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  • rejection compression artefacts

    Hi all,

    I got two pics rejected for jpg compression artefacts.
    c.f.: http://www.jetphotos.net/viewreject_b.php?id=1952584

    But I don't know what I did wrong:

    - is the size wrong?
    - or was there loss of quality due to resizing?

    I always save the pics with best quality (= 12 in Photoshop). Is that wrong?
    And after that I resize to 1024 x 683 (I thought, but am not sure, it's not 1024x768, that I know).

    What should I alter in the process?

    Greets from the Netherlands,

    Hinkelbein

  • #2
    You should always resize THEN save the image at best quality , this avoids loss of quality...

    Comment


    • #3
      Ah, I guess I go wrong there

      Thanks!

      Comment


      • #4
        Could someone show me where the artefact is, maybe by putting a circle around or an arrow at it?

        Not that I mistrust the judgement of the screeners involved, but just so I know what to pay attention on next time(s).

        Thanks.

        http://www.jetphotos.net/viewreject_b.php?id=1954787

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        • #5
          You can see the artefacts around any line you choose in the picture. It is also visible in the sky as a "blocky" pixellation.

          JPG compression causes lines and edges of titles ( in the case of your picture, the registration, KLM titles, window and door edges ) to have what looks like "stirred up" pixels immediately adjacent to these edges. As a result of this the line edges become very blurry. If you equalise the image you will see this effect become even worse with the titles becoming almost unreadable. Note also how the sky pixels in the equalised image also becomes very "blocky".

          Possibly the best way to describe the effect is to take an image with, lets say 8 million pixels in total in it, remove 2 million pixels, and then expect the remainder to occupy the same space with no loss of detail or sharpness. It simply isn't going to happen.
          If it 'ain't broken........ Don't try to mend it !

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          • #6
            Ah, thanks again, Brian. Now I know what the term compression artefact actually means.

            I still have the original, taken with EOS400D. It must be possible to make a decent acceptable photo from it for the database?

            I will read again in one of my previous started topics on how to deal with sharpening and cropping/resizing.

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            • #7
              Originally posted by Hinkelbein
              Could someone show me where the artefact is, maybe by putting a circle around or an arrow at it?

              Not that I mistrust the judgement of the screeners involved, but just so I know what to pay attention on next time(s).

              Thanks.

              http://www.jetphotos.net/viewreject_b.php?id=1954787
              To add to Brians post, an obvious area is around the Rego at the back and the KLM logo at the front. Theres a lot of noise around those logos (sharp edges are hard to compress. Theres a complex explanation, but it would take a fair bit of typing)

              And the posterisation in the sky is a dead giveaway.....

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              • #8
                I guess when I first crop/resize and then sharpen I can get a good picture from it? I still have the original off course.

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                • #9
                  In adddition to Brian and Stephen, jpeg artifacts can also been found round the windows. Another way to describe artifactsis that is has appearance of a cloud of mosquitoes.Here is a website I always find very ilustrative:http://www.landiss.com/compression.htm

                  Best to save in a lossless file type like tiff, only save a jpeg in the final stage, before uploading.

                  cheers, Pamela

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