Are there any fast alternatives to SURF and SIFT for scale-invariant feature extraction?
SURF is patented, as is SIFT. ORB and BRIEF are not patented, but their features are not scale-invariant, seriously limiting their usefulness in complex scenarios.
Are there any feature extractors that can extract scale-invariant features as fast as SURF and are not so strictly patented as SURF and SIFT?
Answer
Although you already choose BRISK, you might find FREAK interesting. Author claims to have better results than BRISK and ORB. I should also add that ORB is scale-invariant but has some problems in that area. So I would still recommend it for someone to try it.
The FREAK source code is compatible with OpenCV (how easy it is to merge them I do not know) and the author is working on submitting it to the OpenCV project.
EDIT:
FREAK is now part of opencv feature detectors / descriptors.
You can read here more about the differences between several feature detectors/extractors, and also a series of benchmarks which includes FREAK and other popular ones.