How to extract data from *.bag?

edit

You easily can use the rosbag C++ or python API to directly load data from the bag. You find a tiny example on the rosbag C++ API documentation:

http://www.ros.org/doc/api/rosbag/html/c++/

just for the fun of it, here is a bag2csv one-liner for bash

for topic in `rostopic list -b bagfile.bag` ; do rostopic echo -p -b bagfile.bag $topic >bagfile-${topic//\//_}.csv ; done

I could not find an easy way to save all topics to a csv file for post-processing, so I wrote a python script to do it. I hope others will find it useful:

http://www.speal.ca/bag2csv.py (bag2csv.py)

Usage1 (for one bag file):

python bag2csv.py filename.bag

Usage 2 (for all bag files in current directory):

python bag2csv.py

I wasn't sure if this would be the right place to share this script for others to use. Can anybody else please advise?

Please let me know if you find this useful, or if you have any suggestions for how to improve it.

I have written a really simple script to output ros topics to a text file. you can find the script here; https://gist.github.com/831611

for example, we need the timestamp, width and height of sensor_msgs::PointCloud2;

$ ./topics_to_ssv -d " " hoge.ssv /camera/depth/points2/header/stamp/secs /camera/depth/points2/header/stamp/nsecs /camera/depth/points2/width /camera/depth/points2/height
$ cat hoge.ssv
/camera/depth/points2/header/stamp/secs /camera/depth/points2/header/stamp/nsecs /camera/depth/points2/width /camera/depth/points2/height 
1297944909 267601871 640 480 
1297944909 310369177 640 480 
...

I hope it help you...

Not sure if this will be useful for your specific case, but in one of the packages in starmac-ros-pkg there is a tool called bag2mat that reads in bag files for use in Python and can also save to a binary .mat file:

http://ros.org/doc/fuerte/api/starmac_tools/html/bag2mat_8py_source.html

Edit, because the answer by Lorenz is better than my previous suggestion: If you want to parse a bagfile yourself this might be interesting: http://www.ros.org/wiki/Bags/Format/2.0

Btw: It would be really cool if there was some kind of converter from bag-files to yaml or xml files.

I followed guide at http://www.ros.org/doc/api/rosbag/html/c++/ when i run the code it tells me "ImportError: No module named rosbag"

Traceback (most recent call last): File "/home/user/ros_workspace/beginner_tutorials/nodes/read_bag.py", line 4, in <module> import rosbag ImportError: No module named rosbag

please tell me how to fix this!

Make sure you have set the path variable and add these two lines to the top of your main python script to avoid any dependency errors.

import roslib; roslib.load_manifest('your_package_name')

then write your rest of the script. Please keep in mind that this does not need to be in every python script but just in the main entry point.

Check a relevant answer on reading bag file in python here: https://answers.ros.org/question/2883...