Hacker News Reader

Comment by godelski | Hacker News Reader

The model uses a GAN which does not learn the exact PDF. So not lossless, but as you can see from the images it gets extremely visually accurate results.

From the README

> The generator is trained to achieve realistic and not exact reconstruction. It may synthesize certain portions of a given image to remove artifacts associated with lossy compression. Therefore, in theory images which are compressed and decoded may be arbitrarily different from the input. This precludes usage for sensitive applications. An important caveat from the authors is reproduced here:

> "Therefore, we emphasize that our method is not suitable for sensitive image contents, such as, e.g., storing medical images, or important documents."

View on HN

> "Therefore, we emphasize that our method is not suitable for sensitive image contents, such as, e.g., storing medical images, or important documents."

As an example of this going wrong previously, xerox had once implemented compression based on deduplicating duplicate parts of documents. Obviously numbers contains tons of duplicate symbols (digits). The problem was that the scanner software deduplicated different numbers with each other, leading to wrong numbers.

http://www.dkriesel.com/en/blog/2013/0802_xerox-workcentres_...

>The model uses a GAN which does not learn the exact PDF. So not lossless, but as you can see from the images it gets extremely visually accurate results.

Yes, I understand this is a lossy compression method - what I was proposing is to have the compressor as a final pass take the predicted output image, and subtract it from the original pixels. This gives you a delta between the predicted image and the original image. You can then compress that delta losslessly, and store it alongside the output of this model - if the predicted image is close enough to the original image then you've significantly reduced the amount of entropy in the delta, making it highly compressible.

This is how some domain-specific lossless compression algorithms work, e.g. DTS-HD Master Audio

Yes, the model is not lossless as this would require learning the PDF in the original input space.

However, the model does learn a conditional probability distribution over a lower-dimensional representation of the original image - this is unavoidable as entropy coding requires a distribution over discrete symbols. The GAN is almost auxiliary and not a central component of the model - in fact, you can get very good results without the GAN, but does seem to result in visually superior reconstructions.

Reply on news.ycombinator.com