In one of my first entries, I spoke of the need to avoid accumulation of errors caused by odometry. In that entry I discussed very briefly the concept of loop detection and loop closure for a typical GraphSLAM application. During this time, I've been implementing this functionality.
For the loop detection step (front-end) I've made a simple implementation based on the number of inliers resulting from the keyframe matching. This implementation has an advantage and a drawback. The main advantage is that it is easy to implement. However, the disadvantage that arises is that only gives acceptable results for small graphs and environments with abundant and distinguishable textures. The reason for having made this implementation is that the problem of detecting loops in an efficient and robust way is a research field in itself and I preferred to opt for a simple solution that would address the problem of GraphSLAM completely.
For the optimization part of the graph of poses (back-end), I've integrated two different implementations. The first uses the graph-slam module of the MRPT library, while the second uses the G2O library and represents the state of the art in graph optimization.
Here's a video showing the resulting maps from the optimization of the graph of poses. As can be seen, the resulting maps after optimization are more consistent, especially in the area that has been revisited after a while.
During this time I've also been doing other equally important tasks. In recent weeks I've been writing part of my Final Year Project Report and I've read several articles and technical reports related to GraphSLAM. I've also added the ability to reject visual outliers using the fundamental matrix. In my tests, however, the outlier rejection seems to perform better using the homography matrix instead of the fundamental matrix.
Also, this week my Preliminary Project Report has been accepted, so I hope to present my Final Year Project before February. After many months of hard work, I now see the end of an era and the beginning of a new one.