Apollo Planning学习（3）-------LANE_CHANGE_DECIDER

LANE_CHANGE_DECIDER

在这个decider可能会有很多人陷入误区，认为Apollo在规划中换道的时候是有一个主动请求的，这里引用知乎上iGear大佬的解释：Apollo的都是自己计算换道时机和换道安全，一般没有主动换道请求，一般人可能会认为换道要有明确的时机，类似于有一个明确的状态，这个状态位true了就要换道。但Apollo里面没有明确的换道时机，就像人开车一样，觉得安全且有必要就去执行某一个动作。这里大家可以反复看看Apollo的规划流程。这节的lane_change_decider只是计算换道状态，后面我们还会计算换道的boundary，然后再规划换道轨迹，不是一个逻辑或者一个task就能实现换道的。就算计算出来了换道boundary也不一定就能生成换道轨迹，生成不了换道轨迹还是实现不了换道。不是先有了换道的必要性才去计算这些，而是根据这些计算来判断最终是否安全且有必要去换道。（个人被这句话点醒，膜拜大佬）
也就是说可以将该decider的大致作用是对记录ADC此时的换道状态，并不决定ADC是否进行lanechange。车的换道状态信息存于 injector_->planning_context()->mutable_planning_status()->mutable_change_lane()中，此dicider中出现最多的就是以下代码

// 此函数的主要作用是更新状态
void LaneChangeDecider::UpdateStatus(double timestamp,
                                     ChangeLaneStatus::Status status_code,
                                     const std::string& path_id) {
  auto* lane_change_status = injector_->planning_context()
                                 ->mutable_planning_status()
                                 ->mutable_change_lane();
  lane_change_status->set_timestamp(timestamp);
  lane_change_status->set_path_id(path_id);
  lane_change_status->set_status(status_code);
}

// added a dummy parameter to enable this task in ExecuteTaskOnReferenceLine
// 添加了一个伪参数以在ExecuteTaskOnReferenceLine中启用此任务
// 这个ExecuteTaskOnReferenceLine在\modules\planning\scenarios\stage.cc目录下有具体内容
Status LaneChangeDecider::Process(
    Frame* frame, ReferenceLineInfo* const current_reference_line_info) {
  // Sanity checks.
  CHECK_NOTNULL(frame);

  const auto& lane_change_decider_config = config_.lane_change_decider_config();

// 通过frame拿到车辆此时所在的区域参考线个数
  std::list<ReferenceLineInfo>* reference_line_info =
      frame->mutable_reference_line_info();
// 如果没有参考线则提示错误  
  if (reference_line_info->empty()) {
    const std::string msg = "Reference lines empty.";
    AERROR << msg;
    return Status(ErrorCode::PLANNING_ERROR, msg);
  }
// 是否进行强制换道，如果是进入此函数，这里关于这个函数的解析在此之后
  if (lane_change_decider_config.reckless_change_lane()) {
    PrioritizeChangeLane(true, reference_line_info);
    return Status::OK();
  }

  auto* prev_status = injector_->planning_context()
                          ->mutable_planning_status()
                          ->mutable_change_lane();
  double now = Clock::NowInSeconds();
  // 默认设置false
  prev_status->set_is_clear_to_change_lane(false);
//此处判断传进来的referenceLineinfo是否是变道参考线，如果是则通过
// IsChangeLanePath():判断是否是可变车道，如果车不在车道片段上，则该车道为可变道车道。
  if (current_reference_line_info->IsChangeLanePath()) {
//IsClearToChangeLane()检查该参考线是否满足变道条件，
//IsClearToChangeLane只考虑传入的参考线上的动态障碍物，不考虑虚的和静态的障碍物。疑点：为什么只/考虑动态障碍物
// 后面介绍 IsClearToChangeLane（...）
    prev_status->set_is_clear_to_change_lane(
        IsClearToChangeLane(current_reference_line_info));
  }

