Launch cv :: warp

Question

I run the function cv :: warpPerspective () on the image and in order to get the position of some points of the image of the result that I get in the original image, this is how far I came:

int main (){ cv::Point2f srcQuad[4],dstQuad[4]; cv::Mat warpMatrix; cv::Mat src, dst,src2; src = cv::imread("card.jpg",1); srcQuad[0].x = 0; //src Top left srcQuad[0].y = 0; srcQuad[1].x = src.cols - 1; //src Top right srcQuad[1].y = 0; srcQuad[2].x = 0; //src Bottom left srcQuad[2].y = src.rows - 1; srcQuad[3].x = src.cols -1; //src Bot right srcQuad[3].y = src.rows - 1; dstQuad[0].x = src.cols*0.05; //dst Top left dstQuad[0].y = src.rows*0.33; dstQuad[1].x = src.cols*0.9; //dst Top right dstQuad[1].y = src.rows*0.25; dstQuad[2].x = src.cols*0.2; //dst Bottom left dstQuad[2].y = src.rows*0.7; dstQuad[3].x = src.cols*0.8; //dst Bot right dstQuad[3].y = src.rows*0.9; warpMatrix =cv::getPerspectiveTransform(srcQuad,dstQuad); cv::warpPerspective(src,dst,warpMatrix,src.size()); cv::imshow("source", src); cv::imshow("destination", dst); cv::warpPerspective(dst,src2,warpMatrix,dst.size(),CV_WARP_INVERSE_MAP); cv::imshow("srouce 2 " , src2); cv::waitKey(); return 0; 

my problem is that if I select a point from dst , then how can its coordinates be obtained in ** src or src2 **, since the function cv :: warpPerspective t take cv :: Point as a parameter ??

Answer 1

A portable transformation links two points as follows:

 [x'] [m00 m01 m02] [x] [y'] = [m10 m11 m12] [y] [1] [m20 m21 m22] [1] 

Where (x,y) are the original two-dimensional point coordinates, and (x', y') are the transformed coordinates.

In your case, you know (x', y') and want to know (x, y) . This can be achieved by multiplying the known point by the inverse transformation matrix:

 cv::Matx33f warp = warpMatrix; // cv::Matx is much more useful for math cv::Point2f warped_point = dstQuad[3]; // I just use dstQuad as an example cv::Point3f homogeneous = warp.inv() * warped_point; cv::Point2f result(homogeneous.x, homogeneous.y); // Drop the z=1 to get out of homogeneous coordinates // now, result == srcQuad[3], which is what you wanted 

Answer 2

You want the Transform perspective (which works on the points vector), not warpPerspective. Take the inverse of warpMatrix; you may need to adjust the last column.

 vector<Point2f> dstPoints, srcPoints; dstPoints.push_back(Point2f(1,1)); cv::perspectiveTransform(dstPoints,srcPoints,warpMatrix.inv());