How does the Lowe's ratio test work?

Question

Suppose I have a set of N images and I have already computed the SIFT descriptors of each image. I know would like to compute the matches between the different features. I h

Answer 1

Lowe's Ratio test is as follows:

---

1. First, we compute the distance between feature fi in image one and all the features fj in image two.
2. We choose feature fc in image two with the minimum distance to feature fi in image of one as our closest match.
3. We then proceed to get feature fs the feature in image two with the second closest distance to the feature fi.
4. Then we find how much nearer our closest match fc is over our second closest match fs through the distance ratio.

The distance ratio = d(fi, fc)/d(fi, fs) can be defined as the distance computed between feature fi in image one and fc the closest match in image two. Over the distance computed between feature fi and fs, the second closest match in image two.

5. Finally we keep the matches with distance ratio < distance ratio threshold.

We usually set the distance ratio threshold (ρ) to around 0.5, which means that we require our best match to be at least twice as close as our second best match to our initial features descriptor. Thus discarding our ambiguous matches and retaining the good ones.