How can I detect the edges in an image without using method 'edge', with only using mathematical operations (matrix or Derived or div or any other)? Indeed, how can I rewrite the function edge by using the algorithm Canny or sobel or any other?

For example:

pink rectangle 256*256 black rectangle 127*127

Answer:Canny Tutorial

• matlab
• math
• edge-detection
• matlab-guide

You state that you wish to use Canny, Sobel or another algorithm. These can both be used in edge. Try for example:

BW = edge(I,'canny');

where I is your image matrix. If you are interested in finding out how edge works, type

edit edge

into your command window. You will then get to see MATLAB's own implementation.

You may wish to reimplement edge from scratch, to gain a good understanding of how image processing algorithms work. If so, I would direct you towards the following sources:

• The Canny wikipedia page
• The Sobel wikipedia page
• I personally found this book an excellent reference for getting to grips with the basics of things like filters and edge detectors.

For your specific example with the rectangles, it is quite possible to use edge to find the edges. The one trick you have to do is to convert the rgb image to a grayscale one, using rgb2gray. Try for example:

rgb_image = imread('iarLe.png');
gray_image = rgb2gray(rgb_image);
edge_image = edge(gray_image);
imshow(edge_image);