Abstract
Abstract. This paper presents a new methodology to accurately obtain 3D rotational velocities of blocks and fragments. Four high speed cameras are used to capture the scene. An additional two tilted mirrors are used to multiply the number of views. Hence, a total of six different viewing perspectives can be used to track translational and rotational velocities in 3D. The focus in the current work is on the rotational velocities, as tracking of the translation is generally straightforward. A common outline tracking algorithm based on the visual hull is adapted. The visual hull is further meshed using triangular elements to approximate the shape of the object. This 3D reconstruction is then used to track the 3D motion of the object. However, the accuracy of the results strongly depends on the accuracy of the 3D reconstruction which is mainly influenced by the number and position of the available views. In any case, the 3D reconstruction from the visual hull is only an approximation and significant errors can be introduced which influence the tracking accuracy. Hence, an in-house post-processing algorithm based on the knowledge of the real geometry of the object, which can generally be accurately determined after a test, was developed. The improved performance of this new post-processing method is shown by controlled spinning tests. Finally, results of a real example of an impact fragmentation test are discussed.
Highlights
Rockfall is a natural hazard that requires a rigorous and comprehensive assessment to be successfully mitigated
To obtain the rotational velocity, two or more features including the centre of gravity (CofG), have to be tracked (Shum & Komura, 2005)
A postprocessing algorithm based on a more accurate fragment geometry, obtained from scanning post-testing, was developed and combined to the visual hull approach. This paper presents this new post-processing algorithm and its validation, which was achieved through controlled spinning tests and a drop test
Summary
Rockfall is a natural hazard that requires a rigorous and comprehensive assessment to be successfully mitigated. Images and videos recorded by high-speed cameras are commonly used for tracking rockfall trajectories during smallscale laboratory and large-scale field studies (e.g., Asteriou & Tsiambaos, 2018; Azzoni & De Freitas, 1995; Dorren et al, 2005; Giacomini et al, 2012; Giani et al, 2004; Glover, 2015; Spadari et al, 2012; Volkwein & Klette, 2014). To obtain the rotational velocity, two or more features including the centre of gravity (CofG), have to be tracked (Shum & Komura, 2005). This allows estimation of the velocity around the axis of rotation.
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