Apollo planning lane_change

PathReuseDecider 是lanefollow 场景下，所调用的第 1 个 task，它的作用主要是对车辆换道状态进行管理，即对车辆换道状态（IN_CHANGE_LANE 换道中、CHANGE_LANE_FINISHED 换道结束、CHANGE_LANE_FAILED 换道失败）之间的转换进行管理

入口参数：frame、current_reference_line_info

该decider的主要操作是对记录车此时的换道状态，并不决定车是否换道。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

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);
}

可以看到，UpdateStatus()的操作就是通过lane_change_status指针来对车辆换道状态进行实时更新。更新逻辑如下：

//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();
//没有参考线就return，因为apollo的后续所有tasks全部都依赖参考线来做轨迹规划
if (reference_line_info->empty()){const std::string msg = "Reference lines empty.";AERROR << msg;return Status(ErrorCode::PLANNING_ERROR, msg);}
...（省略的地方最后讲）
//判断参考线数量
bool has_change_lane = reference_line_info->size() > 1;
//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) {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，就连我们家小县城的马路都是单向3车道。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 {  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 statusif (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);}}

用图表示切换逻辑（在这里引用知乎的一张图）

PrioritizeChangeLane(const bool is_prioritize_change_lane,std::list<ReferenceLineInfo>* reference_line_info) 解析：

//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
//我很纳闷，这里的形参reference_line_info是从frame类里面拿来的，你在这里任意调换
//referenceLineInfo的位置，就不怕public_road_planner那边对参考线进行遍历的功能吗？
//还有，将referenceLineInfo在链表中的位置来回换的目的我还不清楚，个人粗浅的认为
//此PrioritizeChangeLane（）功能意义不大
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 nowif (!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_lineis_prioritize_change_lane == false: prioritizenon_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();
}

上文...省略的地方解析：

//当配置文件强制要求换道，即lane_change_decider_config.reckless_change_lane()为true，
//则调用PrioritizeChangeLane(true, reference_line_info)，将换道参考线放到链表第一位，
//直接return true。好奇连车的换道状态都不设置了吗？直接return了
if (lane_change_decider_config.reckless_change_lane()) {//若第一个参数为true,则将reference_line_info中的第一个变道车道参考线调到第一位//若第一个参数为false,则将reference_line_info中的第一个非变道车道参考线调到第一位PrioritizeChangeLane(true, reference_line_info);return Status::OK();}
//此处判断传进来的referenceLineinfo是否是变道参考线，如果是则通过
//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？
if (current_reference_line_info->IsChangeLanePath()) {//获取reference_line_info中存储的所有动态obstacle，一一的遍历这些obstacle//如果该障碍物不再reference_line_info所在的车道中，则不考虑//对于在此车道范围内的obstacle，通过分析该障碍物的位置与运动方向，结合车辆的位置来判断是否iscleartochangelaneprev_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));//获取非变道lane的idprev_status->set_last_succeed_timestamp(now);return Status::OK();}

IsClearToChangeLane（）解析：

//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) {//车辆轮廓起点double ego_start_s = reference_line_info->AdcSlBoundary().start_s();//车辆轮廓终点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()) {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 trajectorybool 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 confsstatic 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;}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;
}

总结与疑问：
lane_change_decider，我觉得应该叫车辆换道状态检测器，因为该decider通篇都在通过UpdateStatus来对injector_->planning_context()->mutable_planning_status()

->mutable_change_lane()信息进行实时更新。当然，顺便对proc传进来的current_reference_line_info通过IsClearToChangeLane()做了是否具备换道条件检测，是的话就额外的将injector_->planning_context()->mutable_planning_status()

->mutable_change_lane()->set_is_clear_to_change_lane设置为true,表面车辆此时周围有可以换道的参考线。这一信息很重要，后面的path_bound_decider会依据此标志位来确定regular_path_bound。

该决策器还有一个重点就是多参考线的换道逻辑状态图，这里有疑点:

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 {
//为什么CHANGE_LANE_FINISHED结束后，过个一定时间，又要切换到IN_CHANGE_LANE,
//就不能消停点吗？PrioritizeChangeLane(true, reference_line_info);UpdateStatus(now, ChangeLaneStatus::IN_CHANGE_LANE, current_path_id);ADEBUG << "change lane again after success";}

图片链接：Apollo规划模块详解（五）：算法实现-lane change decider - 知乎

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