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By default, Multipoint particles are added at the same position as their source particles. With this parameter you can define a radius around a particle where the new particles are created. When the radius that is chosen is too big, it is very likely that the new particles interpret (?) interpenetrate solid objects in the render. To mention it again: Multipoint particles are only created at render time and then discarded. This value is given in metres [m].
Radius
Particles are rendered as spheres and here you can determine their radius. The default value is 0.006.
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