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September 28, 2011

Speaker: Luis Zarrabeitia, PhD candidate, University of Ontario Institute of Technology.

Title: Extracting 3D blood flight trajectories from videos for forensic analysis.

Abstract: At present, blood stain pattern analysis (BPA) is a largely qualitative sub-discipline of forensic science. Our ultimate aim is to develop software for quantitative analysis to aid BPA forensic analysts. Towards this end, we are developing a sound physics-based model (i.e., incorporating gravity and air resistance) for bloodstain pattern formation using pre-recorded videos of simulated blood-letting events. The simulations consist of fake blood encased in ballistic gel being splattered by projectiles. The resulting blood flight trajectories are recorded by a high speed camera.

We present a method for extracting the three-dimensional flight trajectories of liquid droplets from video data. A high-speed stereo camera pair records videos of experimental reconstructions of projectile impacts and ensuing droplet scattering. After background removal and segmentation of individual droplets in each video frame, we introduce a model-based matching technique to accumulate image paths for individual droplets. Our motion detection algorithm is designed to deal gracefully with the lack of feature points, with the similarity of droplets in shape, size, and colour, and with incomplete droplet paths due to noise, occlusions, etc. The final reconstruction algorithm pairs two-dimensional paths accumulated from each of the two cameras' videos to reconstruct trajectories in three dimensions.

Traditional forensic methods for reconstructing crime scenes, such as "stringing'', ignore the effects of gravity and drag. Our preliminary results show that gravity and drag play an important role in the trajectories of the droplets. The reconstructed droplet trajectories constitute a starting point for a physically accurate model of blood droplet flight for forensic bloodstain pattern analysis.

This is joint work with Dhavide Aruliah and Faisal Qureshi, based on experiments by Raquel Murray, Paul Prior and Franco Gaspari.

Biography: Luis Zarrabeitia is currently working towards a PhD in Computer Science at the University of Ontario Institute of Technology. He is jointly supervised by Dr. Dhavide Aruliah and Dr. Faisal Qureshi.
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