undistorting/ dewarping a picture with 4 points on a plane

Question

I'd like to dewarp an image using python (opencv/PIL etc.). I have 4 points of which I know that they should form a rectangular on a plane.

In gimp I can manually dewarp the picture usingbackwards-correction, but I'd like to write a program, that doesn't rely on gimp.

All the functions I have found rely on a transformation matrix, so I guess it would be sufficient to give me some pointers on how to calculate the correct matrix.

Greets & thanks for the help

Answer 1

To calculate this matrix, use :

cv2.getPerspectiveTransform(src, dst)

With src a list containing your 4 points and dst a list containing the corners of the new image. Be careful, they must be in the right order

Answer 2

I don't use python but here is your answer in c++ it should be the same:

transformationMatrix= cv::getPerspectiveTransform(source , dst_pnt);  // getting the transformation matrix 

cv::warpPerspective(src, quad, transformationMatrix,perspectiveSize,1); // warping


.........................
unwarping 

cv::Matx33f unwarp = transformMatrix;
    cv::Point3f homogeneous = unwarp.inv() *pTmp; // pTmp is a point in your transformaed frame 

cv::Mat unwarpFrame =  unwarp.inv() * srcFrame;   // in case of a frame 
enter code here