Saturday, March 1, 2008

[Reading] Scale & Affine Invariant Interest Point Detectors

Though I know this paper for a long time, I didn't attempt to read it before. This paper present a method to detect the interest points which are invariant to scale or affine transformations. That is, for each detected point, a transformation matrix is also extracted which transforms the local patch around the detected point into a canonical form for efficient description and matching.

Though the formulation and the method is elegant, it is too slow for many applications. The detection part requires around 50 times computation than SIFT, and the descriptor used in the paper is not as good as the one used in SIFT. I think that's why most applications only apply SIFT. It's interesting that some people use SIFT and this detector to detect the interest points and then use the SIFT descriptor to describe the detected points. These algorithms find different types of interest points and the union of these points cover a boarder class and improve the results. In some applications this is critical: SIFT sometimes give like 10 matchings between images.

One little question is about how to update the affine transformation. The paper uses the second moment matrix (co-variance) around the local patch and many issues are not discussed: the effect of sampling, window size, etc. Nevertheless, this paper still provides me a good example on the utilization of the continuous optimization technique.

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