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The python bag API is actually really easy to use. In your case something like this should work (not tested). I suppose the main question is how you want to synchronize the messages. Here I assume that the new topic should be publishes every time a new topic2 message has arrived and there was a topic1 message (anytime) before. This may not be exactly what you want, but I hope this will help you enough to get going.

import rosbag
from std_msgs.msg import Int32, String

inbag = rosbag.Bag('in.bag')
outbag = rosbag.Bag('out.bag', 'w')

try:
    parent = 0;
    for topic, msg, t in inbag.read_messages(topics=['topic1', 'topic2']):
        if topic == 'topic1':
            parent = msg
        else:
            if parent != 0:
                parent.sub = msg;          
                outbag.write('newtopic', parent)
finally:
    bag.close()

The python bag API is actually really easy to use. In your case something like this should work (not tested). I suppose the main question is how you want to synchronize the messages. Here I assume that the new topic should be publishes every time a new topic2 message has arrived and there was a topic1 message (anytime) before. This may not be exactly what you want, but I hope this will help you enough to get going.

import rosbag
from std_msgs.msg import Int32, String

inbag = rosbag.Bag('in.bag')
outbag = rosbag.Bag('out.bag', 'w')

try:
    parent = 0;
None;
    for topic, msg, t in inbag.read_messages(topics=['topic1', 'topic2']):
        if topic == 'topic1':
            parent = msg
        else:
            if parent != 0:
None:
                parent.sub = msg;          
                outbag.write('newtopic', parent)
finally:
    bag.close()