I took about 1/5th of the png images generated from the Velodyne point cloud data and manually aligned them in Gimp. It's easy to shift-select a bunch of images and open them as individual layers in Gimp, but there are no multiple layer selection capabilities: it's not possible for instance select all the layers and change their transparency.
In all of the images there are a few features, mostly beyond the edges of the road, that can be aligned with the earlier and later images. The closer together the images are in time the easier this is, but I was skipping every 4 images to take the 5th in order to speed up the process- also a gimp image with 300 layers is difficult to handle.
The later portions of the data are all purely translational, only at the very beginning are rotations and translations needed.
I think an automatic process for alignment won't be that hard, but the inherent inconsistency in frame to frame image will make for a lot of error. Translations correspond to phase shifts in the frequency domain, and I think rotations are almost as simple- and there isn't any scaling to account for.
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