5 ms·
It wouldn't be a problem for jpeg (though I believe it is for mp3?). In jpeg the quantisation step controls the degradation in quality - you can run it through
by aidos 14y ago
It wouldn't be a problem for jpeg (though I believe it is for mp3?). In jpeg the quantisation step controls the degradation in quality - you can run it through a 2nd time with the same quantisation without losing further quality.
- Retric 14y agoIn theory that might be true in practice there are issues with using jpeg on a jpeg if you repeat it enough. http://www.codinghorror.com/blog/2006/07/a-comparison-of-jpeg-compression-levels-and-recompression.html http://www.codinghorror.com/blog/2006/07/a-comparison-of-jpe...
- aidos 14y agoIndeed, since I posted that comment I've been looking into it further. I read on wikipedia [0] that: The DCT based form cannot guarantee that encoder input would exactly match decoder output since the Inverse DCT is not rigorously defined I don't get that - I thought the inverse of the DCT was just the linear combination of the components. So...I've been playing around with it myself to see what happens (using graphicsmagick). I converted an image to jpg (quality 70) and then bmp -> jpg again. There are definitely some minor artefacts. I then iterated that step jpg -> bmp -> jpg (quality 70) a dozen times. These further iterations don't add any additional artefacts. So it sort of gets into a stable state. EDIT: reading that quote from wikipedia again it looks as though it's just saying that jpeg is lossless (ie, we apply quantisation). [0] http://en.wikipedia.org/wiki/Lossless_JPEG http://en.wikipedia.org/wiki/Lossless_JPEG
- fnordfnordfnord 14y agoThese guys used to let you run your own images through their filter. It was great for spotting even really good shoops. http://www.errorlevelanalysis.com/ http://www.errorlevelanalysis.com/
- ygra 14y agohttp://fotoforensics.com/ http://fotoforensics.com/ still does.