ROS 传感器消息

在使用ROS各个传感器消息之前，弄清楚各个传感器在ROS是如何表示的显得极为重要。特别是，Laserscan, PointCloud等用了很久之后，感觉已经很熟悉了，但是一些细节行的东西一直没有深究，并且对某些参数难以形成直觉上的认识，很大程度上影响了在与其他算法或者硬件衔接时的问题。这里，我单独的将Laserscan，PointCloud发布在rviz中，通过修改不同的ROS消息参数，直观的观察其在参数的作用。整个工程代码可以在此处下载：https://download.csdn.net/download/ziqian0512/10289936。

ROS 传感器消息之Laserscan

消息定义

首先，查看一下sensor_msgs/LaserScan.msg的定义，见ros.org/LaserScan.“`

Header header
float32 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]

在消息中需要特别说明的主要是ranges和intensities，ranges表示在某一个角度时，扫描到物体离自己的距离，这很容易理解。intensities则表示测量的强度，也就是激光所反射的亮度，通常来说，值越大，光反射越亮。也就可以用intensities做一些假设，推荐扫描到物体的材料。比如，Hokyuo Laser scanner就提供了反射的所有强度，而 Sick S300则只检测是否由反光带。

测试代码

以下为测试Laserscan源码，需要特别关注的num_readings（点数），ranges（距离），intensities（强度）参数在调节过程中，rviz中Laserscan的变化。编译以下代码后，还需要选择sensor_frame坐标系，和消息/scan。效果图如下：代码：

#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 = 10;
  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] = 10*i;
    }
    ros::Time scan_time = ros::Time::now();

    //populate the LaserScan message
    sensor_msgs::LaserScan scan;
    scan.header.stamp = scan_time;
    scan.header.frame_id = "sensor_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;
    ROS_WARN_STREAM("count"<<count);
    r.sleep();
  }
}