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How to crop a square that has a rotational angle?

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I need to crop one square of this image:
As you can see, I can't use imcrop() because it isn't horizontal, the squares has a rotational angle, and the region cropped need to be precisely on the edge of the line.
How can I do it?

Answers (3)

Massimo Zanetti
Massimo Zanetti on 9 Mar 2017
Edited: Massimo Zanetti on 9 Mar 2017
Inspect this: roipoly
Here is an example of what you can do:
%open an image
A = imread('coins.png');
%run roi tool, where you can define your polygon
BW = roipoly(A);
%extract image pixels in the roi
S = regionprops(BW,'ConvexImage','BoundingBox');
r = S.BoundingBox;
Q = A( round(r(2)) + (1:r(4)) , round(r(1)) + (1:r(3)) );
%display cropped image
im = imagesc(Q);
im.AlphaData = S.ConvexImage;
axis image; colormap gray;
In the roipoly window track your points manually to define a polygon, as in the picture:
Then, right-click and "Create Mask", you will get your cropped image:
  1 Comment
Selina on 1 Apr 2019
Is it possible to use rectangle crop using the ROI function? If so, can anyone please send a sample coding?

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cui,xingxing on 5 Mar 2022
I wrote an enhanced 'imcrop' function that supports rotating rectangle interception, but yours is an irregular quadrilateral, but you can also refer to what I wrote with 'estimateGeometricTransform2D' to estimate the conversion matrix, and then bring in the 'imwarp' function can be completed arbitrary shape interception, I hope it helps you

Image Analyst
Image Analyst on 19 Apr 2022
If you need to find the white lines automatically, you can try hough(), houghlines(), or radon().
I'm attaching a demo for how to use the radon transform to find the angle or rotation, and then it uses imrotate() to rotate the image until it's straight. Adapt as needed.

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