//头次进入task，车道换道状态应该为空，默认设置为换道结束状态
  if (!prev_status->has_status()) {
    UpdateStatus(now, ChangeLaneStatus::CHANGE_LANE_FINISHED,
                 GetCurrentPathId(*reference_line_info));
    prev_status->set_last_succeed_timestamp(now);
    return Status::OK();
  }

// 判断参考线数量
  bool has_change_lane = reference_line_info->size() > 1;
  ADEBUG << "has_change_lane: " << has_change_lane;
// 如果只有一条参考线（比如往某个方向只有一条车道），那就通过updatestatus将车辆状态设置为CHANGE_LANE_FINISHED，
// 这也符合我们认知，单向只有一条车道，还换什么道，所以车辆就该一直处于换到结束的状态
  if (!has_change_lane) {
    //没有换道参考线（参考线数量小于1条）：如果上个周期状态是已经换道完成或者换道失败，
    //则返回进入下个task或者下个周期；如果上个周期状态是正在换道，更新换道状态
    const auto& path_id = reference_line_info->front().Lanes().Id();
    if (prev_status->status() == ChangeLaneStatus::CHANGE_LANE_FINISHED) {
    } else if (prev_status->status() == ChangeLaneStatus::IN_CHANGE_LANE) {
      UpdateStatus(now, ChangeLaneStatus::CHANGE_LANE_FINISHED, path_id);
    } else if (prev_status->status() == ChangeLaneStatus::CHANGE_LANE_FAILED) {
    } else {
      const std::string msg =
          absl::StrCat("Unknown state: ", prev_status->ShortDebugString());
      AERROR << msg;
      return Status(ErrorCode::PLANNING_ERROR, msg);
    }
    return Status::OK();
  } 
  // 
//下面的else处理不止一条参考线的情况，正常道路都不止一条参考线，
//主要逻辑为状态切换，实际操作还是通过updatestatus来实时更新车辆的换道状态。
  else {  // has change lane in reference lines.
    auto current_path_id = GetCurrentPathId(*reference_line_info);
    if (current_path_id.empty()) {
      const std::string msg = "The vehicle is not on any reference line";
      AERROR << msg;
      return Status(ErrorCode::PLANNING_ERROR, msg);
    }
    if (prev_status->status() == ChangeLaneStatus::IN_CHANGE_LANE) {
      if (prev_status->path_id() == current_path_id) {
        PrioritizeChangeLane(true, reference_line_info);
      } else {
        // RemoveChangeLane(reference_line_info);
        PrioritizeChangeLane(false, reference_line_info);
        ADEBUG << "removed change lane.";
        UpdateStatus(now, ChangeLaneStatus::CHANGE_LANE_FINISHED,
                     current_path_id);
      }
      return Status::OK();
    } else if (prev_status->status() == ChangeLaneStatus::CHANGE_LANE_FAILED) {
      // TODO(SHU): add an optimization_failure counter to enter
      // change_lane_failed status
      if (now - prev_status->timestamp() <
          lane_change_decider_config.change_lane_fail_freeze_time()) {
        // RemoveChangeLane(reference_line_info);
        PrioritizeChangeLane(false, reference_line_info);
        ADEBUG << "freezed after failed";
      } else {
        UpdateStatus(now, ChangeLaneStatus::IN_CHANGE_LANE, current_path_id);
        ADEBUG << "change lane again after failed";
      }
      return Status::OK();
    } else if (prev_status->status() ==
               ChangeLaneStatus::CHANGE_LANE_FINISHED) {
      if (now - prev_status->timestamp() <
          lane_change_decider_config.change_lane_success_freeze_time()) {
        // RemoveChangeLane(reference_line_info);
        PrioritizeChangeLane(false, reference_line_info);
        ADEBUG << "freezed after completed lane change";
      } else {
        PrioritizeChangeLane(true, reference_line_info);
        UpdateStatus(now, ChangeLaneStatus::IN_CHANGE_LANE, current_path_id);
        ADEBUG << "change lane again after success";
      }
    } else {
      const std::string msg =
          absl::StrCat("Unknown state: ", prev_status->ShortDebugString());
      AERROR << msg;
      return Status(ErrorCode::PLANNING_ERROR, msg);
    }
  }
  return Status::OK();
}

