What's The Difference? <sec>

Discussion in 'Digital Photography' started by secheese, Nov 5, 2004.

  1. secheese

    secheese Guest

    I need to be able to identify the differences (at a pixel level) of two
    otherwise identical photos. Although this is not what I'm doing, an
    astrophotography example will clarify what I'm asking.

    Suppose I have two identical pics of a stary sky. But in one pic, there
    is a streak from meteor as it enters the atmosphere. Does anyone know
    of a utility that will indicate the differences between the photos, thus
    pointing out the streak?

    _____________________________________________________
    He whom God has touched will always be a being apart;
    he is, whatever he may do, a stranger among men;
    he is marked by a sign.
    - Ernest Renan (1823-1892)
    secheese, Nov 5, 2004
    #1
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  2. "secheese" <> wrote in message
    news:...
    >I need to be able to identify the differences (at a pixel level) of two
    > otherwise identical photos. Although this is not what I'm doing, an
    > astrophotography example will clarify what I'm asking.
    >
    > Suppose I have two identical pics of a stary sky. But in one pic, there
    > is a streak from meteor as it enters the atmosphere. Does anyone know
    > of a utility that will indicate the differences between the photos, thus
    > pointing out the streak?


    Alignment may be a problem. Assuming they are perfectly aligned, simply
    invert one of them in Photoshop and superimpose it as a 50%-transparent
    layer over the other. I think that will work.
    Michael A. Covington, Nov 5, 2004
    #2
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  3. secheese

    Quarc Guest

    secheese wrote:

    > I need to be able to identify the differences (at a pixel level) of two
    > otherwise identical photos. Although this is not what I'm doing, an
    > astrophotography example will clarify what I'm asking.
    >
    > Suppose I have two identical pics of a stary sky. But in one pic, there
    > is a streak from meteor as it enters the atmosphere. Does anyone know
    > of a utility that will indicate the differences between the photos, thus
    > pointing out the streak?
    >
    > _____________________________________________________
    > He whom God has touched will always be a being apart;
    > he is, whatever he may do, a stranger among men;
    > he is marked by a sign.
    > - Ernest Renan (1823-1892)


    Interesting problem you have there. As the pictures will not be pixel to
    pixel identical, you will first have to map them exactly on top of each
    other. If this involves rescaling you will loose details, but assuming
    thay are exactly the same scale, it is easier. Some rotation is probably
    needed and that will loose quality too. When that is done you should
    just be able to place them on top of each others and apply a per pixel
    XOR (Exclusive or). Only things visible on one of the photos will become
    visible. This can all be done in Photoshop for instance, but I guess you
    really ask for an automatic tool....and that I have not heard of :-((( I
    am sure that assuming tha photos are very similar, a programmer skilled
    inh picture manipulation could put something together very quickly, so I
    would be sursprised if it has not been built by someone...

    Good luck in your search

    /Q
    Quarc, Nov 5, 2004
    #3
  4. If you are truly willing to work at the pixel level, PSP and several
    other programs will do this.

    First blow (zoom) the image up large so you can see the individual
    pixels. Then use the magic wand selection tool to look at the 'color'
    of the pixel. The program will show you the RGB or HSB values for
    each pixel.

    secheese <> wrote in message news:<>...
    > I need to be able to identify the differences (at a pixel level) of two
    > otherwise identical photos. Although this is not what I'm doing, an
    > astrophotography example will clarify what I'm asking.
    >
    > Suppose I have two identical pics of a stary sky. But in one pic, there
    > is a streak from meteor as it enters the atmosphere. Does anyone know
    > of a utility that will indicate the differences between the photos, thus
    > pointing out the streak?
    >
    > _____________________________________________________
    > He whom God has touched will always be a being apart;
    > he is, whatever he may do, a stranger among men;
    > he is marked by a sign.
    > - Ernest Renan (1823-1892)
    Don Stauffer in Minneapolis, Nov 5, 2004
    #4
  5. secheese

    Bill Hilton Guest

    > I need to be able to identify the differences (at a pixel level) of two
    > otherwise identical photos.
    >
    > Suppose I have two identical pics of a stary sky. But in one pic, there
    > is a streak from meteor as it enters the atmosphere. Does anyone know
    > of a utility that will indicate the differences between the photos, thus
    > pointing out the streak?


