OpenCV Q&A Forum - RSS feedhttp://answers.opencv.org/questions/OpenCV answersenCopyright <a href="http://www.opencv.org">OpenCV foundation</a>, 2012-2018.Wed, 08 Jul 2015 05:43:24 -0500undistortPoints, findEssentialMat, recoverPose: What is the relation between their arguments?http://answers.opencv.org/question/65788/undistortpoints-findessentialmat-recoverpose-what-is-the-relation-between-their-arguments/**TL;DR**: What relation should hold between the arguments passed to `undistortPoints`, `findEssentialMat` and `recoverPose`.
I have code like the following in my program
Mat mask; // inlier mask
undistortPoints(imgpts1, imgpts1, K, dist_coefficients, noArray(), K);
undistortPoints(imgpts2, imgpts2, K, dist_coefficients, noArray(), K);
Mat E = findEssentialMat(imgpts1, imgpts2, 1, Point2d(0,0), RANSAC, 0.999, 3, mask);
correctMatches(E, imgpts1, imgpts2, imgpts1, imgpts2);
recoverPose(E, imgpts1, imgpts2, R, t, 1.0, Point2d(0,0), mask);
I `undistort` the Points before finding the essential matrix. The doc states that one can pass the new camera matrix as the last argument. When omitted, points are in *normalized* coordinates (between -1 and 1). In that case, I would expect that I pass 1 for the focal length and (0,0) for the principal point to `findEssentialMat`, as the points are normalized. So I would think this to be the way:
1. **Possibility 1** (normalize coordinates)
Mat mask; // inlier mask
undistortPoints(imgpts1, imgpts1, K, dist_coefficients);
undistortPoints(imgpts2, imgpts2, K, dist_coefficients);
Mat E = findEssentialMat(imgpts1, imgpts2, 1.0, Point2d(0,0), RANSAC, 0.999, 3, mask);
correctMatches(E, imgpts1, imgpts2, imgpts1, imgpts2);
recoverPose(E, imgpts1, imgpts2, R, t, 1.0, Point2d(0,0), mask);
2. **Possibility 2** (do not normalize coordinates)
Mat mask; // inlier mask
undistortPoints(imgpts1, imgpts1, K, dist_coefficients, noArray(), K);
undistortPoints(imgpts2, imgpts2, K, dist_coefficients, noArray(), K);
double focal = K.at<double>(0,0);
Point2d principalPoint(K.at<double>(0,2), K.at<double>(1,2));
Mat E = findEssentialMat(imgpts1, imgpts2, focal, principalPoint, RANSAC, 0.999, 3, mask);
correctMatches(E, imgpts1, imgpts2, imgpts1, imgpts2);
recoverPose(E, imgpts1, imgpts2, R, t, focal, principalPoint, mask);
However, I have found, that I only get reasonable results when I tell `undistortPoints` that the old camera matrix shall still be valid (I guess in that case only distortion is removed) and pass arguments to `findEssentialMat` as if the points were normalized, which they are not.
Is this a bug, insufficient documentation or user error?
**Update**
It might me that `correctedMatches` should be called with (non-normalised) image/pixel coordinates and the Fundamental Matrix, not E, this may be another mistake in my computation. It can be obtained by `F = K^-T * E * K^-1`themightyoarfishWed, 08 Jul 2015 05:43:24 -0500http://answers.opencv.org/question/65788/