Digital Analysis of Bite Mark Evidence - Softcover

Johansen, Raymond J.; Johansen, Raymond; Bowers, C. Michael

Softcover

ISBN 10: 0967786606 ISBN 13: 9780967786605

Publisher: Forensic Imaging Inst, 2000

Synopsis

This book is the first practical step-by-step description of digital methods necessary to control and analyze physical forensic evidence. Digital imaging is becoming commonplace in our world today and Forensic Dentistry and its allied disciplines are no exception. Bite marks are now being photographed digitally, digitally scanned from photographs, sent via electronic mail, and copied onto floppy, zip, and compact disks. These same applications are used in firearms, ballistics, fingerprints, and toolmark investigation.

It is becoming essential for the Forensic Examiner to be able to work with these digital images when asked to investigate cases. Adobe(r) Photoshop(r) is a powerful software tool that allows for imaging, enhancing, and resizing of the original evidence photographs. Photoshop(r) is also ideal for fabrication of suspect dentition overlays and other forensic exemplars used in physical comparisons.

Topics include scanning of bite mark photographs, enhancement and resizing these images, photographic distortion control, scanning two and three dimensional evidence, fabrication of overlays, and physical comparison techniques within Photoshop(r).

Practice image files that are used throughout the book are included on a 3.5 inch, high-density 1.44MB floppy disk.

About this title

About the Author

Raymond J. Johansen is a practicing dentist in Santa Barbara, California. He is a forensic consultant to the Santa Barbara Coroner' Office.

C. Michael Bowers is a practicing dentist and attorney in Ventura, California. He is a Deputy Medical Examiner for the Ventura County Medical Examiner's Office. He is a Diplomate of the American Board of Forensic Odontology, IAI Senior Crime Scene analyst and a Fellow of the American Academy of Forensic Sciences.

From the Back Cover

Digital imaging is becoming commonplace in our world today and Forensic Dentistry is no exception. Bite marks are now being photographed digitally, digitally scanned from photographs, sent via electronic mail, and copied onto floppy, zip, and compact disks. This technology allows us to instantaneously collect and share information.

It is becoming essential for the Forensic examiner to be able to work with these digital images when asked to analyze bite mark cases.

Adobe(r) Photoshop(r) is a powerful software tool that allows for imaging, enhancing, and resizing of the original injury photographs. Photoshop(r) is also ideal for fabrication of suspect dentition overlays to be used in physical comparison to the bite mark images.

Topics include scanning of bite mark photographs, enhancement and resizing these images, scanning suspect's dentition casts, fabrication of overlays, and comparison techniques within Photoshop(r).

Excerpt. © Reprinted by permission. All rights reserved.

Chapter 4 Correcting for Angular Distortion

The tools within Photoshop(r) can be used to detect and correct for certain angular distortion. This is an extremely important step as it is the foundation for the comparison procedures that follow.

A photograph is a graphical representation of the objects within the range of the camera's lens. The degree to which this graphic exactly reproduces those objects is influenced by many variables. When bite marks are photographed as evidence, attempts are made to carefully control these variables in an effort to obtain an accurate representation of the bite mark for later analysis. Unfortunately, these efforts are not always successful and distortion is often introduced into the image.

Ideally, the bite mark images received for analysis will have the following features:

* A scale present (or some appropriate measuring device) oriented on the same plane as the bite mark. * The bite mark was photographed perpendicular to this scale. * Orientation of the film plane is parallel to the bite mark. This insures no perspective distortion of the image due to off angle camera position. * No perspective distortion of the scale caused by its misplacement away from the plane of the bite mark.

The presence of these features allows us to accurately resize the bite mark photograph without having to rectify the original image.

Often, this is not the case and the image is distorted. Carrying out a meaningful comparison demands an image reconstruction that corrects for this distortion. Photoshop has the ability to do this for all but the most severe angular distortion.

Correction for angular distortion can be difficult until you become familiar with the procedure. Once the steps are mastered you will have no problems rectifying distorted images and it will become a routine part of your bite mark analysis.

In order to determine if the photograph is distorted the scale present in the image must be evaluated for size and shape. The sides of the scale or ruler must be parallel, the incremental lines perpendicular to the sides and equally spaced and, if present, any circular reference shapes must be round in shape (not oval). If a two-dimensional scale is present, the angle formed by the two legs should be 90 degrees. For the purpose of this exercise an ABFO #2 scale, manufactured by Lightning Powder Co., Inc., Salem, Oregon, will be used.

Placing a Grid on the Image

The degree of distortion can be preliminarily checked by placing a Grid over the bite mark image and evaluating the scale's sides, incremental lines, and angles for parallelism. The grid will appear along the x/y axis of the image when activated (use of the grid will be discussed in detail later in this section). In order to check the alignment of the scale to the grid, the bite mark photograph must also have the scale oriented along the x/y axis.

If the scale within the original photograph is not oriented along the x/y axis, the entire image must be rotated in the Photoshop image window to achieve the proper scale orientation. The procedures for rotating the image are detailed below.

SIMPLE ROTATION and Cropping of the Bite Mark Image

Select the Measure Tool from the Tool Bar and draw a line along the horizontal or vertical edge of the scale (the horizontal edge is used in the example). Note that the Info Palette (upper right) the A displays the angle of the drawn line relative to the zero (x) or 90 (y) axis. The D represents the distance or length of the measuring line and is unimportant for this step. (Fig. 8).

Note: to discard the measuring line when finished or to redraw a line, click anywhere on the line between the end points and drag it out of the window.

Next click

Image > Rotate Canvas > Arbitrary

The Rotate Canvas dialog box appears (Fig. 9) and displays the angle that the Measure Tool line is off either the horizontal or vertical axis. This represents the amount of rotation necessary to bring the measuring line parallel to the image border. Photoshop also knows which direction to rotate the image.

oCW = degrees clockwise oCCW = degrees counter clockwise

Click OK to rotate the image.

The image has been rotated so that the edge of the scale is oriented parallel to x/y axis. Now the borders of the photograph can be cropped square to these axes.

Cropping the Image

The Crop Tool is hidden under the Rectangular Marquee tool (Fig. 10). This step removes unnecessary perimeter areas that are of no forensic importance. As a second benefit it will make your image file size smaller.

From the Toolbox, double click the Rectangular Marquee Tool. Hold down the mouse button and drag the cursor from the Rectangular Marquee Tool onto the Crop Tool. Then hold the mouse button down on the Crop Tool icon. It is now activated. Place the cursor back onto the image and place it on the upper left corner of the image. Drag the resulting cropping box diagonally from the upper left to the lower right of the image. Release the mouse button. Note the eight anchor points around the perimeter of the cropping box. These can be moved to resize the cropping box as desired. When you are satisfied with the position of the cropping box press Enter. The image will be cropped and the scale will be oriented parallel to the image borders (Fig. 11).

Save this image as:

Casename-RotatedCropped-Color(or B&W)

Edition details

• Publisher: Forensic Imaging Inst
• Publication date: 2000
• Language: English
• ISBN 10: 0967786606
• ISBN 13: 9780967786605
• Binding: Spiral-bound
• Edition number: 1
• Number of pages: 118