The 3D Digital Capture of 3D Shoeprints

Abstract

Shoeprint evidence is an important part of forensic investigations. The current capture method, Dental Stone, has been used for over 25 years. The rapid pace of technological advances in that time has increased the interest in capturing 3D shoeprints digitally. While several authors have investigated the use of other capture methods, the experiments are generally specific to one scanner in one set of conditions and are based on one-off captures. Additionally, little has been done to assess the use of deviation analysis as a more objective comparison method, and the suitability of various postprocessing packages for forensic use. Six scanners (NextEngine, Faro Focus, Faro ScanArm, Minolta VIVID 910, Artec MH and Artec EVA™) and five photogrammetry programs (iWitness, Photosynth, Photofly, Visual SFM, and Photomodeler Scanner) were investigated and compared to dental stone. Six templates were designed and created: five were created in different light reflectance values (5%, 16%, 39%, 64%, 86%) representing the range of different coloured substrates a 3D shoeprint is likely to be found in. The sixth template contained 22 indentations with 20 unique depth:width ratios (designed to test sensitivity) representing the size range of class, wear and individualizing characteristics that can be present in a shoe outsole. These templates were scanned five times each by each scanner from multiple angles to produce the best results, while still being suitable for field work: it was also cast five times with dental stone. Additionally, three postprocessing software packages, Polyworks, Rapidform and Geomagic were evaluated for use in forensic science. Jpg images were used for all photogrammetry evaluations. There was a wide variation in the capture quality across all eleven capture methods. iWitness wasn’t suitable for shoeprint capture as it did not create a point cloud and was therefore unsuited for shoeprint comparisons as well. Photosynth and Visual SFM created data with high numbers of holes, regardless of the number of photographs used and the light reflectance values, they were also unable to capture the sensitivity template due to blurry sections of the photographs. Photomodeler was complicated and due to limited time with the program no useable results were obtained. The Faro Focus had insufficient sensitivity to capture shoeprint evidence; the Minolta had holes and poor reproducibility, while the Faro ScanArm has high levels of noise. The Artec and EVA™ scanners also produced noisy scans with low sensitivity. The NextEngine scanner and Photofly showed the best repeatability regardless of light reflectance values and also demonstrated a high sensitivity. Dental stone captured all indentations with perfect reproducibility and high sensitivity, however air bubbles prevented ideal capture. Further investigation into the NextEngine scanner and Photofly would be required to determine which would be most suitable for scene work. Geomagic was determined as the best post-processing software due to its user-friendly interface and reproducible results as well as its operating stability.