How to extract data from *.bag?

edit

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

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++/

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...

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.

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

Here is a small python framework that extracts bag information from a bag file and puts it in a pandas dataframe in python. It also has two small scripts to dump it into a csv file or place graph information from a topic. https://github.com/aktaylor08/RosbagP...

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

convert a bag file to .csv format, use

$ rostopic echo /topicname -b bagFileName.bag -p > file.csv

Remember to replace topicname with your topic (/tactile). And bagFileName.bag need to be replace with your bag file that you need to convert. And finally file.csv will be your output file. Instead of having .csv you can try .txt also. But for the best practice .csv file is good.