旋转后如何在图像上找到一个点？

Question

A point(x,y) on image need to be located on same image after rotating it. I have used imrotate, imtransform and imwarp for rotation of image (Code below show implementation of imwarp), but none seems to work for me. The output rotated image is larger than original image. Can anybody advise how to locate the same point on output rotated image.

close all;
I=imread('P.png');
x=339;
y=317;

subplot(2,2,1);
imshow(I),title('Original Image');    
hold on;
plot(x,y,'ro');
hold off;
theta=5;
tform = affine2d([cosd(theta) -sind(theta) 0;sind(theta) cosd(theta) 0; 0 0 1])
RI = imwarp(I,tform);
[x1,y1]=transformPointsForward(tform,x,y);
subplot(2,2,2);
imshow(RI),title('Rotated Image');    
hold on;
plot(x1,y1,'ro');
hold off;

Though the code works if rotated image have same size as original image. but in my case it leads to cropping of original image. I have been able to find answer to this question on various forums, but none seems to be working for me. Kindly answer it.

Answer 1

Call imwarp like this:

[RI, ref] = imwarp(I,tform);

ref is a spacial referencing object of type imref2d , which contains the relationship of the output image's coordinate system to a "world coordinate system", which, in this case, is the coordinate system of the input image. Then you can account for the translation like this:

[x1,y1]=transformPointsForward(tform,x,y);
x1 = x1 - ref.XWorldLimits(1);
y1 = y1 - ref.YWorldLimits(1);