def render_callback(self, frame_num):
    self.render_ground(
        size=[100, 100],
        color=[0.8, 0.8, 0.8, 1.0],
        axis="y",  # utils.axis_to_str(self.motions[0].skel.v_up_env),
        origin=not self.hide_origin,
        use_arrow=True,
    )

    colors = [
        np.array([123, 174, 85]) / 255.0,  # green
        np.array([255, 255, 0]) / 255.0,  # yellow
        np.array([85, 160, 173]) / 255.0,  # blue
    ]
    self._render_characters(colors, frame_num)
    color, depth = self.r.render(self.scene)
    return color