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### 1.2 Detection of the particles orientation UV light imaging
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Due to particles being frictional in most experiments, particles will usually rotate if the granular system undergoes deformation. Hence tracking the rotation of particles is also important, which requires the knowledge of particle orientations. To achieve this, one can draw a bar with UV ink on the surface of the particle. This bar will leave no mark under white light, thus having no interference with particle center detection process as introduced above. Once illuminated by UV light with white light off, only the UV bars are visible and can be recorded by camera. Due to high contrast in this situation (blue for UV bars and black for background), the image can be easily binarized with [Matlab function imbinarize](https://www.mathworks.com/help/images/ref/imbinarize.html) so that the UV bars are 1 and the rest of the particle is 0. With previously determined particle center and diameter, each bar can be associated with its corresponding particle. With the positions of all the pixels inside the bar, a least squares fit with the minimized perpendicular offsets reveals a linear function between the x and y positions in each UV bar. The slope of the line gives the orientation associated with each particle. An example illustrating this procedure is shown below.
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Due to particles being frictional in most experiments, particles will usually rotate if the granular system undergoes deformation. Hence tracking the rotation of particles is also important, which requires the knowledge of particle orientations. To achieve this, one can draw a bar with UV ink on the surface of the particle. This bar will leave no mark under white light, thus having no interference with particle center detection process as introduced above. Once illuminated by UV light with white light off, only the UV bars are visible and can be recorded by camera. Due to high contrast in this situation (blue for UV bars and black for background), the image can be easily binarized with [Matlab function imbinarize](https://www.mathworks.com/help/images/ref/imbinarize.html) so that the UV bars are 1 and the rest of the particle is 0. With previously determined particle center and diameter, each bar can be associated with its corresponding particle. With the positions of all the pixels inside the bar, [a least squares fit with the minimized perpendicular offsets](http://mathworld.wolfram.com/LeastSquaresFittingPerpendicularOffsets.html) reveals a linear function between the x and y positions in each UV bar. The slope of the line gives the orientation associated with each particle. An example illustrating this procedure is shown below.
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![Fig_DisksOri](uploads/9bfdd1555c24d731e6e00793149115df/Fig_DisksOri.png)
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