Tuesday, February 26, 2008

[Reading] Distinctive Image Features from Scale-Invariant Keypoints

I read and implemented this paper long time ago (2006) for a project in the course 'digital visual effect'. I use SIFT to register multiple images into a panorama:

(click the image to the project page, including more results and my SIFT code)

The paper presents a keypoint localization and description method, SIFT. Though these two parts can be considered independently, the author did not discuss this issue much. The keypoint keypoint localization is done by detecting the local extrema in the scale-scale representation of the image, and the description is a processed local gradient histogram around the keypoint.

The concept of these two steps are not hard to understand, but the implementation details are insanely complex. There are many parameters in the system, and the author found the possibly optimal values by experiments (these experiments are extended recently and the related papers are listed here). A large portion of the paper is filled with the figures in tuning these parameters. Even though, many implementation details are skipped and it is very difficult to write a SIFT program identical to the author's version.

Nevertheless, SIFT is a huge success and hence enables many new applications (also here). It is designed only for scale- and illumination-invariant, but, however, it is affine-invariant to some degree and is much faster than those truly affine-invariant keypoint detection methods. Although there are numerous new keypoint localization and description methods every years, I still prefer SIFT due to its efficiency, conciseness (the theory part). Most important of all, many small drawbacks in SIFT bring many interesting research topics.

1 comment:

Neha DD said...

I have implemented SIFT to some extent. But the main problem faced by me is stability of key points. If the key points in two images to be matched are different(due to illumination changes) then how can i get a match?