reprojectImageTo3D() in OpenCV

Question

I\'ve been trying to compute real world coordinates of points from a disparity map using the reprojectImageTo3D() function provided by OpenCV, but the output seems to be inc

Answer 1

Your code seems fine to me. It could be a bug with the reprojectImageTo3D. Try to replace it with the following code (which has the same role):

cv::Mat_<cv::Vec3f> XYZ(disparity32F.rows,disparity32F.cols);   // Output point cloud
cv::Mat_<float> vec_tmp(4,1);
for(int y=0; y<disparity32F.rows; ++y) {
    for(int x=0; x<disparity32F.cols; ++x) {
        vec_tmp(0)=x; vec_tmp(1)=y; vec_tmp(2)=disparity32F.at<float>(y,x); vec_tmp(3)=1;
        vec_tmp = Q*vec_tmp;
        vec_tmp /= vec_tmp(3);
        cv::Vec3f &point = XYZ.at<cv::Vec3f>(y,x);
        point[0] = vec_tmp(0);
        point[1] = vec_tmp(1);
        point[2] = vec_tmp(2);
    }
}

I never used reprojectImageTo3D, however I am using successfully code similar to the snippet above.

[Initial answer]

As it is explained in the documentation for StereoBM, if you request a CV_16S disparity map, you have to divide each disparity value by 16 before using them.

Hence, you should convert the disparity map as follows before using it:

imgDisparity16S.convertTo( imgDisparity32F, CV_32F, 1./16);

You can also directly request a CV_32F disparity map from the StereoBM structure, in which case you directy get the true disparities.