How to find new coordinates of a point after image rotation?

I have an image that has a ROI inside it. I have the coordiantes for the ROI inside the image. Now I am rotating the image with 180 degree and wants to find the new coordinates for the image. I used this code:

P = [31961;29101];    % coordinates of point
alpha = 180;   % angle for rotation
RotatedIm = imrotate(im,alpha);   % rotation of the main image (im)
RotMatrix = [cosd(alpha) -sind(alpha); sind(alpha) cosd(alpha)]; 
ImCenterA = (size(im)/2)';         % Center of the main image
ImCenterB = (size(RotatedIm )/2)';  % Center of the transformed image
RotatedP = RotMatrix*(P-ImCenterA)+ImCenterB;

And the output will be negative values of :

      -31273
      -28305

How to solve this issue?!

Answer 1

[Tarunbir Gambhir]

I am not able to rectify the issue in your case since you have not provided the image. But you can see the results on a sample image below without any issue.

im = imread('peppers.png');
P = [100;200];    % coordinates of point within the image size
alpha = 180;   % angle for rotation
RotatedIm = imrotate(im,alpha);   % rotation of the main image (im)
RotMatrix = [cosd(alpha) -sind(alpha); sind(alpha) cosd(alpha)]; 
ImCenterA = (size(im,1,2)/2)';         % Center of the main image
ImCenterB = (size(RotatedIm,1,2)/2)';  % Center of the transformed image
RotatedP = RotMatrix*(P-ImCenterA)+ImCenterB;

figure('Name','Before Rotation','NumberTitle','off');
imshow(im);
hold on
plot(P(2),P(1),'r+', 'LineWidth', 20);
plot(ImCenterA(2),ImCenterA(1),'k+', 'LineWidth', 20);

figure('Name','After Rotation','NumberTitle','off');
imshow(RotatedIm);
hold on
plot(RotatedP(2),RotatedP(1),'r+', 'LineWidth', 20);
plot(ImCenterB(2),ImCenterB(1),'k+', 'LineWidth', 20);

(note the change made in line 6,7 for the size function is because I used an RGB image)

The thing here to keep in mind is to reverse the coordinates when drawing/plotting on an image. This is the spatial image coordinates which uses the intrinsic coordinate system. You can find more information regarding it here.