JPEG compression options -- can anybody explain?

Discussion in 'Digital Photography' started by Beowulf, Aug 3, 2004.

  1. Beowulf

    Beowulf Guest

    When I save a file as jpeg, I understand basic compression and quality (I
    might save as 90% quality or 10% compression, for example; of course I
    save as TIFF or other lossless format for editing and prints). But there
    are lots of other JPEG compression options I do not understand -- amount
    of "smoothing" (0-100); checkboxes to activate "optimize" and
    "progressive"; "subsampling" choices of: "2x2,1x1,1x1", "2x2,1x1,1x1
    (4:2:2)", "1x1,1x1,1x1",; "DCT Method (speed/quality tradeoff)" choices
    of: Integer, Fast Integer, Floating Point.

    Can anybody explain these option for fine tuning the saving of a JPEG
    image, or direct me to a website that might explain the best such settings
    for JPEG image saving?
     
    Beowulf, Aug 3, 2004
    #1
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  2. Gordon Richardson, Aug 3, 2004
    #2
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  3. Beowulf

    deimos Guest

    Smoothing -- artificially blurs the whole photo such an amount. It can
    help compressibility as JPEG works very well with smooth gradients of
    color; sharp edges tend to work against it.

    Optimize -- does a few reordering and newer optimizations to affect file
    size without affecting quality.

    Progressive -- creates a multi-resolution image that is "progressively"
    downloaded and decoded as to allow a web browser with a slow connection
    to preview part of the graphic. i.e. Goes from no detail and pixelated
    to full resolution as it's downloaded.

    Subsampling -- what blocksize it operates on. 1x1 will give you the
    sharpest image, but may bloat the file size unnecessarily. Use only for
    computer generated and rendered material. 4:2:2 should always be used
    for photos. 2x2 is useful only when either recompressing a previous
    JPEG or when there is little sharp detail at all (like thumbnails or bad
    material).

    DCT Method -- Accuracy of the DCT algorithem. Use Floating Point
    always. Speed of the jpeg algorithem is generally a non-issue and you
    shouldn't ever have to use an integer format unless you're creating a
    grpahic for a integer processor only device. Effects image quality
    also. Colors become blocky and inaccurate with lower accuracy.
     
    deimos, Aug 4, 2004
    #3
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