MATLAB: The threshold of sobel

Image Processing Toolboxsobelthreshold

Hi,
I am trying to find how the sobel threshold is defined:
I found :
determine the threshold; see page 514 of "Digital Imaging Processing" by
    % William K. Pratt
    if isempty(thresh), % Determine cutoff based on RMS estimate of noise
        % Mean of the magnitude squared image is a
        % value that's roughly proportional to SNR
        cutoff = scale*mean2(b);
        thresh = sqrt(cutoff);
scale = 4; % for calculating the automatic threshold
So I looked in the book to find an explanation why the scale is 4? but it is not explained, the book states that the threshold can be scaled linearly with the SNR but it does not explain why scale =4. Can someone explain this? I so that there were other post around this topic but I did not find a solution to this question.
txn, regards, J

Best Answer

You can use imgradient() to get the full, original Sobel-filtered image. Then you can set a threshold equal to 4 or 5 or whatever you want. It doesn't matter what they did. It only matters what you need to do to get the best results for your particular images.
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