    Easy in Photoshop ... flatten the image, make a second copy of the base layer
    and change the blend mode of the top layer to 'difference'. Black means
    there's no difference on a pixel by pixel basis, any differences will show up
    if you look carefully at 100% or higher magnification.

    Dunno about the consumer programs like Elements and Paintshop Pro but I'd guess
    any program with layers and the 'difference' blending mode will let you do
    something similar.
    Bill Hilton, Nov 5, 2004
    #5
  6. Bill Hilton wrote:
    >> I need to be able to identify the differences (at a pixel level) of
    >> two otherwise identical photos.
    >>
    >> Suppose I have two identical pics of a stary sky. But in one pic,
    >> there is a streak from meteor as it enters the atmosphere. Does
    >> anyone know of a utility that will indicate the differences between
    >> the photos, thus pointing out the streak?

    >
    > Easy in Photoshop ... flatten the image, make a second copy of the
    > base layer and change the blend mode of the top layer to
    > 'difference'. Black means there's no difference on a pixel by pixel
    > basis, any differences will show up if you look carefully at 100% or
    > higher magnification.

    [snip]

    And a quick way then of checking if there are any differences whatsoever is to
    do Image > Trim. If it says "this would leave no pixels in the image", you
    know they all have a consistent difference, which is easy to confirm as zero.

    --
    Barry Pearson
    http://www.Barry.Pearson.name/photography/
    http://www.BirdsAndAnimals.info/
    Barry Pearson, Nov 5, 2004
    #6
  7. secheese

    Alan Meyer Guest

    "Quarc" <> wrote in message
    news:cmg3mn$3c5$...
    > secheese wrote:
    >
    > > I need to be able to identify the differences (at a pixel level) of two
    > > otherwise identical photos. Although this is not what I'm doing, an
    > > astrophotography example will clarify what I'm asking.
    > >
    > > Suppose I have two identical pics of a stary sky. But in one pic, there
    > > is a streak from meteor as it enters the atmosphere. Does anyone know
    > > of a utility that will indicate the differences between the photos, thus
    > > pointing out the streak?
    > >

    >
    > Interesting problem you have there. As the pictures will not be pixel to
    > pixel identical, you will first have to map them exactly on top of each
    > other. If this involves rescaling you will loose details, but assuming
    > thay are exactly the same scale, it is easier. Some rotation is probably
    > needed and that will loose quality too. When that is done you should
    > just be able to place them on top of each others and apply a per pixel
    > XOR (Exclusive or). Only things visible on one of the photos will become
    > visible. This can all be done in Photoshop for instance, but I guess you
    > really ask for an automatic tool....and that I have not heard of :-((( I
    > am sure that assuming tha photos are very similar, a programmer skilled
    > inh picture manipulation could put something together very quickly, so I
    > would be sursprised if it has not been built by someone...
    >
    > Good luck in your search


    I suspect that this will be a pretty tricky program to write. As you
    say, there may be rescaling and rotation of the night sky. There
    may be atmospheric effects (e.g., clouds or haze or smog in different
    places on different nights) too.

    But I would think that there have to be programs that do this already.
    Surely the big observatories are using image processing techniques
    to study their images.

    I suggest asking this question in an astronomy newsgroup. I bet
    programs have been written and may even be available as open
    source.

    Alan
    Alan Meyer, Nov 5, 2004
    #7
  8. secheese

    Bill Hilton Guest

    >> Easy in Photoshop ... flatten the image, make a second copy of the
    >> base layer and change the blend mode of the top layer to
    >> 'difference'.


    >From: "Barry Pearson"
    >
    >And a quick way then of checking if there are any differences whatsoever is
    >to do Image > Trim. If it says "this would leave no pixels in the image", you
    >know they all have a consistent difference, which is easy to confirm as zero.


    That's a good idea, thanks for posting it.
    Bill Hilton, Nov 5, 2004
    #8
  9. "Bill Hilton" <> wrote in message
    news:...
    >> I need to be able to identify the differences (at a pixel level) of two
    >> otherwise identical photos.
    >>
    >> Suppose I have two identical pics of a stary sky. But in one pic, there
    >> is a streak from meteor as it enters the atmosphere. Does anyone know
    >> of a utility that will indicate the differences between the photos, thus
    >> pointing out the streak?

