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Thread: jpeg quality?

  1. jpeg quality?

    What difference does it make to adjust the jpeg quality (edit/art rage preferences/export/jpeg quality)? It makes the file size a lot bigger, but doesn't change the pixels; I'm wondering what filesize it makes sense to default to for uploading for prints. My impulse is to go with highest quality, but I'd like to have a better understanding of what that is actually doing. Thanks!

  2. #2
    Join Date
    Nov 2007
    Location
    UK
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    238
    Just check the link here. The more you compress it, the less details you get, which means you will get more mosaics.
    It is not obvious for big pictures (since you always zoom out so the little '1 pixel' details are not that important). However if yo have a, say, 800*600pixel picture, a non-compressed image is way much better than a fully compressed image.
    In Photoshop I like jpeg at level 10, it compress the file size a LOT and the image would still look as detailed.

  3. #3
    Join Date
    Jun 2007
    Location
    Delaware, USA
    Posts
    85

    JPEG Compression

    JPEG (Joint Photographic Experts Group) compression is a lossy compression scheme which discards image information based on the quality of compression you choose. It averages blocks of pixels, which is why the more you compress, the more artifacts show up in your image. The attached image shows the effects. On the left you have an enlargement of a compression at 7% original quality, on the right you have a compression at 85% quality. Always keep your original .ptg file to keep your options opened.
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  4. #4
    Join Date
    Jul 2010
    Location
    New Zealand
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    50
    If you plan on-going edits of your document, avoid .jpg, as your image will quickly degrade.

  5. Thanks for the replies, especially Mikecur, the side-by-side is helpful in showing that yes, the quality is actually better if you set the image quality higher. The Wikipedia article defining jpeg really did not help (things like "The resulting compression ratio can be varied according to need by being more or less aggressive in the divisors used in the quantization phase" ! ).

    I thought that pixels pretty much defined quality, so I am baffled that you can have the same pixels but one is more detailed than the other. I experimented by exporting the same image at 100% quality and at 50% quality and at 400% size can definitely see the difference. I've been using DeviantArt for prints, and they have a pixel to print size guide, so I was trying to understand how jpeg quality fits into that calculation. But I don't need to understand! I'll just stick with highest quality.

    kaveman, no, this is not to work with, but to upload finished images for printing, so I want to export to an image file and will stick with highest quality. Is a different image file format recommended for higher quality (as opposed to jpeg)?

    Thanks again!

  6. #6
    Join Date
    Oct 2007
    Location
    NC, USA
    Posts
    2,868
    If you're exporting for the purposes of printing, you should think about using .Tiff or .PNG, because they are lossless file types (meaning, they will retain the quality they were created at).
    Nothing is easy to the unwilling.

  7. Quote Originally Posted by Someonesane View Post
    If you're exporting for the purposes of printing, you should think about using .Tiff or .PNG, because they are lossless file types (meaning, they will retain the quality they were created at).
    Thanks, I'll do that!

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