Tuesday, 31 December 2013


Got the OpenCL version of the genetic algorithm based classifier working this morning. Actually I think it already worked, I just had a problem with the way I was testing the output. Because I don't have a better sort just yet I forced the image set to be a power of two. I left the breeding code in Java for now so I don't have to deal with random number generation.

With a population of 512 individuals, 4096 total training images (each 32x16), and using 4x4x4 classifiers it executes a little over 100 generates per second. I was hoping for a bit better but I guess a 30x speedup over single-thread cpu is good enough for now - certainly getting to a solution in 10 minutes is better than 5 hours (just a bit).

The largest single processing element is classifying all the images. However; I am re-executing all classifiers, even those which haven't changed. So I can pretty much double the performance with my current generational settings which keeps half each iteration. The only stage which needs to run over all the whole population is the fitness ranking.

Interestingly on one of the first test runs I managed to create a perfect classifier for the training set after about 40K generations (~8 minutes). Apart from data setup i'm not sure how to tweak this to be more general - as there isn't really 'training' involved, just searching for a good solution to fit the data - i can't really cross-verify with a verify set without it just becoming the training set. A multi-stage classifier could use a verification set for adjusting the thresholds though. The quality of the training set is the most critical factor.

I also tried the three different fitness tests again (approximate area under roc, number of true-positives-before-any-false-positive and number of true-negatives before any true-positivies) and it seems the area under the ROC is by far the best measure. I guess it does work after all, and I think I was just looking at bugs before.

I also upped the mutation rate to 5 bits per off-spring. It gives a bit more 'jiggle' as the improvement rate levels off. It still requires multiple runs because each solution is different, but it seems to help it stave off stagnation for a bit longer. I thought I had a problem with breeding making too many clones of the best solution after a while; but after putting in some checking code I was only ending up with 0-10 clones in any given generation which isn't enough to care about.

I guess I will keep working toward some sort of object detector to see if everything holds up in practice, but i'd like to experiment a bit too - finding time is always the problem.

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