Most experiments study a model 2 dimensional (2D) granular system composed of circular discs as grains. While in cases of studying particle shape effect, discs are replaced by other shapes like ellipses, polygons and crosses \cite{}. With a digital camera, the first and one of the most important information that can be obtained is particle positions and particle orientations. The latter are informative even for circular discs due to existence of inter-particle friction in most experiments. Particle detection and tracking alone provide rich information such as particle configuration and flow field, and are necessary for further measurements like force-bearing contact detection and contact force calculation. In this session, a detailed description for detecting particle positions and orientations and tracking particles will be summarized below.
Most experiments study a model 2 dimensional (2D) granular system composed of circular discs as grains. While in cases of studying particle shape effect, discs are replaced by other shapes like ellipses, polygons and crosses \cite{}. With a digital camera, the first and one of the most important information that can be obtained is particle positions and particle orientations. The latter are informative even for circular discs due to existence of inter-particle friction in most experiments. Particle detection and tracking alone provide rich information such as particle configuration and flow field, and are necessary for further measurements like force-bearing contact detection and contact force calculation. In this session, a detailed description for detecting particle positions and orientations and tracking particles in high resolution images will be summarized below. In addition, in images with resolution not high enough to precisely track particles, another method (PIV) will be introduced.
Position detection and tracking
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* Detection of the particles from white light imaging
Thresholding
Hough transform
Circular Hough transform
Convolution method
* Detection of the particle from UV light imaging
Example of the star detection
Though being used most commonly in granular studies, discs are far from grains in reality. Hence there have been numerous studies on granular systems with different shapes other than discs, e.g., ellipses, polygons and star-shape-like particles. Here we choose star particles as an example to illustrate how to detect them from UV light imaging.
* Tracking
Direction measurement
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Thresholding
Hough transform
In order to detect particle orientation, we usually put an artificial UV ink bar on the surface of particles.
