code: public void getLocalMatrix(int boneindex, Matrix4f mtx)
      {
          Bone bone = mBones[boneindex];
  
          if ((bone.Changed & (WORLD_ROT | WORLD_POS)) != 0)
          {
              calcLocal(bone, mSkeleton.getParentBoneIndex(boneindex));
          }
          bone.getLocalMatrix(mtx);
      }
docs: Get the local rotation matrix for this bone (relative to parent).
  @param mtx		where to store bone matrix.
  @param boneindex	zero based index of bone to get matrix for.
  @return local matrix for the designated bone.
  
  @see #getWorldRotation
  @see #getLocalRotation
  @see GVRSkeleton#setBoneAxis