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image-processingoctaveedge-detectionsobel

Why does the Sobel function for edge detection fail to find the contour of a white square in a black background?

Square BW (uint8)

I tryed to apply to the image the following code in octave:

sq = imread("Square BW.jpg"); 
figure(1), imshow(Square);

cont1 = edge(sq,"Sobel");
figure(2), imshow(cont1);

The image I get is:

Edges (logical)

And a similar image appears if I use the Prewitt function. Can anyone explain to me what is happening? The problem is that I can't visualize the process only the result, so I can't understand why the code isn't working.

Solution

  • The problem seems to be how threshold is computed in Octave. You can see how Octave does it by looking at its source by entering type edge at the Octave prompt, or online (I'm not copying the exact code since the code is GPL -- although quite simple)

    To get the border, you will need to set the threshold yourself (hopefully, in future versions of Octave's image package this will be fixed but at the moment it's Matlab incompatible since Matlab documentation on their default is unclear).