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This feature is used to enhance the quality of the fluid's → weight maps (the description is for C4D, but the concept is exactly the same for 3DS).
Weights (3DS, C4D)
If you have loaded two or more particle sequences the RFRK is capable of calculating the amount of interaction between the different fluids. This is of particular interest with mixing fluids, because with the “Weights” channel you will be able to create blending colours when the fluids are mixing. Under → “How to... Use Weight Maps?” you will find a complete workflow description of how to achieve this effect.
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