cartographer重要模块——motion_filter

运动滤波，间隔一段时间、一段距离和角度，对odometry的数据进行关键帧采样。

  absl::optional<MotionFilter> pose_graph_odometry_motion_filter;
  if (trajectory_options.has_pose_graph_odometry_motion_filter()) {
    LOG(INFO) << "Using a motion filter for adding odometry to the pose graph.";
    pose_graph_odometry_motion_filter.emplace(
        MotionFilter(trajectory_options.pose_graph_odometry_motion_filter()));
  }

具体可以看看MotionFilter，判断两帧数据是否相似，以减少重复数据。在静止时，避免插入scan数据等操作。

//间隔时间，间隔距离，间隔角度大于阈值，return false
bool MotionFilter::IsSimilar(const common::Time time,
                             const transform::Rigid3d& pose) {
  LOG_IF_EVERY_N(INFO, num_total_ >= 500, 500)
      << "Motion filter reduced the number of nodes to "
      << 100. * num_different_ / num_total_ << "%.";
  ++num_total_;
  if (num_total_ > 1 &&
      time - last_time_ <= common::FromSeconds(options_.max_time_seconds()) &&
      (pose.translation() - last_pose_.translation()).norm() <=
          options_.max_distance_meters() &&
      transform::GetAngle(pose.inverse() * last_pose_) <=
          options_.max_angle_radians()) {
    return true;
  }
  last_time_ = time;
  last_pose_ = pose;
  ++num_different_;
  return false;
}

这些阈值参数可以在cartographer/configuration_files/trajectory_builder_2d.lua文件中找到

  motion_filter = {
    max_time_seconds = 5.,
    max_distance_meters = 0.2,
    max_angle_radians = math.rad(1.),
  },