pybullet needs setLinearFactor/setAngularFactor to lock axis

[benelot]

We will need this to replicate the 2D RL environments such as humanoid/half-cheetah. I believe it is not yet available. Maybe I will start an attempt to add it. Or is there another trick to achieve this for now?

[erwincoumans]

Please see the MJCF files, it loads the original MuJoCo environments. 2D works automatically, since it fixes the base and adds single DOFs at a time. Try running Bullet ExampleBrowser / Importers / ImportMJCF a few times and watch the various mjcf files. By default the joints are locked (motors with target 0) just use pybullet.loadMJCF and set the motors as desired.

There are also a few other ways to create this effect (using maximal coordinate constraints or btMultiBody and adding the right degrees of freedom using a fixed base).