Pixel Aspect ratio, PhotoShop CS and TIFF CCITT Fax Group 4

Discussion in 'Photoshop Tutorials' started by François Robert, Mar 3, 2004.

  1. Dear NewsGroup,

    I am wondering about the correctness of PhotoShop CS behavior
    in the following scenarios (under Windows):

    With Kodak Imaging, (a TIFF viewer that comes with Windows), I
    save a line art (bitonal) raster as a TIFF CCITT Fax Group 3
    or bitonal uncompressed. PS CS opens the resulting TIFF file
    without complaining. If I further state in Kodak Imaging my
    TIFF has different horizontal and vertical resolutions, and
    open it in PSCS, PSCS does not complain either. Furthermore
    the "Pixel Aspect Ratio" reported by PS CS is "square"... I
    have used tiffinfo and checked that the XRes and YRes are
    indeed different.

    Now, if I save (still with Kodak Imaging) the same raster as a
    TIFF Fax Group 4 (with SAME horizontal and vertical
    resolutions), and open it with PSCS, it complains that "Pixel
    aspect ratio correction is for preview purposes only. Turn it
    off for maximum image quality". And indeed, PSCS reports a
    Pixel Aspect Ratio of 0.5...

    Here are my questions :
    I fully expected that a square pixel would be synonymous with
    having equal X and Y resolutions. Apparently that is not the
    case. So what is pixel aspect ratio then ? And how is it
    connected with resolutions ?
    I also fully expected that choosing a different compression
    scheme would NOT impact the pixel aspect ratio. Again that
    does not seem to be the case. What gives ?

    Any comment (and/or pointers) appreciated.
    _______________________________________________________
    François Robert
    (to mail me, reverse character order in reply address)
     
    François Robert, Mar 3, 2004
    #1
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  2. François Robert

    Chris Cox Guest

    The TIFF standards say that Group 4 encoding should use an 0.5 aspect
    ratio.

    Yes, a square pixel has an aspect ratio of 1.0 (1:1).
    Aspect ratio is not connected to resolution.

    Chris
     
    Chris Cox, Mar 7, 2004
    #2
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  3. Can you point me to the exact clause in the TIFF specifications
    where this is stated ? (Or in the ITU T6 or T4 recommendation,
    since I also have access to those)
    ....
    This I really don't understand, honestly.
    If a pixel is not square, then surely I can pack more of them in
    one direction than in the other, thereby resulting in different
    spatial resolutions. Or is there a definition of resolution that
    is not "pixel count divided by linear dimension" ?
    _______________________________________________________
    François Robert
    (to mail me, reverse character order in reply address)
     
    François Robert, Mar 7, 2004
    #3
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