How to straighten a tilted square shape in an image?

Question

How can I straighten a tilted square shape in an image? I do not know the angle with which it is tilted and the code must calculate it and then rotate it automatically.

For example, I have the following image:

which should be rotated to give the following output image:

Answer 1

A simple way using only the top and bottom corners. Note that this approach relies on the upper and lower most corners:

i = imread('sq.jpg');
i_bw = im2bw(i)==0;
% Modify the strel as required
se = strel('square', 10);
i_ed = imopen(i_bw, se);

limits = sum(i_ed, 2);

top_y = find(limits>0, 1);
bottom_y = find(limits>0, 1, 'last');
top_x = round(mean(find(i_ed(top_y, :)>0)));
bottom_x = round(mean(find(i_ed(bottom_y, :)>0)));    

slope = -1 * (top_y - bottom_y)/(top_x - bottom_x);
rot_angle = 2 * pi * atan(slope);

i2 = imrotate(i, -rot_angle);
imshow(i2)

BEFORE

AFTER