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Thread: Image Rotation - Quality loss?

  1. #1

    Default Image Rotation - Quality loss?

    I have searched the forums but could not find the above.

    For convinence, I usually use the "rotate" function on Win XP to bring my JPG images to vertical. My friend told me that this inevitably reduces the size of the JPG images on disk after rotation. It is correct to assume now the rotated JPG images is of lessor quality? after losing more details, useless or otherwise?

    By this asumption, doing rotation on Adobe or other high-end software retains all the original JPG quality (if any). I am trying very hard to stay away from RAW for all non-paid images.

    Anyone can point me to where I can get more information? Or just pen us some of the infor you already know.

    Opinions also can.

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  3. #3
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    Default Re: Image Rotation - Quality loss?

    well according to those links. It seems that microsoft windows image viewer IS a loseless rotater. so I guess its ok?

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    Member bigbun's Avatar
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    Default Re: Image Rotation - Quality loss?

    Quote Originally Posted by SpitFir3
    well according to those links. It seems that microsoft windows image viewer IS a loseless rotater. so I guess its ok?
    Hmmm where did u read that?
    i honestly don't think think WindowExplorer/MyComputer has lossless rotation features..

    everytime i rotate, it;s quite obvious the jpeg pic is further compressed.
    so now i mostly use PS to rotate... but only in 90 degrees..

    anyway here's a list of application with lossless rotation features compiled by JPEGclub.
    http://sylvana.net/jpegcrop/losslessapps.html

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    Default Re: Image Rotation - Quality loss?

    PS doesn't do lossless rotation.

    the loss in details come because they uncompress the file, open it to memory space, does the necessary transformation, recompress it, and save it. any rotation doesn't affect anything.
    the compression causes you to lose more details.

    As long as you have to open it, then it's not lossless.

    The lossless rotation applications does rotation of the file data itself. though not exactly, but the simple idea is that they change the data in the file so that it's not uncompressed and compressed again.

  7. #7

    Default Re: Image Rotation - Quality loss?

    Quote Originally Posted by Caspere
    I have searched the forums but could not find the above.

    For convinence, I usually use the "rotate" function on Win XP to bring my JPG images to vertical. My friend told me that this inevitably reduces the size of the JPG images on disk after rotation. It is correct to assume now the rotated JPG images is of lessor quality? after losing more details, useless or otherwise?

    By this asumption, doing rotation on Adobe or other high-end software retains all the original JPG quality (if any). I am trying very hard to stay away from RAW for all non-paid images.

    Anyone can point me to where I can get more information? Or just pen us some of the infor you already know.

    Opinions also can.
    If windows did not prompt you that the quality might be degraded, it should be a lossless rotation. There are some physical resolution which cannot be rotated properly and windows will prompt you that the image cannot be rotated losslessly.

    Then in such a case, you can use Rota.

    Actually what you can do is a test,
    1) make a copy of the image
    2) rotate the copy of the image 4 times in the same direction so that it's back to the original orientation
    3) open the original file in GIMP or PS
    4) open the rotated copy in another layer and do a layer subtraction or difference.

    If the rotation is lossy, it will show up as a difference, otherwise, the resultant is just one black frame. You might need to flatten the subtracted image and do a auto levels.

    Here is the link for Rota information
    http://park2.wakwak.com/~tsuruzoh/Computer/Soft/Rota/readme-e.html
    Here's the link for download.
    http://www.webattack.com/get/rota.html
    Last edited by lsisaxon; 17th April 2006 at 03:09 PM.

  8. #8

    Default Re: Image Rotation - Quality loss?

    Quote Originally Posted by bigbun
    Hmmm where did u read that?
    i honestly don't think think WindowExplorer/MyComputer has lossless rotation features..

    everytime i rotate, it;s quite obvious the jpeg pic is further compressed.
    so now i mostly use PS to rotate... but only in 90 degrees..

    anyway here's a list of application with lossless rotation features compiled by JPEGclub.
    http://sylvana.net/jpegcrop/losslessapps.html
    Yes, Windows picture viewer is capable of lossless rotation of JPG, provided the dimensions is divisible by 16, eg 3008 x 2000 are each divisible by 16.

    As long as one of the dimension (in pixels) is not divisible by 16, once you click rotate, Windows will prompt you that it is not a lossless operation. The image dimension (2272x1704) of the Minolta G400 is one such case. 2272 is divisible by 16 but 1704 is not.

    You can try creating JPGs of various dimensions to try and you'll be amazed. Rota is able to do a partial decompression and then do a rotation for any JPG dimensions losslessly.

  9. #9
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    Default Re: Image Rotation - Quality loss?

    IIRC, windows viewer does NOT do lossless rotation.

    As long as the image is loaded up into memory space, decompression has been done. It has been extracted into a buffer space which no longer contains any details of jpeg compression.

    The mere action of opening a jpg and saving it is lossy, you don't even have to do any rotation. Doing 1 rotation, or 2 rotation or even 1000 with an image in the memory space will always be lossless.

    As mentioned, the loss is when you save the file, not when you rotate.

    when you use windows viewer, you view it 10 times, it'll load 10 times. you rotate it once, it'll open the file, rotate, then SAVE the file (at goodness knows what setting). Data will be lost no matter what.

    However, the loss of details is not appreciable right away. try open save close the file about 10 times, at say level 6 in photoshop.. you will see the loss of details very soon.
    Last edited by unseen; 17th April 2006 at 03:19 PM.

  10. #10

    Default Re: Image Rotation - Quality loss?

    Quote Originally Posted by unseen
    IIRC, windows viewer does NOT do lossless rotation.

    As long as the image is loaded up into memory space, decompression has been done. It has been extracted into a buffer space which no longer contains any details of jpeg compression.

    The mere action of opening a jpg and saving it is lossy, you don't even have to do any rotation. Doing 1 rotation, or 2 rotation or even 1000 with an image in the memory space will always be lossless.

    As mentioned, the loss is when you save the file, not when you rotate.

    when you use windows viewer, you view it 10 times, it'll load 10 times. you rotate it once, it'll open the file, rotate, then SAVE the file (at goodness knows what setting). Data will be lost no matter what.

    However, the loss of details is not appreciable right away. try open save close the file about 10 times, at say level 6 in photoshop.. you will see the loss of details very soon.
    Windows picture viewer does not work that way. It opens and decompress the file just for display. When you rotate, if the pixel resolution on each axis is divisible by 16, it will just transform the coordinates in the original file without the need to decompress and recompress the jpeg. It doesn't need to re-encode the file like you do if you open with photoshop.

    However, if the pixel dimension on either of the axis is not divisible by 16, it will need to decompress the image, transform then recompress therefore will be lossy. Before it does this, it will prompt you first to see if you allow that operation.

    Rota 0.7 is able to do a lossless rotation regardless of the pixel dimension because it decodes the JPG halfway but stopped short of decompressing it and then use a lossless transform on the compressed data, and re-encodes the jpg file. This way, it does not go through re-encoding and therefore is lossless.

    Don't have to wonder so much about it. All you need to do is make a copy of a file, rotate it several times and put in PS or GIMP as a different layer and do a subtraction from the original image in another layer. If details have been lost in the rotation, it will show up as a difference, usually most prominent at the edges of the features.

    Here is an article..
    http://www.impulseadventure.com/phot...-rotation.html
    Last edited by lsisaxon; 20th April 2006 at 05:45 PM.

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