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I just recently got openni_tracker working but don't have the hardware in front of me right now, making it difficult to give specifics. Assuming it is actually working for you, it sounds to me like you just need to become more familiar with TF. (I say that because, rather than 'rostopic echo /tf', you probably want to try using 'rosrun tf tf_echo <source_frame> <target_frame>'.)

What would likely be most instructive is to run RViz and add the TF display, setting the 'Global Settings -> Fixed Frame' to the appropriate frame (again, sorry, I don't have it in front of me and can't recall the default frame name). If it's working, you'll be able to see a 'skeleton' of connected 3-axis display objects, where each axis object is a joint being tracked.

In terms of actual code, you likely want to spend more time on the Writing a tf listener tutorial. The various skeleton joint frames will move relative the sensor's fixed frame and can be accessed using a transform listener's lookupTransform method.