The problem with lossy compression in a competition is that in order to compare different approaches to lossy compression you have to define a metric for the qu

The problem with lossy compression in a competition is that in order to compare different approaches to lossy compression you have to define a metric for the quality of different lossy reconstructions of the data. This is practically impossible to do in an unbiased way. As soon as you define a quality metric then the competition becomes cheating the metric instead of useful compression. So how do you define a metric that can't be cheated? You add an arithmetic coder after your lossy compressor, which turns it into a lossless compressor, and the size of the losslessly compressed data is your metric for the quality of your lossy compressor. It's the only metric that definitively can't be cheated.