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Working with logs

✎

Modified 2019-10-01 by Rohit Suri

Robotics is innately married to hardware. However, when we develop and test our robots’ software, it is often the case that we don’t want to have to waste time to test on hardware after every small change. With bigger and more powerful robots, it might be the case that a software can result in a robot actuation that breaks it or even endanger human life! But if one can evaluate how a robot or a piece of code would behave before deploying on the actual platform then quite some headaches can be prevented. That is why working in simulation and from logs is so important in robotics. In this section you will learn how to work with logs in ROS.

Laptop setup (unknown ref opmanual_duckiebot/laptop-setup)

warning (41 of 57) index

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Created by function n/a in module n/a.

Docker poweruser skills

Developer knowledge of ROS

Reading and processing bag files

Section 3.1 - Rosbag
Section 3.2 - Rosbag: Recording
Section 3.3 - Rosbag Python API: Reading
Section 3.4 - Rosbag Python API: Writing
Section 3.5 - Exercises

Rosbag

✎

Modified 2020-10-13 by arsimone

A bag is a file format in ROS for storing ROS message data. Bags, named so because of their .bag extension, have an important role in ROS. Bags are typically created by a tool like rosbag, which subscribes to one or more ROS topics, and stores the sequence of messages in a file as it is received. These bag files can be played back in ROS ith the same topics that were recorded, or even using remapping to new topics. When a bag file is replayed the temporal order of the different messages is always kept.

Please go through this link for more information.

Rosbag: Recording

✎

Modified 2020-10-13 by arsimone

You can use the following command to record bag files

$ rosbag record TOPIC_1 TOPIC_2 TOPIC_3

or simply

$ rosbag record -a

to record all messages being published.

Be careful on recording all the messages published in a ROS system. There might be quite a lot of topics creating very by bag files very quickly, especially using images.

Rosbag Python API: Reading

✎

Modified 2019-09-29 by Aleksandar Petrov

The following code snippet is a basic usage of the rosbag API to read bag files:

import rosbag
bag = rosbag.Bag('test.bag')
for topic, msg, t in bag.read_messages(topics=['chatter', 'numbers']):
    print msg
bag.close()

Rosbag Python API: Writing

✎

Modified 2019-09-29 by Aleksandar Petrov

The following code snippet is a basic usage of the rosbag API to create bag files:

import rosbag
from std_msgs.msg import Int32, String

bag = rosbag.Bag('test.bag', 'w')

try:
    s = String()
    s.data = 'foo'

    i = Int32()
    i.data = 42

    bag.write('chatter', s)
    bag.write('numbers', i)
finally:
    bag.close()

Exercises

✎

Modified 2019-10-01 by Rohit Suri

All containers in the exercises below should be run on your laptop, i.e. without -H MY_ROBOT.local.

Record bag file

✎

Using the following concepts,

create a Docker container on your laptop with a folder mounted on the container. You can use the image duckietown/dt-ros-commons:daffy. This time, however, instead of exporting the ROS_MASTER_URI and ROS_IP after entering the container, do it directly with the docker run command specifying environment variables. You already know how from here.

Run the lane following demo (unknown ref opmanual_duckiebot/demo-lane-following)

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Created by function n/a in module n/a.

. Once your Duckiebot starts moving, record the camera images and the wheel commands from your Duckiebot using rosbag in the container you just created (the one with the folder mounted). In order to save the bags once the container is stopped you should record them in the mounted folder. To do that navigate to the mounted folder using the cd command and then run

laptop-container $ rosbag record /MY_ROBOT/camera_node/image/compressed /MY_ROBOT/wheels_driver_node/wheels_cmd

Record the bag file for 30 seconds and then stop the recording using Ctrl+C. Use the rosbag info filename.bag command to get some information about the bag file. If the bag does not have messages from both the topics, check if you ran the container correctly(you can easily check that a topic is published using the rostopic echo functionality from within the container).

Stop the demo before proceeding.

Analyze bag files

✎

Download this bag file.

Start by creating a new repository from the template, like in section C-2. Inside, the ./packages folder, create a python file for this exercise. You do not need to create a ros package for this, you can simply launch a python script as you did in RH2. This is because reading a bag file does not actually require ROS, however, you can still choose to do so if you want. Using the following concepts,

create a Docker image which can analyze bag files and produce an output similar to the one shown below. The min, max, average, and median values printed are statistics of the time difference between two consecutive messages. The NNN and N.NN are just placeholders, eg. NNN could be 100 and N.NN could be 0.05.

/tesla/camera_node/camera_info:
  num_messages: NNN
  period:
    min: N.NN
    max: N.NN
    average: N.NN
    median: N.NN

/tesla/line_detector_node/segment_list:
  num_messages: NNN
  period:
    min: N.NN
    max: N.NN
    average: N.NN
    median: N.NN

/tesla/wheels_driver_node/wheels_cmd:
  num_messages: NNN
  period:
    min: N.NN
    max: N.NN
    average: N.NN
    median: N.NN

Make sure to mount the folder containing the bag file to the Docker container, instead of copying it.

Run the same analysis with the bag file you recorded in the previous exercise.

Processing bag files

✎

Use the bag file which you recorded earlier for this exercise. Using the following concepts,

create a Docker image which can process a bag file. Essentially, you will extract some data from a bag file, process it, and write the results to a new bag file. Once again, create a new repository, and the necessary python file for this exercise inside the ./packages folder. For the image message in the bag file, do the following:

Extract the timestamp from the message
Extract the image data from the message
Draw the timestamp on top of the image
Write the new image to the new bag file, with the same topic name, same timestamp, and the same message type as the original message

The new bag file should be generated in the mounted folder.

To verify your results, create a docker container exactly like you did in exercise 18. Make sure you place your processed bag file in the folder being mounted. Run the following command:

laptop-container $ rosbag play processed_bag.bag --loop /MY_ROBOT/camera_node/image/compressed:=/new_image/compressed

In a new terminal, use dts start_gui_tools to open a container connected to your robot and run rqt_image_view inside it. Can you see /new_image/compressed?

Stop the rosbag play using CTRL+C and now run the following command inside the same container:

laptop-container $ rosbag play processed_bag.bag --loop

Again, use start_gui_tools but this time check /MY_ROBOT/camera_node/image/compressed. What’s going on? Why? What does the last part of the first command do?