny more than others I guess the following links are of interest for you (esp. the first one):

• http://dsp.stackexchange.com/questions/4716/differences-between-opencv-canny-and-matlab-canny
• http://stackoverflow.com/questions/4292249/automatic-calculation-of-low-and-high-thresholds-for-the-canny-operation-in-open
• http://answers.opencv.org/question/5247/gaps-in-canny-detected-edges/