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This is based on the paper "Detail Preserving Continuum Simulation of Straight Hair" (McAdams, Selle, Ward, 2009) The main difference is that hair line segments are used rather than only rasterizing velocity at the vertices. This gives a much better coverage of the hair volume grid, otherwise gaps can be produced at smaller grid cell sizes and the distribution is uneven along the hair curve. The algorithm for rasterizing is a variation of Bresenham's algorithm extended onto 3D grids. Conflicts: source/blender/physics/intern/BPH_mass_spring.cpp