When Soft Cloth Physics are applied to cloth that has multiple materials, the system requests that you select a weight map for each material. This is fine. However, the system uses the same weight map validation as it would use for a single material which is not correct.
When dealing with a single material, a weight map that is completely black does not make sense because then the entire cloth is rigid and thus there are no soft cloth physics to be applied. Agreed. As such the system has a validation which check to see if the weight map is completely black and does not allow it if it is.
However, the same is not true for a cloth of multiple materials. For example, a fringe skirt which has been divided into two materials (one for the base of the skirt and one for the fringe) would likely has a totally black weight map for the based part of the skirt and the totally white or gradient white for the fringe material. Currently, however, the system does not allow you to apply a totally black weight map.
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Bowser and Blue, Busting The Breast