Removing perspective (photographic) distortion with photoshop.

[Myfanwy]

Hi! I'm a new member, but unfortunately not a photographer!

I am doing some research on perspective distortion correction with photoshop in relation to bitemarks, ie marks/bruising normally left on skin by the teeth in violent interactions. In order to present any photo evidence in court that as been enhanced you would need to explain with a fair bit of detail what has happened to the pixels within the image.

I have had a very trying couple of days attempting to get some info out of Adobe, to no avail!

I am using the transform box with the anchors that you can use to change the perspective and align the 'L shaped' scale within the photograph to a number of vertical and horizontal guides.

Does anyone know of any publications that give an algorithm or describe in detail what is happening to the image? As I have to reference any information I put into my study unfortunately I need the information to have come from a 'formal' source, though I'm sure many of you clever photographers have a clear understanding of what is happening.

Any help would be incredible.

A very stressed Myfanwy!

[tclune]

"Perspective transformation" is a technical term that is not specific to Photoshop. Any decent graphics programming text will discuss it in detail. I'm a bit of a dinosaur, so my usual references are probably not considered au courant, but this stuff hasn't changed since Euclid. My "go-to" math text for this sort of thing is "Mathematical Elements for Computer Graphics (2nd edition)" By David F. Rogers and J. Alan Adams McGraw Hill, 1990. Perspective transformations are discussed in chapter 3, which includes various 3D transforms.

Rogers' companion volume, "Procedural Elements For Computer Graphics,' is often a good source for pseudocode when it comes time to program up what you want to do, but in this case there is no good discussion of coding issues. So my fallback choice would be an excellent accessible volume, "A Simplified Approach to Image Processing" by Randy Crane (Prentice Hall, 1997). The section on perspective transformation is, appropriately enough, directly after the discussion on affine transforms. The most salient comments in his discussion include, "The perspective transformation, also known as the projective transformation, preserves lines in all orientations. Perspective transformations are a superset of affine transformations...The perspective transformation cannot directly map a (non-rectilinear) quadrilateral to another (nonrectilinear) quadrilateral. If you must map a quadrilateral to a quadrilateral, you can break the sequence up into two transformations. The first transforms a quadrilateral to a square. The second transforms the square to the second quadrilateral." (pp. 211-212)

If you need real help on this, go to any computer science department and you can be referred to someone who can provide expert assistance. This is all pretty standard stuff.

[Myfanwy]

Thank you Tom!

That gives me a good starting point. It all sounds foreign to me at the moment but I'm sure with a bit of reading all will be made clear.

Cheers, Ceri

[rpcrowe]

This is a very interesting web site for anyone interested in evidence photography. Although it is aimed at U.K. evidence; I am sure much of the information is applicable to U.S. and other nations.

http://www.crime-scene-investigator.net/csi-photo.html

Canon sells a "Data Verification Kit" which is supposed to provide verification that the image has not been altered. However, I have recently read (but have not saved the source) that some hacker has been able to get around the security of at least one of the Canon Data Verification Kit systems. There are at least three different versions of ths data Verification Kit: DVK-E1, DVK-E2 and OSK-E3:

http://www.google.com/#q=canon+data+...w=1120&bih=482