video stabilization crashes with a blank video (Bug #3023)

Here is my modified global_motion.cpp of 2.4.4.

Sorry that I did not have time to setup git and do a proper pull request.

I still noticed some problems in other edge cases when there are not enough features. the previous solution seemed ok in VS2012, but had a problem in VS2005.

The correct solution is:

if (points0.size() < 5 || points1.size() < 5)
return Mat::eye(3, 3, CV_32F);

Just to explain in more detail the use case: runn image stabilization on a camera that at some point can become temporarily obstructed (i.e. cover the webcam with you hand), resulting in no features to track.

Found some more issues:

Just to be on the safe side, we need at least 6 points, since the number of parameters is 6. so I've made it at least 10 points.
If 5 points are encountered (rarely, but it can happen), then it ends up in an infinite loop in the lines below, while trying to find potential indices.


if (points0.size() < 10 || points1.size() < 10)
return Mat::eye(3, 3, CV_32F);


Also..there needs to be a check before invoking the optical flow. If there are no points, that fails:

Hence I've introduced the check:

if (pointsPrev_.size() > 0)
optFlowEstimator_->run(frame0, frame1, pointsPrev_, points_, status_, noArray());
else
return Mat::eye(3, 3, CV_32F);