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google / brax

Massively parallel rigidbody physics simulation on accelerator hardware.
Apache License 2.0
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`act_size` (v2) larger than the number of actuators #323

Closed Theo-Cheynel closed 1 year ago

Theo-Cheynel commented 1 year ago

Hi,

I noticed something unexpected while playing with the humanoid environment in BRAXv2. The MJCF file defines 17 actuators (linked to 17 joints + a free joint for the root of the character). The link_types string of this system looks like f2131312121, which can be read as "free joint, 2 joints, 1 joint, etc". That is logical, as 2+1+3+1+3+1+2+1+2+1 = 17. Now, the act_size of the system is expected to be 17 as well, since there are 17 motors associated to 17 hinge joints with 1DoF each. However, the act_size of this system is not 17 but 35, counting 2DoFs for each joint in a two-joints configuration, and 3DoFs for each joint in a three-joints configuration. Indeed, 2+2 + 1 + 3+3+3 + 1 + 3+3+3 + 1 + 2+2 + 1 + 2+2 + 1 = 35.

I struggle to see if this is the expected behaviour, it seems to me that the act_size should be the sum of the number of DoFs of each joint, in our case, 17. Can someone explain if my understanding is correct ?

Thanks in advance

btaba commented 1 year ago

Hi @Theo-Cheynel thanks for pointing this out, should be fixed now!

btaba commented 1 year ago

Closing for now, but feel free to re-open if there are more issues

Theo-Cheynel commented 1 year ago

Thanks ! I'll have a look :smile:

In the meatime, I added pseudo bodies in between all my joints, to convert 3DoF joints to three 1 DoF joints :laughing:

btaba commented 1 year ago

Ah got it, that is indeed annoying to have to do, sorry for the delay!