What is .pbstream and can I read it

[myu20]

Hi, I initially installed Cartographer in June 2017. At that time the trajectory output was a .pb file, from which I could get a human readable version with the command protoc --decode cartographer.mapping.proto.Trajectory -I. $(find . -name '*.proto') < ~/.ros/map.pb. The output was a timestamped trajectory of the form

node {
  timestamp: 636324486985516000
  pose {
    translation {
      x: 0.0011769832389950886
      y: 0.015897276467976743
      z: 0.00029333615062883135
    }
    rotation {
      x: 0.000467514026206124
      y: -0.010101004818702106
      z: -0.0072682970219754535
      w: 0.99992245848900119
    }
  }
}

Today I used git pull to update Cartographer and now Cartographer outputs a .pbstream. Does the .pbstream also contain a timestamped trajectory and is there a command to get a human readable version of it?

Thank you!

[wohe]

Hi, yes and no. The .pbstream files contain much more information now, it is written by MapBuilder::SerializeState(). You can read it using the ProtoStreamReader in cartographer/io/proto_stream.h. The first proto message it contains is a cartographer.mapping.proto.SparsePoseGraph which can contain one or more trajectories. I hope this helps. Let me know if there are more questions.

[myu20]

After I get a mapping::proto::SparsePoseGraph graph using the ProtoStreamReader, do I access the trajectories using graph.trajectory_size() and graph.trajectory(0)?

[wohe]

Yes, for example. It's a normal protocol buffer message: https://developers.google.com/protocol-buffers/docs/reference/cpp-generated#repeatedmessage

[myu20]

Great, thanks!

[litaws]

So reading this into python will be a little more intricate?

[wohe]

But only a tiny bit. Using Python's gzip and struct modules, an implementation of ProtoStreamReader in Python should be rather straightforward. Let me know if you have specific questions.

[woshidaye]

@myu20 Hi, I want to extract data from pbstream. How do you extract trajectory from pbstream? Can you share your code for extracting data? I look forward to your reply.

[myu20]

@woshidaye Hi, I used the following code:

cartographer::io::ProtoStreamReader reader(pbstream_filename);
cartographer::mapping::proto::PoseGraph pose_graph;
reader.ReadProto(&pose_graph);

You can access the trajectories with pose_graph.trajectory(i), and from the trajectory you can access the trajectory nodes with pose_graph.trajectory(i).node(j).

Relevant protos: pose_graph trajectory

[woshidaye]

@myu20 Thank you for your answer. I have another question.I want to save the submaps and pose-graph. Does the pbstream file contain this data? If not, what are your suggestions for saving this information? Looking forward to your reply again. Thank you.

[myu20]

@woshidaye As I mentioned in my previous comment, you can read the pose graph with

cartographer::io::ProtoStreamReader reader(pbstream_filename);
cartographer::mapping::proto::PoseGraph pose_graph;
reader.ReadProto(&pose_graph);

I imagine you can read the submaps in a similar way. I would suggest looking at the serialization proto and the WritePbStream method to see what you can read from the pbstream file. All of the relevant protos are probably in cartographer/cartographer/mapping/proto.

[woshidaye]

@myu20 Thank you very much.

[woshidaye]

@myu20
I wrote a script according to your method. What file should I put this script under? Do we need to write the script as a ROS node? How do I run this script file to extract trajectory data?

[Flandoe]

@woshidaye Have you solved the problem? I met the same problem to compile the code. Thanks!

[meyerjo]

https://github.com/meyerjo/cartographer-read-pbstream this shows how one can read the pbstream file