OpenCV: findHomography generating an empty matrix

Question

When using findHomography() :

Mat H = findHomography( obj, scene, cv::RANSAC , 3, hom_mask, 2000, 0.995 );

Sometimes, for some image, the resulting H matrix stays empty (H is a UINT8, 1x0x0). However, there is clearly a match between both images (and it looks like good keypoint matches are detected), and just a moment before, with two similar images with similar keypoint responses, a relevant matrix was generated. Input parameters "obj" and "scene" are both a vector of Point2f containing various coordinates.

Is this a common issue? Or do you think a bug might lurk somewhere? Personally, I have processed hundreds of images where a match exists and while I have seen sometime poor matches, it is the first time I get an empty matrix...

EDIT : This said, even if my eyes think that there should be a match in the image pairs, I realize that it might confuses some portion of the image with an other one and that maybe there is indeed no "good" match.

So my question would be: How does findHomography() behave when it is unable to find a suitable Homography? Does it return an empty matrix or will it always give a homography, albeit a very poor one? I just want to know if I encounter standard behaviour or if there is a bug in my own code.

Answer 1

Well you see, cv::findHomography() function could return empty homography matrix (0 cols x 0 rows) starting approximately from 2.4.5 release. According to some opinion this seems happen only when cv::RANSAC flag is passed.

See the issue reported here :

It likely happened because we put in new experimental version of Levenberg-Marquardt solver, which does not work that well (maybe due to some bugs)

I suggest to check the computed homography before using it anywhere:

cv::Mat h = cv::findHomography(...)
if (!h.empty())
{
    // Use it
}