Mirror Image

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Features matching and geometric consistency.

Here I want to talk about matching in image registration. We are doing registration in 3D or 2D, and using feature points for that. Next stage after extraction of feature points from the image is finding corresponding points in two(or more) images. Usually it’s done with descriptors, like SIFT, SURF, DAISY etc. Sometimes randomized trees are used for it. Whatever methods is used it usually has around .5% of false positives. False positives create outliers in registration algorithm. That is not a big problem in planar trackers or model/marker trackers. It could be a problem for Structure From Motion though. If CPU power is not limited the problem is not very serious. Heavy-duty algorithms like full-sequence bundle adjustment and RANSAC cope with outliers pretty well. However even for high-end mobile phones such algorithms are problematic. Some tricks can help – Georg Klein put full-sequence bundle adjustment into separate thread on PTAM tracker to run asynchronously, but I’m trying to do local, 2-4 frames bundle adjustment here. The problem of false positives is especially difficult for images of patterned environment, where some image parts are similar or repeated.
Here mismatched correspondence marked with blue line (points 15-28).

As you can see it’s not easy for any descriptor to tell the difference between points 13(correct) and 15(wrong) on the left image – their neighborhood is practically the same:


Such situations could easily happen not only indoor, but also in cityscape, industrial, and others regular environments.
One solution for such cases is to increase descriptor radius, to process a bigger patch around the point, but that would create problems of its own, for example too much false negatives.
Other approach is to use geometric consistency of the image points positions.
There are at least two ways to do it.
One is to consider displacements of corresponding points between frames. Here is example from paper by Kanazawa et al “Robast Image Matching Preserving Global Geometric Consistency”

This method first gathering local displacement statistic around each points, filter out outliers and and apply smoothing filter. Here are original matches, matches after applying consistency check and matches after applying smoothing filter.

However this method works best for dense, regular sets of feature points. For small, sparse set of points it does not improving situation much.
Here is a second approach. Build graph out of feature points for each frame.

Local topological structure of the two graphs is different because of false positives. It’s easy to find graph vertices/edges which cause inconsistency – edges marked blue.They can be found for example by signs of crossproducts between edges. After offending vertices found they are removed:

There are different ways to build graph out of feature points. Simplest is nearest neighbors, but may be Delaney triangulation or DSP can do better.

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11, December, 2009 - Posted by | Coding AR, computer vision | , , , , , , , ,

2 Comments

  1. You said : ” It’s easy to find graph vertices/edges which cause inconsistency ”

    Can you tell us more about that i can’t think of a good way of doing this.

    Comment by maxime caron | 4, January, 2010

  2. I just use sign of crossproducts of neighboring edges. If signs of crossproducts are different for two frames I drop offending edges.

    Comment by mirror2image | 4, January, 2010


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