这里关于参考线不止一条时的主要换道逻辑参考知乎iGear大佬的一张逻辑图如下：

//当is_prioritize_change_lane为true,则遍历存储referenceLineInfo的链表，把
//当前车辆不位于的的那条（俗称换到参考线）放到链表的第一个位置
//当is_prioritize_change_lane为false,则遍历存储referenceLineInfo的链表，把
//当前车辆所位于的的那条放到链表的第一个位置
//首先获取第一条参考线的迭代器，然后遍历所有的参考线
//注意，可变车道为按迭代器的顺序求取，一旦发现可变车道，即推出循环。
//很多博主认为此函数意义不大，本人没什么见解只是学习ing
void LaneChangeDecider::PrioritizeChangeLane(
    const bool is_prioritize_change_lane,
    std::list<ReferenceLineInfo>* reference_line_info) const {

  if (reference_line_info->empty()) {
    AERROR << "Reference line info empty";
    return;
  }

  const auto& lane_change_decider_config = config_.lane_change_decider_config();

  // TODO(SHU): disable the reference line order change for now
  if (!lane_change_decider_config.enable_prioritize_change_lane()) {
    return;
  }
  auto iter = reference_line_info->begin();
  while (iter != reference_line_info->end()) {
    ADEBUG << "iter->IsChangeLanePath(): " << iter->IsChangeLanePath();
    /* is_prioritize_change_lane == true: prioritize change_lane_reference_line
       is_prioritize_change_lane == false: prioritize
       non_change_lane_reference_line */
    if ((is_prioritize_change_lane && iter->IsChangeLanePath()) ||
        (!is_prioritize_change_lane && !iter->IsChangeLanePath())) {
      ADEBUG << "is_prioritize_change_lane: " << is_prioritize_change_lane;
      ADEBUG << "iter->IsChangeLanePath(): " << iter->IsChangeLanePath();
      break;
    }
    ++iter;
  }
  reference_line_info->splice(reference_line_info->begin(),
                              *reference_line_info, iter);
  ADEBUG << "reference_line_info->IsChangeLanePath(): "
         << reference_line_info->begin()->IsChangeLanePath();
}

解析函数LaneChangeDecider::IsClearToChangeLane(…)

//调选出位于该referenceline上的动态障碍物，结合障碍物的运动方向和车的运动方向，
//检查每个障碍物与车的前后距离，看是否都满足安全阈值。只要有一个动态障碍物不满足条件
//该referenceline就不满足换道条件。prev_status->set_is_clear_to_change_lane(false)
bool LaneChangeDecider::IsClearToChangeLane(
    ReferenceLineInfo* reference_line_info) {
  double ego_start_s = reference_line_info->AdcSlBoundary().start_s();
  double ego_end_s = reference_line_info->AdcSlBoundary().end_s();
  double ego_v =
      std::abs(reference_line_info->vehicle_state().linear_velocity());

  for (const auto* obstacle :
       reference_line_info->path_decision()->obstacles().Items()) {
    // 排除虚拟障碍物和静态障碍物
    if (obstacle->IsVirtual() || obstacle->IsStatic()) {
      ADEBUG << "skip one virtual or static obstacle";
      continue;
    }

    double start_s = std::numeric_limits<double>::max();
    double end_s = -std::numeric_limits<double>::max();
    double start_l = std::numeric_limits<double>::max();
    double end_l = -std::numeric_limits<double>::max();

    for (const auto& p : obstacle->PerceptionPolygon().points()) {
      SLPoint sl_point;
      reference_line_info->reference_line().XYToSL(p, &sl_point);
      start_s = std::fmin(start_s, sl_point.s());
      end_s = std::fmax(end_s, sl_point.s());

      start_l = std::fmin(start_l, sl_point.l());
      end_l = std::fmax(end_l, sl_point.l());
    }

    if (reference_line_info->IsChangeLanePath()) {
      double left_width(0), right_width(0);
      reference_line_info->mutable_reference_line()->GetLaneWidth(
          (start_s + end_s) * 0.5, &left_width, &right_width);
    //只考虑在reference_line_info所在的车道的障碍物
      if (end_l < -right_width || start_l > left_width) {
        continue;
      }
    }

