Moving an image around until it exactly matches another one underneath

Discussion in 'Photoshop' started by Ramon F Herrera, Oct 12, 2007.

  1. Let's say I have scanned 2 documents which are very similar and I need
    to quickly find out the parts (words) in which they differ. A very
    useful technique is to alternatively display both images and the eye
    will quickly spot the differences. The catch is that the similar parts
    of the images have to be aligned as precisely as possible. I could
    attempt to do this by trial an error, moving one of the images around,
    measuring pixels, etc., but I bet there is a better way.

    Is there?


    Ramon F Herrera, Oct 12, 2007
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  2. Ramon F Herrera

    Rob Guest

    Rob, Oct 12, 2007
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  3. Ramon F Herrera

    John Guest

    Put the two images on separate layers. Reduce the transparency of the
    top layer so you can see partly through it. then just nudge the layers
    around to get the best match. Reset the transparency of the top layer
    to 100% and switch the top layer on and off

    John, Oct 12, 2007
  4. Ramon F Herrera

    Ed Hannigan Guest

    Actually, if both scans are exactly the same size, you may not have to
    move the layers around at all.

    If you set the top layer to Difference, the pixels that are identical
    will turn black. So, when the layers are perfectly aligned the whole
    image will be black except for any parts that are different, which makes
    the differences easy to pick out.
    Ed Hannigan, Oct 12, 2007
  5. It is unlikely that this will work with two different scans. Even if the
    images are the same size, there will be scanning differences which are
    irrelevant for what the OP is looking for, but will interfere in this
    Johan W. Elzenga, Oct 12, 2007
  6. Ramon F Herrera

    Joel Guest

    I don't see why 2 scans can't line up to each other, even with different
    DPI. Cuz the only different would be quality, size (you have option to fit
    the canvas) which wouldn't cause much or any problem.

    Cuz one of several tricks to remove the old film gain from old photo is
    scanning the photo (1) top-to-bottom (2) bottom-to-top to get different
    result from the light-movement of the scanner, then rotate 1 photo then
    blend 2 scans together etc..
    Joel, Oct 12, 2007
  7. Yes, there's a very simple way to do this.

    Instead of pasting to layers, rotating images by half a degree, resizing by
    a pixel or two and shunting left and right until everything lines up - a
    task which could take hours to do correctly, why not let your computer do
    all the hard work instead. It can do the exact same job for you in a matter
    of seconds.

    Open both images, and then click on File>Automate>Photomerge
    When it asks you which files you want to use, select the option "Use Open"
    and tick the box labelled "Attempt to automatically align images". Now click
    okay to proceed.
    Photoshop will now present you with a composite image with both the source
    images correctly aligned over each other.
    On the right hand side of this window, place a tick in the box labelled
    "Keep as layers", and then click Okay.
    You should now have a new image open in your workspace, with two separate
    layers. Each layer contains one of the original source images, and they
    should be perfectly aligned.

    Now just toggle the top layer on and off to highlight any changes between
    the two.
    \(not quite so\) Fat Sam, Oct 12, 2007
  8. Cuz the originals are two different printed documents, and then scanned
    individually. It's unlikely that these will give you pixel perfect
    copies of eachother, because both the printer and the scanner has
    certain tolerances. Most likely the scans will be ever so slightly
    different, making it impossible to align all the text pixel perfect. You
    can align some of the text, but then another part will be slightly
    misaligned. For an old photo that isn't so important, because it will
    only slightly blur the blended result. But to see the difference between
    the scans by using the 'Difference' layer mode, you need pixel perfect
    Johan W. Elzenga, Oct 12, 2007
  9. I just realised a far more important problem, making the 'difference'
    layer mode method clearly useless: Let's say that there is only ONE
    different word, and this word is in the beginning of the text. If the
    new word is shorter or longer than the word it replaces, it will cause
    the entire text to reflow. In the rest of the text, each word will be in
    a different position, even though the word itself is still the same. The
    'difference' layer mode will show that the entire text is different,
    only because the words are in a different place now.
    Johan W. Elzenga, Oct 12, 2007
  10. Ramon F Herrera

    Joel Guest

    Well, it seems like the OP just drop his last will here then hang hiself
    or whatever but we haven't seen a sight of him and his thought yet <bg>. I
    don't know what you see at your end but if the OP says they are similar, and
    hard to be able to spot the difference then it must be almost if identical.

    And I hate the conversation over the death of the OP <bg>
    Joel, Oct 12, 2007
  11. Ramon F Herrera

    Avery Guest

    Very good!
    Avery, Oct 13, 2007
  12. IIRC the original writer wanted to compare two text documents to see if they
    were the same. I assume he was talking about paper documents, in which case it
    would be far better to OCR the two documents and then use a file compare utility
    to find the differences. I use the free Beyond Compare
    ( as a plug-in with Dreamweaver, but I expect that it
    could also be used with a word processor. This will immediately show the
    slightest difference between two documents, even if it is only a missing comma
    in a 1000 line document.

    James McNangle
    James McNangle, Oct 14, 2007
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