Law Enforcement Usage of Photoshop

[Dave Hamer]

Typically, in my line of work. We have been asked to explain it in what
I refer to as laymans terms. No one is expecting us to be able to go
into a lot of specific details because the Judge and certainly the
members of the jury would all think we were speaking geek and would
quickly be lost. If the challenge is between the defence and the
pocecutor then usually the experts from the company who supply the
products are forced to attend by subpoena. When you receive a subpoena,
there is not much you can do. If you explain to the Judge that you want
to protect your source, then often they consent by way of a voire dire.

Dave

[David Knoerlein]

Sam, Thats exactly what was done in the courtroom. I demonstrated each step of the process and provide the court with a detailed report of the enhancement history. Again I must say that the court excepted the process as a valid forensic tool. What I'm looking for is assistance in where I can find logical explanations of what exactly happens to the "Pixels" as each enhancement is applied.

[ID. Awe]

David:

I would show a physical example of how the enhancements detailed the image, eg. photo 1: original fingerprint scan, photo 2: level adjustment etc. this would show the jury in real terms how you arrived at the final image that was used to link the suspect to the crime.

If the jury has a question about how a particular enhancement step (eg: "I noticed certain changes in the fingerprint during this enhancement"), you can focus on that concern, rather than other steps that do not give rise to any concern.

[Dave Hamer]

ID. Awe

Agreed. In regular film photography, one has to be prepared to show the
negative. With no negative available to digital, one has to be prepared
to show the original image and the then the enhancement and be able to
describe how it was enhanced and why. This is the way it is being
presently accepted in Canada, at least according to my RCMP Ident
specialist.

Dave

[David Knoerlein]

A word about "Alterations". In accordance with the SWIGT guidelines and my own SOP's, only tools that do not add or remove pixels will be utilized to enhance images of evidence. To ensure security and to manage the images I use a (Forensic Image Tracking Software) that applies an encryption code to each original image at the time it was acquired into the system. A dupelicate of the original (also encrypted) is launched into Photoshop, and afetr being enhanced is returned to the tracking software database. The encryption code is then verified. If a single pixel has been added or deleted the code will be different, thereby displayed as a corupt images. This and many other security measures have become policy for all images of evidentual value that are captured ditally.

[Gustavo Sanchez]

Excuse my curiosity,

What does it mean here 'adding or deleting' pixels? I mean, does that include changing its values beyond a given range?

Thanks.

[David Knoerlein]

Gustavo, increasing or decreasing the pixel count for that image would be an alteration. Adusting the values is concidered a enahncement. I explain it this way in court. When I do enhancement to an image its very simular to adjusting an image on a television set. I adjust brightness, hue, even volume to give me what I beleive is the best image for viewing. I do not change the contents of the image on being broadcasted to my TV, I just make it more clear and easier to see details. This is just like the enhancement of digital images, which is a (SUBJECTIVE Process)and up to the analsyt to decide what is used and how much of a change to make.

[Phosphor]

No worries about the spelling and grammar, David. It's clear those are just hasty mistakes, given your grasp of the subject and depths to which you've gone to share your knowledge.

You can click the "Edit" text link just below the body of your message for up to 30 minutes to make corrections, addendums or to clarify.

Just for gits and shiggles, please consult Seymour Tipps <http://www.adobeforums.com/Images/help/edithelp.html>.

[Lawrence Hudetz]

Some operations can unwittingly remove a pixel, no?

If you adjust the values to far, the image will pixelize, thereby removing the pixels from that channel.

Is this correct? Does your encryption software catch that kind of dropout?

On second thought, the pixel remains; it's value is zero. Hmmm.

[Gustavo Sanchez]

David,

Thanks. I asked because I think a lawyer could rightfully argument that changing the values of the pixels may be even more important than adding or substracting one.

That is: If a digital picture is basicly a big spreadsheet of data, the content in each cell and how they interrelate is as important as how many cells there are.

Perhaps it would be better not to get into unneeded details, as it was mentioned before ;)

[YrbkMgr]

Most of these issues are pointed to by the book Rising Sun by Michael Crichton. Again, image manipulation in any fashion can be used for good or evil, whether intentional or unwittingly.

I think David has a most excellent handle on the issues. One of the main issues is his documentation of his methods. Those demonstrate the rationale and will be hard to argue with. They DO change the image, but it's not difficult to establish that they are techniques that enhance data already in the image and removes "noise", similar to the techniques used with analog video stills.

So A)establish the protocols as sound techniques and B)prove that you followed those protocols. That's really what is at issue.

[Gustavo Sanchez]

Neat, Tony.