Returning to the SOM

Posted: February 16, 2009 at 10:06 pm

After capturing some test images from the motivated camera I’ve been working on the SOM structure. The quality of the SOM is very interesting (when using typical linearly decreasing learning and neighbourhood functions) when the camera provides images that are already in clusters. This cluster structure is complex enough that the resulting SOM is quite complex. Following is a representation of the memory field, its Umatrix and the Umatrix of the codebooks (neuron weights):

montageA.jpg

Umatrix of images stored in the memory field:

histumatrixA.png

Umatrix of codebooks:

codebooksumatrix.png

In comparison here are the memory field resulting from the same learning settings, except images are fed in their original order, and due to slow learning rate are run for 30,000 iterations:

montageB.jpg

This clearly shows that the order in which images are presented is highly significant in the foldedness of the resulting SOM. This is problematic considering the basis of the camera motivation is making sublte variations on the camera’s position, based on the visual scene. I could explore using very large multipliers in the motivation, but that would loose some of the quality of the camera following features of the scene. It would appear to just be randomly jumping between points. Another approach could be to use a two stage SOM. An initial SOM would simply store images (the number yet to be determined) as a first effort at organization. This would be highly folded, as seen above. The question is whether a second SOM, trained on only those images stored by the first SOM, would produce a more organized result. The first SOM would have to be trained on a cyclic function (to integrate new data), possibly a sawtooth function. The second would read the images in a random order and retrain between each dream. I wonder how quickly the second SOM could be trained. The size of both SOMs is also interesting. If the first SOM was larger (in terms of the number of units) and the second smaller, this could be an analog to longer and shorter term memory.