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Using GIMP to find changes between images

Jul 1, 2011, 20:21
Post: #1
Using GIMP to find changes between images
Hi guys,
I have a bit of a technical question. I have two satellite images that I am comparing to look for changes in topography between the two dates at which the images were taken.
I was going to use the Layers Divide Mode, but the problem is that any brightness changes AND non-changes show up as white (the algorithm gimp uses is 256*Image1/(image2+1) ), so you can see if the pixels of image1 and 2 are the same, it'll give a pixel with value 256, and if image 1 has brighter pixel than image 2, it will give a value greater than 256...which gets converted back to 256.
I've also looked into the Difference and Subtract modes, but they take absolute values or sets anything negative to 0, which skews my results.
Basically, I'm wondering if there is something in gimp which will allow me to compare the two images, kind of like creating a ratio image in ENVI where darker colors stand for darkening over time, whiter colors stand for brightening, and greyish colors show non-changes. I want to divide one pixel value by the other, and use that value to create a new ratio image.
If there is nothing in GIMP, could you suggest another program?
Thanks so much!
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Jul 1, 2011, 21:11
Post: #2
RE: Using GIMP to find changes between images
In GIMP I use two layers in difference mode but my use for this is mostly to detect JPEG artifacts. Detecting differences between satellite images is a very different matter: the images have to be registered (they will always have slightly different distortions), compensated for exposure, seasons, light angle... This is indeed work for a full software suite like ENVI.

There is also a GIMP plugin to produce the difference image of two images. It's not meant for satellite photo may it may work in your case: http://gimp-registry.fargonauten.de/node/25020

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Jul 1, 2011, 22:09
Post: #3
RE: Using GIMP to find changes between images
(Jul 1, 2011 21:11)ofnuts Wrote:  In GIMP I use two layers in difference mode but my use for this is mostly to detect JPEG artifacts. Detecting differences between satellite images is a very different matter: the images have to be registered (they will always have slightly different distortions), compensated for exposure, seasons, light angle... This is indeed work for a full software suite like ENVI.

There is also a GIMP plugin to produce the difference image of two images. It's not meant for satellite photo may it may work in your case: http://gimp-registry.fargonauten.de/node/25020

How will this react with negative values? Because Difference mode in the layers takes the absolute value which messes up my comparisons.
And I am not too worried about exposure, seasons, light angle etc. It's kind of a rough comparison that I'm looking for. If it was possible, I'd love to just literally divide one image by the other (without multiplying by 256), and then having that result be scaled up to 255 colors or whatnot, because multiplying by 256 is what's giving me the problem with wayy too much of a white image because of a larger brightness in one of them.
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Jul 2, 2011, 00:02
Post: #4
RE: Using GIMP to find changes between images
(Jul 1, 2011 22:09)cp05 Wrote:  How will this react with negative values? Because Difference mode in the layers takes the absolute value which messes up my comparisons.
1) Try it... it's free.
2) Get in touch with the author, your need may be pretty close to what it already has and he could be able and willing to do the change.
3) Grab the source code and do it yourself: form the look of it the plugin performs some operation between the corresponding pixels of each image, so you would only have to change the inner operation.

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