3D Footprints: Making a Positive Impression
Written by Eugene Liscio   

-Sponsored- As with many other forms of physical evidence, footprints have long been recognized as an important piece of crime scene evidence. So long as gravity remains a factor at crime scenes, footprints will forever remain a commonly found piece of evidence. Although at times they may be difficult to interpret due to a difficult substrate or distortion in the print, they are still a valuable piece of evidence to tie a suspect to a particular crime.

There are a few techniques that are commonly used to assist an examiner in making a footprint comparison. Most of these techniques require a semi-transparent overlay that may be done with Photoshop (or some other image editing program) or transparent plastic sheets in order to make a flat, two dimensional comparison with an overlay of the images. However, at times it is important to consider the full 3D profile of the impression. This is especially so when the substrate is ideal and has a good impression left behind. An older or worn pair of shoes can leave a very distinguished set of prints which may specific or common with other shoes that are owned by the wearer. Photographing a 3D footprint as a 2D image removes the depth signature and therefore, it may be more valuable to treat these as full 3-D objects and not as 2D prints.

Some of the challenges with scanning technologies has been a limited resolution. The digital camera is rather unique because it is scale invariant. It’s possible to photograph objects in a very wide range of sizes so long as a suitable lens is chosen. For small features, using a macro lens can provide great detail over a small portion of the shoe tread to make a very high resolution examination. However, depth of field may be an issue at high magnifications and perspective distortion may be an issue when the object cannot be visualized completely at 90° due to curvature.

With the advent of newer scanning technologies such as structured light scanners and laser line scanners, it is important to consider a full 3D analysis as another tool in the toolbox for a forensic footprint examiner. 3D analysis is not a replacement for current techniques but is an additional tool which can aid in analyzing depth features to provide a very logical and measurable result. Of course, much work needs to be done in this area, but the documentation tools and analysis software are critical.

Most 3D technologies have a fixed resolution and therefore are limited in what they can capture. Therefore, it is very important to start with a high accuracy instrument that can capture 3D information quickly. Currently, metrology grade instruments such as the FARO® Edge and ScanArm HD are suitable for this type of application in a lab environment. They can easily detect 3D geometry well below 0.001" and therefore when looking at typical wear patterns in both the shoe and the impression/cast, it is possible to make comparisons in three dimensions. A typical scan of a shoe sole or cast of a print may take somewhere on the order of 10-15 minutes from start to finish. An enormous amount of data can be captured in this short amount of time and the results can be digitally stored indefinitely.

Figure 1. Sole of running shoe and its imprint in sand. Scanned with FARO® Edge and ScanArm HD.

There are a number of programs available which will allow for a deviation analysis. These programs may be Polyworks, GeoMagic, and even the freely available CloudCompare. By importing point cloud or meshed objects from a metrology grade 3D scanner it is possible to compare a shoe and a print. Differences in distance between the closest matching points are detected and then displayed in a color histogram based on the relative distance. For example, in the image below colors which are blue are closest together while colors which are white and then red are farthest away. All the points in each of the point clouds or mesh can be grouped based on their distance and a statistical measure of how well the points "match" can be accomplished.

Figure 2. A deviation analysis in CloudCompare showing matching regions in blue while white and red indicate areas of greater difference.

There is still much research and validation testing required to move this new area forward, but the outlook is quite promising since it presents a new paradigm to how footprint comparisons have traditionally been made. The technology to document footwear and their respective prints is currently available in metrology grade instruments and will no doubt continue to improve. Therefore, much of the work on the road ahead is to look at 3D workflows, comparison techniques and deviation analysis methodology which can move towards a truly measurable and statistical approach.

About the Author

Eugene Liscio is President of AI2-3D.


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