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Compression level: Since compression is not lossy, the only reason to use a compression level less than 9 would be a too long time to compress file on a slow computer. Nothing to fear from decompression: it is as quick whatever the compression level.

https://docs.gimp.org/2.2/en/gimp-images-out.html

GIMP suggests using compression for lossless .png images, "compressed" images are saved using the same file type. If I set compression level to 0 when saving the image is several times larger than the original image. How does "lossless compression" work and do I lose quality by using it?

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Lossless compression is part of the PNG standard and is similar to using the compression portion of ZIP or a utility like GZip on an image. PNG compression does not cause loss of quality. However, the amount of compression is highly dependent on the type of image (photograph vs. cartoon) and the content. The resulting file will also normally be much larger than a file generated by a lossy algorithm like JPEG where the reduction of space comes at the cost of image quality.
And none of this is specific to GIMP - any image manipulation program that can save as PNG or JPEG, e.g. Adobe Photoshop, will do the same thing with similar results.

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  • So the only drawback is longer time to process the saved image? With archived content I generally need to extract the files before using.
    – user598527
    Jul 17 '16 at 15:47
  • 1
    Correct, since compression is part of the PNG standard, the application displaying it will transparently decompress the image without an additional step. The archive portion of ZIP is what I was trying to exclude in the answer.
    – kronenpj
    Jul 17 '16 at 15:52

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