OpenCV findFundamentalMat strange behaviour

Question

I am trying to get the fundamental Matrix for two matched point-sets using openCVs find findFundamentalMat(). The images are distorted, then I detect Keypoints and match those. I thought using undistortPoints would give me better results for the fundamental matrix(I know the intrinsic parameters of my camera), but after undistortPoints, findFundamentalMat gives strange results. First, in the resulting mask array, all points are considered inliers. Second, the error is very high. I compute the error like this:

        vector<Point2f> points1Raw; //Raw points from Keypoints
        vector<Point2f> points1; //Undistorted points
        vector<Point2f> points2Raw;
        vector<Point2f> points2;
        for(int k=0; k<matches.size(); k++) {
            points1Raw.push_back(keypoints1[matches[k].queryIdx].pt);
            points2Raw.push_back(keypoints2[matches[k].trainIdx].pt);
        };

        undistortPoints(points1Raw, points1, cameraMatrixm, distCoeffsm);
        undistortPoints(points2Raw, points2, cameraMatrixm, distCoeffsm);

        vector<uchar> states;

        Mat f = findFundamentalMat(points1, points2, FM_RANSAC, 3, 0.99, states);

            //For all k matches
            Mat p1(3, 1, CV_64F);
            p1.at<double>(0, 0) = points1[k].x;
            p1.at<double>(1, 0) = points1[k].y;
            p1.at<double>(2, 0) = 1;
            Mat p2(1, 3, CV_64F);
            p2.at<double>(0, 0) = points2[k].x;
            p2.at<double>(0, 1) = points2[k].y;
            p2.at<double>(0, 2) = 1;

            Mat res = abs(p2 * f * p1); // f computed matrix

            if((bool)states[k]) //if match considered inlier (in my strange case all)
                err = err + res.at<double>(0, 0); //accumulate errors

The total resulting error is like 100 to 1000 or even more. But by manually checking the matches before computing the fundamental matrix, most of them seem right. What am I doing wrong? :/

Answer 1

How many points do you have? To be counted as inlier, an error up to (in your case) 3 pixels is allowed,... so if you have several hundred points, it's clear that the cumulated error is big.