When calling simplify(), always include the root joint.

Summary: Because we don't pass any arguments to simplify() and it just calls parametersToJoints, we need it to do the right thing and include the root joint regardless of whether it's being driven.  Otherwise the simplifySkeleton call fails and this is not an expected result.  Note that this wasn't a problem before because we always had parameters driving the root but it's no longer the case for Tinity v4 and above since all transform parameters have been moved to b_root.

Reviewed By: jeongseok-meta

Differential Revision: D68297713

fbshipit-source-id: 18a80cab0d1b63012fb786066c85aaefa931532b

momentum/character/character.cpp

@@ -369,7 +369,16 @@ CharacterT<T> CharacterT<T>::simplifyParameterTransform(const ParameterSet& para
 
 template <typename T>
 CharacterT<T> CharacterT<T>::simplify(const ParameterSet& activeParams) const {
-  return simplifySkeleton(parametersToActiveJoints(activeParams));
+  auto activeJoints = parametersToActiveJoints(activeParams);
+  // always keep the root joint to ensure valid simplification.
+  // This is needed because while previously we generally parametrized the root joint with
+  // the root transform, modern skeletons sometimes have a body_world above b_root that is
+  // not parametrized, but if we throw it out we end up with an invalid character.
+  if (!activeJoints.empty()) {
+    activeJoints[0] = true;
+  }
+
+  return simplifySkeleton(activeJoints);
 }
 
 template <typename T>