Monday, May 22, 2006

Matching Keypoints

Today, I matched soda-can keypoints to each other. Conveniently, C-code for doing this is provided with David Lowe's Keypoint generation code. Using the above set of images, I found keypoints for each image and matched them with the keypoints from all of the other images. The images with a prefix of "s" are the images without an "s" that have been scaled down to 3% of their original size and then scaled up by 600%. The resulting images with prefix "s" are of a size of around 150x240 pixels. The other images have a size of about 1100x2100 pixels. This was done as a test to see how much of an effect the resolution of the images has on extracting keypoints, which angles have the most matchable keypoints, and if similar resolution soda-can images match better than dissimilar resolution ones. I will post the results from this small experiment along with a table and a graph on my next blog entry.

This is how keypoint matching works between two images, referred to as A and B:

1. keypoints are found in A.
2. keypoints are found in B.
3. The closest matching keypoints are found between the images. Each keypoint is a 128 dimension vector. To find the distance between two keypoints, the Euclidean distance is found between the feature vector s belonging to the keypoints. Each keypoint in A is compared with each keypoint in B by Euclidean distance to find the closest matching keypoint. A match is determined as follows:

Let a' be a point in A. Let b' and b'' be the first and second closest matching points in B to the point a'. Let D(x,y) represent the Euclidean distance between its arguments x and y.

If D(a', b')< D(a', b'')*0.6 then the closest point is chosen.
Else, no points are chosen (no matching keypoint).

According to Lowe, finding merely the closest matching keypoint does not yield a reliable match. Lowe claims that a more reliable match is determined by comparing the second-closest matching keypoint. He does this by using the second-closest matching keypoint as a threshold.


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