Create a seperate _sift_detect() function

[lessthanoptimal]

Right now the only way to detect a SIFT feature is to also compute the descriptor. It would be helpful when comparing the your SIFT detector against others, if you could just call the detector without wasting CPU time computing descriptor also. This functionality is easily added. I would submit a patch, but it would probably take longer for you to inspect the code than to do it yourself.