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How to create topics and synchronize them when recording a bagfile.

asked 2021-06-18 03:56:21 -0500

Ifx13 gravatar image

updated 2021-06-18 05:04:02 -0500

Hello everyone,

thanks for taking the time to check my post. So let me describe what I want to do.

  • For starters I have a bag file containing a topic which carries a directory path at each message, the important part of the topic are the timestamps.
  • The path leads to a video file.
  • I take the frames of the video and create an image topic. Note there are 4 videos so the output is 4 Image topics.

The question is how to get them synchronized. I tried creating 4 topics with the same timestamp at each topic but when I recorded them they seem not to be in sync. Here is my code:

#!/usr/bin/env python
import rosbag
import rospy
from sensor_msgs.msg import Image
from cv_bridge import CvBridge
import cv2
import message_filters

# Declare the bag file which recorded the data paths.
bag = rosbag.Bag("econ_vid_paths.bag")

Initialize the node with the name "Image_publisher" and the publishers that will 
contain the image messages
cam_0_publisher = rospy.Publisher('/econ/image_raw/cam_0', Image, queue_size = 100)
cam_1_publisher = rospy.Publisher('/econ/image_raw/cam_1', Image, queue_size = 100)
cam_2_publisher = rospy.Publisher('/econ/image_raw/cam_2', Image, queue_size = 100)
cam_3_publisher = rospy.Publisher('/econ/image_raw/cam_3', Image, queue_size = 100)

# Create an object that converts between OpenCV images and ROS image messages.
bridge = CvBridge()

# Open video files.
cap0 = cv2.VideoCapture('/mnt/28007d94-f5ac-4179-b807-b9ec1cf293c7/econVideo_2021-06-11-17-13-51_0.mkv')
cap1 = cv2.VideoCapture('/mnt/28007d94-f5ac-4179-b807-b9ec1cf293c7/econVideo_2021-06-11-17-13-51_1.mkv')
cap2 = cv2.VideoCapture('/mnt/28007d94-f5ac-4179-b807-b9ec1cf293c7/econVideo_2021-06-11-17-13-51_2.mkv')
cap3 = cv2.VideoCapture('/mnt/28007d94-f5ac-4179-b807-b9ec1cf293c7/econVideo_2021-06-11-17-13-51_3.mkv')

# Create frame_number variable, this will work as a counter for the frames.
frame_number = 0;

for topic, msg, t in bag.read_messages(topics=['/sync_vid_publisher/econ_vid']):
    # Take retrieve value (retX) and frames, if retX is 0 then there is no valid frame.
    ret0, frame0 =
    ret1, frame1 =
    ret2, frame2 =
    ret3, frame3 =  

    if ret0+ret1+ret2+ret3==4: # Check if every camera has a valid frame.
        timestamp_secs = msg.header.stamp.secs
        timestamp_nsecs = msg.header.stamp.nsecs

        video_message_0 = bridge.cv2_to_imgmsg(frame0, encoding="bgr8")
        video_message_0.header.stamp.secs = timestamp_secs
        video_message_0.header.stamp.nsecs = timestamp_nsecs

        video_message_1 = bridge.cv2_to_imgmsg(frame1, encoding="bgr8")
        video_message_1.header.stamp.secs = timestamp_secs
        video_message_1.header.stamp.nsecs = timestamp_nsecs

        video_message_2 = bridge.cv2_to_imgmsg(frame2, encoding="bgr8")
        video_message_2.header.stamp.secs = timestamp_secs
        video_message_2.header.stamp.nsecs = timestamp_nsecs

        video_message_3 = bridge.cv2_to_imgmsg(frame2, encoding="bgr8")
        video_message_3.header.stamp.secs = timestamp_secs
        video_message_3.header.stamp.nsecs = timestamp_nsecs

        print('Frame :',  frame_number, 'published.')

By doing this I thought that the topics are going to be synchronized as they have the same timestamp but when I recorded the messages and looked at the contents in rqt I noticed that they are not synchronized. Although, when I echo each timestamp from the topics (rostopic echo /econ/image_raw/cam_1/header/stamp) they are exactly the same. So why are they not recorded in sync ? I record them using the following: rosbag record -b 0 /econ/image_raw/cam_0 /econ/image_raw/cam_1 /econ/image_raw/cam_2 /econ/image_raw ... (more)

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I edited the title to make it less vague.

Ifx13 gravatar image Ifx13  ( 2021-06-18 05:04:53 -0500 )edit

1 Answer

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answered 2021-06-18 04:20:12 -0500

Webadone gravatar image

Hey! I did a quick search and found that topic which might be of interest for you:

So the comment on the answer states that

"The rosbag writers and readers do not look at the header timestamp within the messages, and does not guarantee that messages are ordered by those timestamps."
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Thank you! Your comment got me to the solution, I know now what I should be searching and I've found this. Now they are indeed synchronized. Thanks again!

Ifx13 gravatar image Ifx13  ( 2021-06-18 05:03:16 -0500 )edit

Ah perfect, good to hear!

Webadone gravatar image Webadone  ( 2021-06-18 05:58:30 -0500 )edit

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Asked: 2021-06-18 03:56:21 -0500

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Last updated: Jun 18 '21