teddy_controller_advance.dart

  @override
  bool advance(FlutterActorArtboard artboard, double elapsed) {
    super.advance(artboard, elapsed);
    Vec2D targetTranslation;
    if (_hasFocus) {
      // Get caret in Flare world space.
      Vec2D.transformMat2D(_caretWorld, _caretGlobal, _globalToFlareWorld);

      // To make it more interesting, we'll also add a sinusoidal vertical offset.
      _caretWorld[1] +=
          sin(new DateTime.now().millisecondsSinceEpoch / 300.0) * 70.0;

      // Compute direction vector.
      Vec2D toCaret = Vec2D.subtract(Vec2D(), _caretWorld, _faceOrigin);
      Vec2D.normalize(toCaret, toCaret);
      Vec2D.scale(toCaret, toCaret,
          _projectGaze); //increase or decrease the projectgaze according to the extent of expressions you want

      // Compute the transform that gets us in face "ctrl_face" space.
      Mat2D toFaceTransform = Mat2D();
      if (Mat2D.invert(toFaceTransform, _faceControl.parent.worldTransform)) {
        // Put toCaret in local space, note we're using a direction vector
        // not a translation so transform without translation
        Vec2D.transformMat2(toCaret, toCaret, toFaceTransform);
        // Our final "ctrl_face" position is the original face translation plus this direction vector
        targetTranslation = Vec2D.add(Vec2D(), toCaret, _faceOriginLocal);
      }
    } else {
      targetTranslation = Vec2D.clone(_faceOriginLocal);
    }

    // We could just set _faceControl.translation to targetTranslation, but we want to animate it smoothly to this target
    // so we interpolate towards it by a factor of elapsed time in order to maintain speed regardless of frame rate.
    Vec2D diff =
        Vec2D.subtract(Vec2D(), targetTranslation, _faceControl.translation);
    Vec2D frameTranslation = Vec2D.add(Vec2D(), _faceControl.translation,
        Vec2D.scale(diff, diff, min(1.0, elapsed * 5.0)));

    _faceControl.translation = frameTranslation;

    return true;
  }