Ros 消息结构1
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1、ROS的消息头信息
#Standard metadata for higher-level flow data types#sequence ID: consecutively increasing ID uint32 seq #Two-integer timestamp that is expressed as:# * stamp.secs: seconds (stamp_secs) since epoch# * stamp.nsecs: nanoseconds since stamp_secs# time-handling sugar is provided by the client librarytime stamp #Frame this data is associated with# 0: no frame# 1: global framestring frame_id
以上是标准头信息的主要部分。seq是消息的顺序标识,不需要手动设置,发布节点在发布消息时,会自动累加。stamp 是消息中与数据相关联的时间戳,例如激光数据中,时间戳对应激光数据的采集时间点。frame_id 是消息中与数据相关联的参考系id,例如在在激光数据中,frame_id对应激光数据采集的参考系。
2.1、激光消息的结构
针对激光雷达,ROS在sensor_msgs 包中定义了专用了数据结构来存储激光消息的相关信息,成为LaserScan。LaserScan消息的格式化定义,为虚拟的激光雷达数据采集提供了方便,在我们讨论如何使用他之前,来看看该消息的结构是什么样的:
## Laser scans angles are measured counter clockwise, with 0 facing forward# (along the x-axis) of the device frame# Header headerfloat32 angle_min # start angle of the scan [rad]float32 angle_max # end angle of the scan [rad]float32 angle_increment # angular distance between measurements [rad]float32 time_increment # time between measurements [seconds]float32 scan_time # time between scans [seconds]float32 range_min # minimum range value [m]float32 range_max # maximum range value [m]float32[] ranges # range data [m] (Note: values < range_min or > range_max should be discarded)float32[] intensities # intensity data [device-specific units]
备注中已经详细介绍了每个参数的意义。
2.2、通过代码发布LaserScan消息
使用ROS发布LaserScan格式的激光消息非常简洁,下边是一个简单的例程:
#include <ros/ros.h>#include <sensor_msgs/LaserScan.h> int main(int argc, char** argv){ ros::init(argc, argv, "laser_scan_publisher"); ros::NodeHandle n; ros::Publisher scan_pub = n.advertise<sensor_msgs::LaserScan>("scan", 50); unsigned int num_readings = 100; double laser_frequency = 40; double ranges[num_readings]; double intensities[num_readings]; int count = 0; ros::Rate r(1.0); while(n.ok()){ //generate some fake data for our laser scan for(unsigned int i = 0; i < num_readings; ++i){ ranges[i] = count; intensities[i] = 100 + count; } ros::Time scan_time = ros::Time::now(); //populate the LaserScan message sensor_msgs::LaserScan scan; scan.header.stamp = scan_time; scan.header.frame_id = "laser_frame"; scan.angle_min = -1.57; scan.angle_max = 1.57; scan.angle_increment = 3.14 / num_readings; scan.time_increment = (1 / laser_frequency) / (num_readings); scan.range_min = 0.0; scan.range_max = 100.0; scan.ranges.resize(num_readings); scan.intensities.resize(num_readings); for(unsigned int i = 0; i < num_readings; ++i){ scan.ranges[i] = ranges[i]; scan.intensities[i] = intensities[i]; } scan_pub.publish(scan); ++count; r.sleep(); }}
我们将代码分解以便于分析:
#include <sensor_msgs/LaserScan.h>
首先我们需要先包含Laserscan的数据结构。
ros::Publisher scan_pub = n.advertise<sensor_msgs::LaserScan>("scan", 50);
创建一个发布者,以便于后边发布针对scan主题的Laserscan消息。
unsigned int num_readings = 100; double laser_frequency = 40; double ranges[num_readings]; double intensities[num_readings]; int count = 0; ros::Rate r(1.0); while(n.ok()){ //generate some fake data for our laser scan for(unsigned int i = 0; i < num_readings; ++i){ ranges[i] = count; intensities[i] = 100 + count; } ros::Time scan_time = ros::Time::now();
