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your Homography Mat is empty() !

yes, that can happen, if you did not have enough "good" points for findHomography().

you have to add a check here:

Mat H = findHomography( obj, scene, RANSAC );
if (H.empty()) {
    cerr << "bad homography." << endl;
    return 1;
}

your Homography Mat is empty() !

yes, that can happen, if you did not have enough "good" points for findHomography().findHomography(), it returns an empty Mat.

you have to add a check here:

Mat H = findHomography( obj, scene, RANSAC );
if (H.empty()) {
    cerr << "bad homography." << endl;
    return 1;
}