Thursday, September 20, 2007
Forensic Uses of Adobe's Photoshop
Forensic Video / Image Analysts, Questioned Document Examiners, Latent Print Examiners ... there is something in Photoshop for everyone. Creative Suite got things moving. Now, with CS3, we are doing more than ever with this robust suite of programs.
Clarify video stills, convert formats, remove unwanted colours or artifacts to reveal hidden info, compare and authenticate ... all this for the price of one program. Quite the bargain.
Working in the field of Forensics, we are tasked with being able to prove our work and with providing a repeatable result. In Forensic Video Analysis, the examiner can use a multitude of tools to capture and analyze video retrieved from crime scenes. There are many vendors competing in the video capture market. From Avid to StarWitness and everywhere in between, somewhere in the examination workflow there is a common thread among examiners. More often than not, an examiner will use Adobe's Photoshop.
When working non-destructively within Photoshop, the examiner will employ layers. In looking back at how far we’ve come in such a short time, remember that layers were introduced when version 3 shipped in 1994. Upon opening the image, the examiner will duplicate the background layer and work with the resulting copy, thus protecting the original. From this point, any number of adjustments can be made to clarify the image.
Introduced along with the layers functionality in 1994 was layer blending modes. Within Photoshop there are a variety of blending modes used to combine the color of the pixels in the upper layer with those in layer or layers beneath it. In forensics, we are blending with a copy of the background layer, also known as a self blend. A layer's blending mode determines how its pixels blend with the pixels of any underlying layers. Some blending modes have tremendous value in making time saving corrections to color and lighting within an image. The results are there in an instant and your eyes are the judge of when the image looks right. But what is happening underneath? Do we know how the blending modes work and how to check their accuracy? While countless books and articles have been written about Photoshop’s blending modes, very few have focused on their underlying mathematics. We verify, prove and document so much about our workflow; how do we prove our results when we use blending modes? In the next installment, we'll take a look at the math behind the blending modes.