Digitizing Device to Capture Track Impression Grant Number 2010-DN-BX-K145

In crime scene investigations it is necessary to capture images of impression evidence such as tire track or shoe impressions. Currently, such evidence is captured by taking two-dimensional (2D) color photographs or making a physical cast of the impression in order to capture the three-dimensional (3D) structure of the information. This project aims to build a digitizing device that scans the impression evidence and generates (i) a high resolution three-dimensional (3D) surface image, and (ii) a co-registered two-dimensional (2D) color image. The method is based on active structured lighting methods in order to extract 3D shape information of a surface. A prototype device was built that uses an assembly of two line laser lights and a high-definition video camera that is moved at a precisely controlled and constant speed along a mechanical actuator rail in order to scan the evidence. A prototype software was also developed which implements the image processing, calibration, and surface depth calculations. The methods developed in this project for extracting the digitized 3D surface shape and 2D color images include (i) a self-contained calibration method that eliminates the need for pre-calibration of the device; (ii) the use of two colored line laser lights projected from two different angles to eliminate problems due to occlusions; (iii) the extraction of high resolution color image of the impression evidence with minimal distortion; and (iv) a super resolution method that takes advantage of the model of the rail motion in order to further increase the resolution of the extracted color image. The resulting digitized 3D image has a resolution of 0.0438mm in the Y-axis (along the rail motion direction), 0.2369 mm along the Xaxis (limited by the HD video camera resolution) and 0.5mm along the Z-axis (depth). The resulting digitized 2D color image has a resolution of 1900 pixels in the X-axis direction and up to 4500 pixels in the Y-axis direction (depending on the speed of the rail motion and length of scan). The Y-axis resolution can be further increased up to 9000 pixels using super resolution techniques. The scan of a long tire track (about 1.75m) at the lowest scan speed takes about 20 minutes at the slowest rail speed and processing the resulting video to generate the 3D image and the 2D color image takes less than 1 hour on a regular PC.