OpenCV stitching output is unstable

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I am using opencv stitching class to make panorama. I am using the simplest code for stitching, just giving images, making pointer to stitcher class, creating default one and calling composePanorama function.
With every sequence if images I am getting unstable results, for example the dimensions of ouput image is different for each time of function works.
Can someone help me to figure out the reason?
I have looked to source codes and forums, I think maybe function finds different keypoints each time ,thus homography is different and the results differ.
Also I have found out, that SURF feature finder and matcher algorithms are robust than other, but that having that module need an additional license.