Canny thresholding issues

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Hi, I have some unclarities about thresholding in Canny's algorithm. Let's take this:

This is an image with max and min luminance of 255 and 0 respectively, and the type is set to be CV_8U. I have added a trackbar to adjust threshold levels on the fly. The results are:

• for aperture size of 3, the edge is being detected only up to ~780 for both thresholds. I thought it could be because of the gaussian blur, but now I have some doubts, as pre-adding gaussian blur to the image doesn't lower the 780 value at all. I would expect the edge to be found till 1020, as the gradient is equal only to the horizontal part.
• for aperture size of 5, the edge is being detected up to... ~7800.

I have no idea what's going on, could you please help me?