How to perform a thresholding operation on the gradient magnitude?

This is the rules:

A simple edge detector using gradient magnitude

1. Compute gradient vector at each pixel by convolving image with horizontal and vertical derivative filters

2. Compute gradient magnitude at each pixel

3. If magnitude at a pixel exceeds a threshold, report a possible edge point.

But I don't know how to apply threshold manually in matlab (step 3.). Can anyone help me?

      im1 = rgb2gray(imread('dog.jpg'));
      [gx,gy] = imgradientxy(im1);
      [gmag,gdir] = imgradient(gx,gy);
      imshow(gmag,[]);

Answer 1

[Image Analyst]

I think they wanted you to use imfilter() or conv2() to do it "manually". If you do want to go to a higher level, you should use imgradent() not imgradientxy - that gives you the magnitude directly. Then you can threshold it by adding this line:

binaryImage = gMag > someThresholdValue; % Threshold the image.

You don't need to do both imgradientxy() and imgradient().

[Eren]

Thank you very much, it's really helpful.