这里的例程中我们虚拟一些激光雷达的数据,如果使用真是的激光雷达,这部分数据需要从驱动中获取。
//populate the LaserScan message sensor_msgs::LaserScan scan; scan.header.stamp = scan_time; scan.header.frame_id = "laser_frame"; scan.angle_min = -1.57; scan.angle_max = 1.57; scan.angle_increment = 3.14 / num_readings; scan.time_increment = (1 / laser_frequency) / (num_readings); scan.range_min = 0.0; scan.range_max = 100.0; scan.ranges.resize(num_readings); scan.intensities.resize(num_readings); for(unsigned int i = 0; i < num_readings; ++i){ scan.ranges[i] = ranges[i]; scan.intensities[i] = intensities[i]; }
创建scan_msgs::LaserScan数据类型的变量scan,把我们之前伪造的数据填入格式化的消息结构中。
scan_pub.publish(scan);
数据填充完毕后,通过前边定义的发布者,将数据发布。
3.1、点云消息的结构
#This message holds a collection of 3d points, plus optional additional information about each point. #Each Point32 should be interpreted as a 3d point in the frame given in the header Header header geometry_msgs/Point32[] points #Array of 3d points ChannelFloat32[] channels #Each channel should have the same number of elements as points array, and the data in each channel should correspond 1:1 with each point
如上所示,点云消息的结构支持存储三维环境的点阵列,而且channels参数中,可以设置这些点云相关的数据,例如可以设置一个强度通道,存储每个点的数据强度,还可以设置一个系数通道,存储每个点的反射系数,等等。
3.2、通过代码发布点云数据
ROS发布点云数据同样简洁:
#include <ros/ros.h>#include <sensor_msgs/PointCloud.h> int main(int argc, char** argv){ ros::init(argc, argv, "point_cloud_publisher"); ros::NodeHandle n; ros::Publisher cloud_pub = n.advertise<sensor_msgs::PointCloud>("cloud", 50); unsigned int num_points = 100; int count = 0; ros::Rate r(1.0); while(n.ok()){ sensor_msgs::PointCloud cloud; cloud.header.stamp = ros::Time::now(); cloud.header.frame_id = "sensor_frame"; cloud.points.resize(num_points); //we'll also add an intensity channel to the cloud cloud.channels.resize(1); cloud.channels[0].name = "intensities"; cloud.channels[0].values.resize(num_points); //generate some fake data for our point cloud for(unsigned int i = 0; i < num_points; ++i){ cloud.points[i].x = 1 + count; cloud.points[i].y = 2 + count; cloud.points[i].z = 3 + count; cloud.channels[0].values[i] = 100 + count; } cloud_pub.publish(cloud); ++count; r.sleep(); }}
分解代码来分析:
#include <sensor_msgs/PointCloud.h>
首先也是要包含sensor_msgs/PointCloud消息结构。
ros::Publisher cloud_pub = n.advertise<sensor_msgs::PointCloud>("cloud", 50);
定义一个发布点云消息的发布者。
sensor_msgs::PointCloud cloud;cloud.header.stamp = ros::Time::now();cloud.header.frame_id = "sensor_frame";
为点云消息填充头信息,包括时间戳和相关的参考系id。
cloud.points.resize(num_points);
设置存储点云数据的空间大小。
//we'll also add an intensity channel to the cloudcloud.channels.resize(1);cloud.channels[0].name = "intensities";cloud.channels[0].values.resize(num_points);
设置一个名为“intensity“的强度通道,并且设置存储每个点强度信息的空间大小。
//generate some fake data for our point cloud for(unsigned int i = 0; i < num_points; ++i){ cloud.points[i].x = 1 + count; cloud.points[i].y = 2 + count; cloud.points[i].z = 3 + count; cloud.channels[0].values[i] = 100 + count; }
将我们伪造的数据填充到点云消息结构当中。
cloud_pub.publish(cloud);
最后,发布点云数据。
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