Subtraction Technique - Forensic Imaging

[dwarrington]

Hello, I work in forensic science and am teaching a lab at a university using the subtraction method for footwear and fingerprints.

Effectively I have 2 photographs taken using a tripod and the same lighting and camera settings (I have JPEG and NEF files for both photographs)
 - 1 is of a dusty partial footwear mark on a wooden floor
 - 1 is of the wooden floor but with the footwear mark completely cleaned away after the first photograph

I want to subtract the background (cleaned substrate) from the photograph showing the footwear mark, leaving only the footwear mark which will hopefully reveal more detail that was visibly lost in the wood grain background. The background can often interfere with visualising the print, especially with fingerprints and so the removal of the background can reveal fine details in ridges, valleys and pores etc.

I have experience using the subtraction method in Photoshop but as this is a university course I am unable to get multiple licences due to funding etc and so I have started playing around with GIMP instead.

I have tried a few methods I found online loading the images as layers and playing around with the subtraction and difference modes with some success but was wondering if there were any other settings/changes I could use to improve this? 

Also, is there an auto-align function in GIMP? e.g. solid lines between floorboards are aligned, not just the photographs themselves. As much as I try not to knock the tripod between photographs there sometimes can be very slight movement.

Thanks for your help/advice, David

[Blighty]

(12-12-2018, 07:28 PM)dwarrington Wrote: Also, is there an auto-align function in GIMP? e.g. solid lines between floorboards are aligned, not just the photographs themselves. As much as I try not to knock the tripod between photographs there sometimes can be very slight movement.

Have a look at this thread
https://www.gimp-forum.net/Thread-How-To...-On-A-Form

Post #6 mentions ofn-layer-aligner and where to download it.

Post #8 gives a brief description of how to use it. (ofn-layer-aligner comes with some docs too - be sure to read those)

[Ofnuts]

As much as I like layer-aligner I'm not convinced it is the right tool for this, because it is not accurate enough, and won't apply the right transform. When you knock the camera you slightly change it's orientation, so the projection of the room on the sensor is made along a slightly different axis. Making the pictures overlap would be more like using perspective. AFAIK the best tool for this is Hugin (and once it is done in Hugin you can transfer the image to Gimp).

[rich2005]

The term is 'registration' there is an old Gimp (compiled) plugin for this

https://sourceforge.net/projects/gimp-im...ion-0.5.5/

There is a 32 bit Windows plugin there, should work with the Gimp 2.10.8 from gimp.org

edit: There is a 64 bit compiled .exe in this collection: https://samjcreations.blogspot.com/2018/...-bits.html if you decide to try it.

Years since I last tried so, a quick compile in linux and it takes two layers, a base layer for registration and layer to transform (edit: remembering why it is years since, either works or totally off..)

A made up pair, same patch of floor + a bit of film. Opacity reduced to show the displacement. https://i.imgur.com/qPHZNxM.jpg

Very slow plugin, 10 minutes 1024 iterations on a 4896x3672 pix image. Differencing the layers and a comparison before/after registration. https://i.imgur.com/qyFVoE5.jpg

Might be worth a try.

Apart from Hugin, another application is NIP2. There is a Windows installer https://github.com/libvips/nip2/releases. Bit of a learning curve with this one.

and an example of use: https://www.researchgate.net/figure/44-I..._267266175

[Ofnuts]

Wondering what does the "Align layers" filter in GMIC?

[dwarrington]

Hi all, thanks for all the information. I've tried the align script and it does the job nicely. Since it is typically the tripod that is bumped and being on a flat surface the perspective of the image in not usually changed, just the location of the footwear pattern in the image.
Still playing around with the various manipulation modes but the results I am getting are the same if not better than what Photoshop was producing.
Thanks heaps