    >
    > Easy in Photoshop ... flatten the image, make a second copy of the base
    > layer
    > and change the blend mode of the top layer to 'difference'. Black means
    > there's no difference on a pixel by pixel basis, any differences will show
    > up
    > if you look carefully at 100% or higher magnification.
    >
    > Dunno about the consumer programs like Elements and Paintshop Pro but I'd
    > guess
    > any program with layers and the 'difference' blending mode will let you do
    > something similar.


    Paintshop Pro lets you do Image Arithmatic - addition, subtratcion, or
    multiplication, pixel by pixel.
    >
    >
    Marvin Margoshes, Nov 5, 2004
    #9
  10. secheese

    GeneR Guest

    My first test would be to make stereo slides of the suspected
    areas. When viewed this way any differences pop out like magic.
    Gene http://www.photoprojects.net


    secheese wrote:

    > I need to be able to identify the differences (at a pixel level) of two
    > otherwise identical photos. Although this is not what I'm doing, an
    > astrophotography example will clarify what I'm asking.
    >
    > Suppose I have two identical pics of a stary sky. But in one pic, there
    > is a streak from meteor as it enters the atmosphere. Does anyone know
    > of a utility that will indicate the differences between the photos, thus
    > pointing out the streak?
    >
    > _____________________________________________________
    > He whom God has touched will always be a being apart;
    > he is, whatever he may do, a stranger among men;
    > he is marked by a sign.
    > - Ernest Renan (1823-1892)
    GeneR, Nov 5, 2004
    #10
  11. secheese <> writes:

    > I need to be able to identify the differences (at a pixel level) of two
    > otherwise identical photos. Although this is not what I'm doing, an
    > astrophotography example will clarify what I'm asking.
    >
    > Suppose I have two identical pics of a stary sky. But in one pic, there
    > is a streak from meteor as it enters the atmosphere. Does anyone know
    > of a utility that will indicate the differences between the photos, thus
    > pointing out the streak?


    ImageMagick has a compare function that takes 2 inputs and produces an output
    file of the differences. Note, you should use TIFF or a RAW format, since I
    tried it initially on two JPG files, and it highlighted the areas that the JPG
    compression algorithm considered equal.

    ImageMagick's web site is: http://www.imagemagick.org.

    --
    Michael Meissner
    email:
    http://www.the-meissners.org
    Michael Meissner, Nov 6, 2004
    #11
  12. secheese

    secheese Guest

    Your suggestion was bang on! I did the "difference" function, and
    voila... the result was spectacular! Thanks.



    On Fri, 5 Nov 2004 14:05:37 -0500, "Marvin Margoshes"
    <> wrote:

    >
    >"Bill Hilton" <> wrote in message
    >news:...
    >>> I need to be able to identify the differences (at a pixel level) of two
    >>> otherwise identical photos.
    >>>
    >>> Suppose I have two identical pics of a stary sky. But in one pic, there
    >>> is a streak from meteor as it enters the atmosphere. Does anyone know
    >>> of a utility that will indicate the differences between the photos, thus
    >>> pointing out the streak?

    >>
    >> Easy in Photoshop ... flatten the image, make a second copy of the base
    >> layer
    >> and change the blend mode of the top layer to 'difference'. Black means
    >> there's no difference on a pixel by pixel basis, any differences will show
    >> up
    >> if you look carefully at 100% or higher magnification.
    >>
    >> Dunno about the consumer programs like Elements and Paintshop Pro but I'd
    >> guess
    >> any program with layers and the 'difference' blending mode will let you do
    >> something similar.

    >
    >Paintshop Pro lets you do Image Arithmatic - addition, subtratcion, or
    >multiplication, pixel by pixel.
    >>
    >>

    >


    _____________________________________________________
    He whom God has touched will always be a being apart;
    he is, whatever he may do, a stranger among men;
    he is marked by a sign.
    - Ernest Renan (1823-1892)
    secheese, Nov 7, 2004
    #12
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