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RealFlow | Cinema 4D's fluid and material engines are cell-based and therefore, colliding objects have to be rasterized internally. The smaller the cells, the better and more accurate the collision, but at the cost of longer simulation times. By default, "Cell Size" is calculated automatically based on the "Collision Geometry Detail" level. For manual control uncheck "Auto". With "Auto" turned off, "Cell size" can be animated. This allows you to refine collision detection over time and speed up the simulation, e.g. from coarse to accurate.
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