C++ ways to get MjDataBodyViews

[Trinkle23897]

Hi,

First, thanks for this great project!

I'm going to translate the existing python bindings in gym to C++, to get the best performance for vectorized environments envpool and minimize the overhead. openai/gym#2762 has made huge progress in integrating the newest version of mujoco. However, I cannot find a related document for MjDataBodyViews.

In short, https://github.com/openai/gym/blob/0e2077bb636fbac979a4edd0c8608235742a8f2a/gym/envs/mujoco/mujoco_env.py#L268-L272

    def get_body_com(self, body_name):
        if self._mujoco_bindings.__name__ == "mujoco_py":
            return self.data.get_body_xpos(body_name)
        else:
            return self.data.body(body_name).xpos  # <-- this line

self.data is a mjData structure. But I cannot find a related pybind binding function of mjData that has a body method. Later on I expand the indexers.[h|cc] to see the code after expansion, by

g++ -E indexers.cc -I ../../include/ $(python3 -m pybind11 --includes) > /tmp/indexer_expand.cc

indexer_expand.txt

However, after going through the expanded code, I'm still quite confused about how to write the same functionality (data.body(body_name).xpos) in C++. Could you please kindly provide some help? Thanks in advance!

Update:

For Ant Env (sail-sg/envpool#74):

• data.xpos[1] is data.body("torso").xpos, mj_name2id(model_, mjOBJ_BODY, "torso")

For Reacher Env (sail-sg/envpool#81):

• data.xpos[-2] is data.body("fingertip").xpos, mj_name2id(model_, mjOBJ_BODY, "fingertip")
• data.xpos[-1] is data.body("target").xpos, mj_name2id(model_, mjOBJ_BODY, "target")

For Pusher Env (sail-sg/envpool#82):

• data.xpos[-3] is data.body("tips_arm").xpos, mj_name2id(model_, mjOBJ_BODY, "tips_arm")
• data.xpos[-2] is data.body("object").xpos, mj_name2id(model_, mjOBJ_BODY, "object")
• data.xpos[-1] is data.body("goal").xpos, mj_name2id(model_, mjOBJ_BODY, "goal")

[saran-t]

I'm not 100% clear on what you're asking here. Are you trying to replicate the Python API of the mujoco package in C++?

[Trinkle23897]

I'm trying to replicate gym's way of using mujoco in C++. For example, this is Ant-v4 env written in pure C++: https://github.com/Trinkle23897/mjc-mwe/blob/master/cc/ant.cc

[saran-t]

In that case my current recommendation would be to code against MuJoCo's C API directly. You can use mj_name2id to look up the numerical index that corresponds to any given entity name.

If you want nice syntactic sugar for named access (e.g. data.body("torso").xpos) in C++ you'll need to roll your own class that implements this interface. The code in python/indexers.cc shows one example on how to go about it.

[Trinkle23897]

Thanks for the answer!