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Thread: jpg lossless rotation - what is a 'perfect' transformation?

  1. #1

    Default jpg lossless rotation - what is a 'perfect' transformation?

    When doing lossless jpg rotation in thumbnails view, you get the option to do perfect transformations only. What does this mean? And what are the consequences of setting/unsetting it?

    Thanks!

  2. #2
    Plugin Author
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    Simplified answer because there is a lot math behing. Jpeg does not store pixels. The conversion to jpeg works on tiles of a given size. A lossless jpeg operation does not modify the stored image data instead you are just manipulating the transformation. The size of a perfect jpeg is an exact multiple of the tile size. If you rotate a non perfect jpeg you will modify the resulting size.

    But maybe I'm totally wrong ;-)

  3. #3
    Multiple User boarder's paradise's Avatar
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    IrfanView 4.20

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    original file:
    (239x239 pixels)



    SHIFT+J (jpeg lossless operations) -> rotate clockwise - 90 degrees

    result:
    (224x239 pixels)



    see how the chin is now right at the edge of the picture and the red border is much thinner?
    not exactly "lossless", right?


    again:
    SHIFT+J (jpeg lossless operations) -> rotate clockwise - 90 degrees

    (224x224 pixels)


    oooops ... something cropped. again.

    SHIFT+J (jpeg lossless operations) -> rotate clockwise - 180 degrees

    (224x224 pixels)


    actually, this last image SHOULD be identical to the first one, if all operations were lossless, but you obviously see they aren't.

    explanation: http://forums.adobe.com/message/3641727#3641727
    Attached Thumbnails Attached Thumbnails Click image for larger version. 

Name:	hayden239+border [rotation1].jpg 
Views:	434 
Size:	15.5 KB 
ID:	2707   Click image for larger version. 

Name:	hayden239+border [rotation2].jpg 
Views:	415 
Size:	15.1 KB 
ID:	2708   Click image for larger version. 

Name:	hayden239+border.jpg 
Views:	420 
Size:	15.9 KB 
ID:	2706   Click image for larger version. 

Name:	hayden239+border [rotation3].jpg 
Views:	421 
Size:	15.2 KB 
ID:	2709  

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