Announcement

Collapse
No announcement yet.

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

Collapse
X
 
  • Filter
  • Time
  • Show
Clear All
new posts

    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
    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 ;-)

    Comment


      #3
      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 Files

      Comment

      Working...
      X