MATLAB Answers

how to crop an image

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Dhandapani.S
Dhandapani.S on 23 Jan 2015
I have an image like the one below. i have to crop the image eliminating the extra white space outside the black rectangle box. so that i can also reduce the size of the image. i have to crop until i encounter the first pixel row and column wise.
it should be like this attachement after cropping

Answers (2)

Matz Johansson Bergström
Matz Johansson Bergström on 23 Jan 2015
Edited: Matz Johansson Bergström on 23 Jan 2015
In my answer I throw away the green and blue channels of the PNG, because it is black/white. If you want to use a color image, the technique is similar.
im = imread('cropp.png');
image(im)
im2 = 255 - im(:,:,1); %check only red component and map white=0
%x limits
xlim = sum(im2); %sum the columns
x(1) = find(xlim, 1, 'first') %x min
x(2) = find(xlim, 1, 'last') %x max
%y limits
ylim = sum(im2, 2); %sum the rows
y(1) = find(ylim, 1, 'first') %y min
y(2) = find(ylim, 1, 'last') %y max
%verification: line([x(1), x(2), x(2), x(1), x(1)], [y(1),y(1),y(2),y(2),y(1)],... 'color','red','linewidth',1)
%the cropping in practice: cropped = im(y(1):y(2), x(1):x(2));
figure
image(cropped)
I basically just find the limits of the image by summing along the x and y axis and finding the index of the non-zero first and last occurance of these elements.
I just added a call to "line" In my example, so you can follow.
  2 Comments
Matz Johansson Bergström
Matz Johansson Bergström on 24 Jan 2015
Yes, good point. I named a variable sum once and it got very confusing very quickly.

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Shoaibur Rahman
Shoaibur Rahman on 23 Jan 2015
I don't see any image attached. Anyway, you can try with:
roipoly to select the image region and crop manually, or imcrop to let the program to do that if you already know the image 'coordinates' to crop.

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