Rotating back points from a rotated image in OpenCV

Question

I’m having troubles with rotation. What I want to do is this:

• Rotate an image
• Detect features on the rotated image (points)
• Rotate back the p

Answer 1

I had the same problem.

For a transform M and point pp in the rotated image, we wish to find the point pp_org in the coordanates of the original image. Use the following lines:

cv::Mat_ iM;
cv::invertAffineTransform(M, iM);
cv::Point2f pp_org = iM*pp;

Where the operator * in the above line is defined as:

cv::Point2f operator*(cv::Mat_ M, const cv::Point2f& p)
{ 
    cv::Mat_ src(3/*rows*/,1 /* cols */); 

    src(0,0)=p.x; 
    src(1,0)=p.y; 
    src(2,0)=1.0; 

    cv::Mat_ dst = M*src; //USE MATRIX ALGEBRA 
    return cv::Point2f(dst(0,0),dst(1,0)); 
} 

Note: M is the rotation matrix you used to go from the original to the rotated image