PBDFluids Doesn't use XPBD
PBDFluids constraint is basically a global constraint for efficiency it reimplements the base class API for computeValueAndGradient and projectConstraint and was never upgraded to use XPBD.
PBDFluids constraint is basically a global constraint for efficiency it reimplements the base class API for computeValueAndGradient and projectConstraint and was never upgraded to use XPBD.