LOAM_velodyne学习（四）

TransformMaintenance

来到了最后一个模块，代码不是很长，我们在看完代码之后，再详细说明这个模块的功能

依然主函数开始

int main(int argc, char** argv)
{
  ros::init(argc, argv, "transformMaintenance");
  ros::NodeHandle nh;

  //订阅了两个节点
  ros::Subscriber subLaserOdometry = nh.subscribe<nav_msgs::Odometry> 
                                     ("/laser_odom_to_init", 5, laserOdometryHandler);

  ros::Subscriber subOdomAftMapped = nh.subscribe<nav_msgs::Odometry> 
                                     ("/aft_mapped_to_init", 5, odomAftMappedHandler);

  //发布一个节点
  ros::Publisher pubLaserOdometry2 = nh.advertise<nav_msgs::Odometry> ("/integrated_to_init", 5);
  pubLaserOdometry2Pointer = &pubLaserOdometry2;
  laserOdometry2.header.frame_id = "/camera_init";
  laserOdometry2.child_frame_id = "/camera";

  tf::TransformBroadcaster tfBroadcaster2;
  tfBroadcaster2Pointer = &tfBroadcaster2;
  laserOdometryTrans2.frame_id_ = "/camera_init";
  laserOdometryTrans2.child_frame_id_ = "/camera";

  ros::spin();

  return 0;
}

主函数非常简单，可以看出integrated_to_init消息是由发布器pubLaserOdometry2发布的，实际上就是由发布器pubLaserOdometry2Pointer发布的。我们找到pubLaserOdometry2Pointer，发现回调函数laserOdometryHandler就是pubLaserOdometry2Pointer的发布函数，这意味着，发现每次接收到laser_odom_to_init消息并调用回调函数laserOdometryHandler时，就发布一次integrated_to_init消息。

看来重点是这个回调函数laserOdometryHandler，我们来仔细看看

void laserOdometryHandler(const nav_msgs::Odometry::ConstPtr& laserOdometry)
{
  double roll, pitch, yaw;
  //对收到的消息进行解析，得到transformSum
  geometry_msgs::Quaternion geoQuat = laserOdometry->pose.pose.orientation;
  tf::Matrix3x3(tf::Quaternion(geoQuat.z, -geoQuat.x, -geoQuat.y, geoQuat.w)).getRPY(roll, pitch, yaw);

  transformSum[0] = -pitch;
  transformSum[1] = -yaw;
  transformSum[2] = roll;

  transformSum[3] = laserOdometry->pose.pose.position.x;
  transformSum[4] = laserOdometry->pose.pose.position.y;
  transformSum[5] = laserOdometry->pose.pose.position.z;

  //位姿更新
  transformAssociateToMap();

  //位姿信息进行存储，准备发布
  geoQuat = tf::createQuaternionMsgFromRollPitchYaw
            (transformMapped[2], -transformMapped[0], -transformMapped[1]);

  laserOdometry2.header.stamp = laserOdometry->header.stamp;
  laserOdometry2.pose.pose.orientation.x = -geoQuat.y;
  laserOdometry2.pose.pose.orientation.y = -geoQuat.z;
  laserOdometry2.pose.pose.orientation.z = geoQuat.x;
  laserOdometry2.pose.pose.orientation.w = geoQuat.w;
  laserOdometry2.pose.pose.position.x = transformMapped[3];
  laserOdometry2.pose.pose.position.y = transformMapped[4];
  laserOdometry2.pose.pose.position.z = transformMapped[5];
  pubLaserOdometry2Pointer->publish(laserOdometry2);

  laserOdometryTrans2.stamp_ = laserOdometry->header.stamp;
  laserOdometryTrans2.setRotation(tf::Quaternion(-geoQuat.y, -geoQuat.z, geoQuat.x, geoQuat.w));
  laserOdometryTrans2.setOrigin(tf::Vector3(transformMapped[3], transformMapped[4], transformMapped[5]));
  tfBroadcaster2Pointer->sendTransform(laserOdometryTrans2);
}

但是，这里还是有个小坑的。这个节点接收了两个消息，分别是laserOdometry节点和laserMapping节点发布的，而这两个节点发布的频率不同，那么是怎么处理的呢？

我们仔细看一看剩下的一个回调函数

void odomAftMappedHandler(const nav_msgs::Odometry::ConstPtr& odomAftMapped)
{
  double roll, pitch, yaw;
  geometry_msgs::Quaternion geoQuat = odomAftMapped->pose.pose.orientation;
  tf::Matrix3x3(tf::Quaternion(geoQuat.z, -geoQuat.x, -geoQuat.y, geoQuat.w)).getRPY(roll, pitch, yaw);

  transformAftMapped[0] = -pitch;
  transformAftMapped[1] = -yaw;
  transformAftMapped[2] = roll;

  transformAftMapped[3] = odomAftMapped->pose.pose.position.x;
  transformAftMapped[4] = odomAftMapped->pose.pose.position.y;
  transformAftMapped[5] = odomAftMapped->pose.pose.position.z;

  transformBefMapped[0] = odomAftMapped->twist.twist.angular.x;
  transformBefMapped[1] = odomAftMapped->twist.twist.angular.y;
  transformBefMapped[2] = odomAftMapped->twist.twist.angular.z;

  transformBefMapped[3] = odomAftMapped->twist.twist.linear.x;
  transformBefMapped[4] = odomAftMapped->twist.twist.linear.y;
  transformBefMapped[5] = odomAftMapped->twist.twist.linear.z;
}

也是很简单的解析函数，作用是在接收到了laserMapping的消息后，更新位姿，这里注意，laserMapping发布的是优化过后的位姿!看到这里，就逐渐能明白作者如何完成两个不同频率之间的协调了。

当接收到laserMapping的消息后，立马更新位姿，这样得到了优化的结果；而这个优化结果会被回调函数laserOdometryHandler​​​​​​​里的transformAssociateToMap这一个函数一直利用来建图，一直到下一次接收到laserMapping​​​​​​​的消息，再一次更新位姿，我们画图来说明：

也就是说，最后采用的位姿是TransformMaintenance发布的integrated_to_init消息。而且由上面的分析可知，TransformMaintenance的发布频率和laserOdometry的发布频率是一致的。