private static FrameVector3D difference(double dt, ReferenceFrame frame, FrameTuple3DBasics before, FrameTuple3DBasics after) { after.checkReferenceFrameMatch(before); FrameVector3D v = new FrameVector3D(after.getReferenceFrame()); v.sub(after, before); v.scale(1 / (2.0 * dt)); v.changeFrame(frame); return v; }