    // Raw estimation on whether same direction with ADC or not based on
    // prediction trajectory
    // 基于预测轨迹的与ADC方向是否相同的原始估计
    bool same_direction = true;
    if (obstacle->HasTrajectory()) {
      double obstacle_moving_direction =
          obstacle->Trajectory().trajectory_point(0).path_point().theta();
      const auto& vehicle_state = reference_line_info->vehicle_state();
      double vehicle_moving_direction = vehicle_state.heading();
      if (vehicle_state.gear() == canbus::Chassis::GEAR_REVERSE) {
        vehicle_moving_direction =
            common::math::NormalizeAngle(vehicle_moving_direction + M_PI);
      }
      double heading_difference = std::abs(common::math::NormalizeAngle(
          obstacle_moving_direction - vehicle_moving_direction));
      same_direction = heading_difference < (M_PI / 2.0);
    }

    // TODO(All) move to confs
    static constexpr double kSafeTimeOnSameDirection = 3.0;
    static constexpr double kSafeTimeOnOppositeDirection = 5.0;
    static constexpr double kForwardMinSafeDistanceOnSameDirection = 10.0;
    static constexpr double kBackwardMinSafeDistanceOnSameDirection = 10.0;
    static constexpr double kForwardMinSafeDistanceOnOppositeDirection = 50.0;
    static constexpr double kBackwardMinSafeDistanceOnOppositeDirection = 1.0;
    static constexpr double kDistanceBuffer = 0.5;

    double kForwardSafeDistance = 0.0;
    double kBackwardSafeDistance = 0.0;
    if (same_direction) {
      kForwardSafeDistance =
          std::fmax(kForwardMinSafeDistanceOnSameDirection,
                    (ego_v - obstacle->speed()) * kSafeTimeOnSameDirection);
      kBackwardSafeDistance =
          std::fmax(kBackwardMinSafeDistanceOnSameDirection,
                    (obstacle->speed() - ego_v) * kSafeTimeOnSameDirection);
    } else {
      kForwardSafeDistance =
          std::fmax(kForwardMinSafeDistanceOnOppositeDirection,
                    (ego_v + obstacle->speed()) * kSafeTimeOnOppositeDirection);
      kBackwardSafeDistance = kBackwardMinSafeDistanceOnOppositeDirection;
    }

/*
根据前面计算的阈值，判断障碍物是否安全，采用的是滞回区间的方法，如果障碍物小于安全距离，laneChangeBlocking为true。
如果障碍物大于安全距离，laneChangeBlocking为false。通过滞回区间进行滤波。
一旦发现有block的障碍物，函数就返回，就认为该Reference 非clear(安全)。
*/
    if (HysteresisFilter(ego_start_s - end_s, kBackwardSafeDistance,
                         kDistanceBuffer, obstacle->IsLaneChangeBlocking()) &&
        HysteresisFilter(start_s - ego_end_s, kForwardSafeDistance,
                         kDistanceBuffer, obstacle->IsLaneChangeBlocking())) {
      reference_line_info->path_decision()
          ->Find(obstacle->Id())
          ->SetLaneChangeBlocking(true);
      ADEBUG << "Lane Change is blocked by obstacle" << obstacle->Id();
      return false;
    } else {
      reference_line_info->path_decision()
          ->Find(obstacle->Id())
          ->SetLaneChangeBlocking(false);
    }
  }
  return true;
}

关于LaneChangeDecider::HysteresisFilter(…)

bool LaneChangeDecider::HysteresisFilter(const double obstacle_distance,
                                         const double safe_distance,
                                         const double distance_buffer,
                                         const bool is_obstacle_blocking) {
  if (is_obstacle_blocking) {
    return obstacle_distance < safe_distance + distance_buffer;
  } else {
    return obstacle_distance < safe_distance - distance_buffer;
  }
}