Artefacts in red painted flat areas in JPEGs

Discussion in 'Digital Photography' started by Michael Taylor in Canada, Feb 25, 2006.

  1. I posted this to the Microsoft group and they suggested I try here for an
    explanation of why I only get this effect in red painted flat areas.

    ------------------------------------------------------------------

    I'm using Digital Image Pro 10

    When I save a photo around 135K (900 pixels by 600) using Quality of 74 ,
    then I get artefacts in the red areas as in
    www.michaeltaylor.ca/218-219-mt.jpg .These are between the white band and
    the silver grille.

    At 100% image quality the image is good as in the cropped version at 145K:
    www.michaeltaylor.ca/218-219-mt-a.jpg

    Questions:
    1) Is there a work around this problem as I would like to keep images at 900
    by 600 and around the 130K size

    2) Why do I only get this effect in the red areas? On blue locomotives
    everything is OK.


    --
    Regards
    Michael Taylor
    Hamilton, Ontario

    www. MichaelTaylor. ca
    (delete the blanks, 'cos some anti-spam programmes block all e-mails with a
    URL)
     
    Michael Taylor in Canada, Feb 25, 2006
    #1
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  2. Michael Taylor in Canada

    Tesco News Guest

    Hi.

    Sorry, but the only real way to avoid Jpeg Artifacts is to avoid saving
    images in Jpeg Format, or only to save as a Jpeg as the very last step in
    the Workflow.

    Roy G
     
    Tesco News, Feb 25, 2006
    #2
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  3. Michael Taylor in Canada

    tomm42 Guest

    You don't say what you are starting with, but if you are starting with
    a camera jpeg, shrinking the image to 900x 600 then saving as a jpeg,
    you are asking for jpeg atrifacts. Try this for a work flow, change the
    camera file to a TIF file, right from the camera, shrink that, save as
    a TIF, then convert to a jpeg. That's the best advice I can give you
    with out your full work flow.

    Tom
     
    tomm42, Feb 25, 2006
    #3
  4. Michael Taylor in Canada

    frederick Guest

    The trial version of Digital Image Pro I downloaded lacks any option to
    set resampling algorithm. Photoshop or The Gimp (free) will allow this,
    and resampling using (bi)cubic interpolation (not linear or "none") may
    be part of the answer. Don't use sharpening in-camera if possible, or
    sharpen the image at all before resampling. Set camera to best quality /
    large size. If you don't use RAW capture, so can't adjust
    colour/saturation etc in 16 bit, then make colour adjustments after
    resampling.
    Cubic interpolation gives a smoother less jagged resampling, but the
    images may appear a little softer, and may benefit from some sharpening
    / USM after resampling.

    Maybe this effect is more evident with the reds, as they are closer to
    "clipping". When compressing the nearly clipped areas, the software
    sees adjacent pixels as nearly the same, so calls them the same to save
    file size. The effect varies according to % compression. x%
    compression in one software program does not equal x% in other software.
    You need to experiment, and reopen saved files to view the effect of
    compression.
     
    frederick, Feb 26, 2006
    #4
  5. Michael Taylor in Canada

    Martin Brown Guest

    It will be occurring in hard saturated blue areas too, but bright red
    trains and busses are more common. Most blues are much less saturated or
    blue green. Deep royal blue is also darker and the errors there will
    look less offensive.
    You can probably avoid these chroma subsampling artefacts in JPEG
    compression by changing from default 2x2 subsampling to no subsampling
    of the colour image (several programs offer this level of control)

    Irfan views is free. It is also possible that your image causes certain
    buggy codecs to mangle the colour data.
    Which software are you using?
    Sharp edge transitions in luminance next to flat uniform areas of
    saturated red or deep saturated blue are worst case scenarios for
    triggering chroma subsampling related artefacts.

    Your best bet is to try saving one with chroma sampling turned off and
    then vary the compression to get the target filesize.

    Some artefacting is inevitable in lossy compression - the trick is in
    finding a tolerable compromise between size and image quality.

    Regards,
    Martin Brown
     
    Martin Brown, Feb 28, 2006
    #5
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