ROS series: Chapter 5 common components rosbag

Five, ROS common components

2.rosbag

In ROS, a special tool is provided for data retention and reading implementation: rosbag;

concept:

A toolset for recording and playback of ROS topics.

effect:

Data reuse is realized, which is convenient for debugging and testing

Nature:

The essence of rosbag is also a node of ros. When recording, rosbag is a subscription node, which can subscribe topic messages and write the subscribed data to disk files; when playing back, rosbag is a publishing node, which can read disk files and publish files topic news;

2.1 rosbag use - command line

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Requirements: ROS built-in turtle, use rosbag to record the operation process, and realize replay after recording.

accomplish:

1. Create a directory to save the recorded files

mkdir ./bags
cd bags

2. Start recording

roscore 
rosrun turtlesim turtlesim_node
rosrun turtlesim turtle_teleop_key

Start the turtle GUI and the keyboard control node;

rosbag record -a -o bags/hello

Operate the little turtle to run for a period of time, and use ctrl+c to end the recording, and a bag suffix file will be generated in the created directory;

3. View files

rosbag info bags/hello_2022-11-16-21-32-41.bag 

path:        bags/hello_2022-11-16-21-32-41.bag
version:     2.0
duration:    21.3s
start:       Nov 16 2022 21:32:41.60 (1668605561.60)
end:         Nov 16 2022 21:33:02.89 (1668605582.89)
size:        216.7 KB
messages:    2902
compression: none [1/1 chunks]
types:       geometry_msgs/Twist [9f195f881246fdfa2798d1d3eebca84a]
             rosgraph_msgs/Log   [acffd30cd6b6de30f120938c17c593fb]
             turtlesim/Color     [353891e354491c51aabe32df673fb446]
             turtlesim/Pose      [863b248d5016ca62ea2e895ae5265cf9]
topics:      /rosout                    3 msgs    : rosgraph_msgs/Log  
             /turtle1/cmd_vel         265 msgs    : geometry_msgs/Twist
             /turtle1/color_sensor   1317 msgs    : turtlesim/Color    
             /turtle1/pose           1317 msgs    : turtlesim/Pose

4. Play back files

rosbag play bags/hello_2022-11-16-21-32-41.bag 

Restart the tortoise node, and you can see that the tortoise runs according to the track recorded in the bag file

Note: rosbag is also a node, so rosrun can also be used.

2.2 rosbag use - encoding

The command line is not flexible enough, so use the encoding mode to enhance flexibility

1. Write bag

Implementation process:

Initialization;
create rosbag object;
open file stream;
write data;
close file stream;

code:

#include "ros/ros.h"
#include "rosbag/bag.h"
#include "std_msgs/String.h"
/*
    需求：使用rosbag 向磁盘文件写入数据(话题+消息)
    流程：
        1、导包
        2、初始化;
        3、创建rosbag对象
        4、打开文件流
        5、写数据
        6、关闭文件流
*/

int main(int argc, char  *argv[])
{
    
    
    // 2、初始化;
    setlocale(LC_ALL,"");
    ros::init(argc,argv,"bag_write");
    ros::NodeHandle nh;
    // 3、创建rosbag对象
    rosbag::Bag bag;

    // 4、打开文件流
    bag.open("hello.bag",rosbag::BagMode::Write);
    // 5、写数据
    std_msgs::String msg;
    msg.data="hello  xxxx";

    bag.write("", ros::Time::now(), msg);
    bag.write("", ros::Time::now(), msg);
    bag.write("", ros::Time::now(), msg);
    bag.write("", ros::Time::now(), msg);

    // 6、关闭文件流
    bag.close();
    return 0;
}

2. Read bag

Implementation process:

Similar, essentially operating disk files

code:

#include "ros/ros.h"
#include "rosbag/bag.h"
#include "rosbag/view.h"
#include "std_msgs/String.h"

/*
    需求：使用rosbag 读取磁盘上的bag文件
    流程：
        1、导包
        2、初始化;
        3、创建rosbag对象
        4、打开文件流(以读的方式打开)
        5、读数据
        6、关闭文件流
*/

int main(int argc, char *argv[])
{
    
    
    // 2、初始化;
    setlocale(LC_ALL,"");
    ros::init(argc,argv,"bag_read");
    ros::NodeHandle nh;
    // 3、创建rosbag对象
    rosbag::Bag bag;
    // 4、打开文件流(以读的方式打开)
    bag.open("hello.bag", rosbag::BagMode::Read);
    // 5、读数据
    //因为磁盘文件保存数据类型读也要读出来
    //取出时间戳，话题和消息内容
    //先获取消息的集合，再迭代取出消息的字段具体

    for (auto &&m : rosbag::View(bag))
    {
    
    
        //解析
        std::string topic = m.getTopic();
        ros::Time time = m.getTime();
        std_msgs::StringPtr p = m.instantiate<std_msgs::String>();
        ROS_INFO("解析的内容,话题:%s,时间戳:%.2f,消息值:%s",topic.c_str(),time.toSec(),p->data.c_str());
    }

    // 6、关闭文件流
    bag.close();
    return 0;
}

An error occurred:

terminate called after throwing an instance of 'ros::InvalidNameException'
  what():  Character [ ] at element [7] is not valid in Graph Resource Name [dwadawd 1].  Valid characters are a-z, A-Z, 0-9, / and _.
已放弃 (核心已转储)

Cause Analysis: There is a naming error in the cpp file, delete the space or change it to an underscore

Solve the problem: After inspection, it is found that during the initialization process, the name of the topic is arbitrarily typed into dwadawd 1, and there are spaces. After changing it to bag_read, recompile and run;