Three-dimensional imaging aids evidence collection
--Andrew Wilson
For police forces in countries that endure harsh winter conditions, foot and tire prints are important types of evidence that are difficult to collect using conventional casting methods. In Ontario, Canada, for example, the Provincial Police Force must often obtain footprint evidence even though snow covers the ground for half the year. Under freezing temperatures, stone-casting techniques prove unsuitable. Instead, sulfur-based techniques are frequently used.
In this approach to collecting print data, sulfur is heated until it is molten and then poured into the footprint impression. Upon contact with the snow, the sulfur hardens and forms a solid replica of the evidence. Though adequate, such print-collection techniques are costly, time-consuming, hazardous, and tedious.
Developing phototype system
To overcome these problems, the Ontario Provincial Police approached The Turing Institute (Glasgow, Scotland) for help in developing a prototype system to gather forensic print evidence using three-dimensional (3-D) imaging techniques. Based on the institute`s proprietary C3D 2020 3-D imaging technology, a prototype system has been developed that can image foot and tire prints in a fraction of a second.
In operation, the system uses two cameras, a frame-grabber board, and an off-the-shelf personal computer to capture stereo image pairs of the desired evidence. Images are captured and processed using the PC to produce 3-D models with a spatial accuracy of 0.5 mm. After capturing the images, the institute`s proprietary software automatically reconstructs digital 3-D view models of the objects or scenes. These views can then be visualized as polygon, wire-frame, shaded, or naturally rendered images. The 3-D models are stored and processed as variable-mesh polygons or integrated with third-party 3-D graphics software.
However, acceptable 3-D models can be built only when a large number of clearly visible and focused points are seen by both cameras. If an object contains no visible texture, such as impressions in snow, no points of reference are available for generating a three-dimensional model.
Ensuring operation
To ensure that the prototype system can operate under snow conditions, textured paint is first sprayed onto the imprints. Images are then taken using a 35-mm-film camera with a Pentax beamsplitter attachment. After the images are digitized onto a photo compact disk, they are matched using the institute`s C3D package to produce a 3-D model. The 3-D model is rendered using OpenGL software as part of the C3D package.
"By matching the texture-projected images, it is possible to guarantee the three-dimensional reconstruction accuracy," says Colin Urquhart, research and development engineer at The Turing Institute.
