Converting RAW images to lossless jpeg using GIMP?

Question

I have a Raw image which I want to get printed. I need to upload this to a website which expects to be in jpeg format.

I use GIMP for photo editing and used UFRaw to view RAW images.

GIMP cannot read Raw files directly, so I used UFRaw to view RAW image and save option saved it in ppm format, then I opened it in GIMP, did some editing and wanted to save the image. Saving in jpeg format compressed size from 10Mb to 3.6Mb

I did not want any compression of the image, I had option of saving in png, but I want lossless jpeg. How one can save in lossless jpeg format.

I use Ubuntu, GIMP, have UFRaw.

Answer 1

Almost nobody uses lossless JPEG -- it's used within the DNG file format, and for some medical imaging applications, but that's about it. The probability that a file created with a JPG or JPEG extension in lossless JPEG format will be readable at the other end approaches 0 with a high degree of confidence. As for compression, well, almost all image file formats other than uncompressed TIFF will exhibit file size compression.

Unless all of your details are 1 pixel in size, a superfine JPEG compression (the highest quality the editor will permit -- that would be "quality level 12" in Photoshop, quality level 100 in the GIMP would be similar) will have so little loss that you wouldn't notice it without doing a pixel-by-pixel mathematical comparison (image subtraction). The alternative, if your service permits it, would be TIFF, but you'd want to compress that with the LZW option (lossless compression) to make the file size manageable. (It will still be much larger than a JPEG.)

Answer 2

The only reason to want a truly lossless compression is if you're later planning to edit the file again or if the file is composed of flat colors that are easier to recompress than if there is compression artefacts (which is the domain of PNG).

For final products, e.g. prints, you don't need a truly lossless compression, a lossy compression on high quality will be able to preserve enough color that any artefacts wouldn't be noticable by human eyes. In any case, with a high quality JPEG, irregularities in the printing equipments would probably introduce more artefacts than the compression.

Answer 3

JPEG compression that goes from 10MB RAW to 3.6 MB is practically lossless. Altough you do not mention what kind of printer or technology you plan to use, I think it is sufficient for pretty much anything. I'd like to see someone who can instantly make difference between prints made from such a JPEG and a print made from equally dimensioned TIF.

Answer 4

You can't, at least not easily. There is a lossless extension to the JPEG format, but GIMP doesn't support it. There may be a few things you can do to get around that, but the more important point is that the service you're intending to submit to almost certainly won't.

The practical advice is to save the image at the highest possible quality settings, and don't stress about it too much. I'll be amazed if you can detect any artifacts in the final output at this level, even if you go over it carefully with a loupe.