how to get boxes to collide / rest on top of each other

[grez72]

Hi,

I'm having some trouble understanding how to correctly specify colliders. In the test below, I'm placing 1m boxes at z positions 1 and 3, and expect them to fall to z positions .5 and 1.5 (with Box2 resting on top of Box1), but Box2 passes through Box1. Is there something more I should specify to make the boxes collide?

import brax
from google.protobuf import text_format
from jax import numpy as jnp

_CONFIG = """
  dt: 1.5 substeps: 1000 friction: 0.6 baumgarte_erp: .1
  gravity { z: -9.8 }    
  bodies { name: "Ground" frozen: { all: true } colliders { plane {}}}
  bodies {    
    name: "Box1" mass: 1
    colliders { box { halfsize { x: 0.5 y: 0.5 z: 0.5 }}}
    inertia { x: 1 y: 1 z: 1 }
  }
  bodies {
    name: "Box2" mass: 1
    colliders { box { halfsize { x: 0.5 y: 0.5 z: 0.5 }}}
    inertia { x: 1 y: 1 z: 1 }
  }
  
"""

def test_boxes_fall_on_one_another():
  """Box1 falls onto the ground and stops, Box2 falls and rests on top of Box1."""
  sys = brax.System(text_format.Parse(_CONFIG, brax.Config()))
  qp = brax.QP(
      pos=jnp.array([[0, 0, 0],[0., 0., 1.],[0., 0., 3.]]),
      rot=jnp.array([[1., 0., 0., 0.], [1., 0., 0., 0.], [1., 0., 0., 0.]]),
      vel=jnp.array([[0., 0., 0.], [0., 0., 0.], [0., 0., 0.]]),
      ang=jnp.array([[0., 0., 0.], [0., 0., 0.], [0., 0., 0.]]))
  qp, _ = sys.step(qp, jnp.array([]))  
  return sys,qp

sys,qp = test_boxes_fall_on_one_another()
print(qp.pos)

# observed
#[[0.         0.         0.        ]
# [0.         0.         0.49988964]
# [0.         0.         0.49988964]]

# expected
#[[0.         0.         0.        ]
# [0.         0.         0.5]
# [0.         0.         1.5]]

[cdfreeman-google]

Ah, we currently don't have box-box collisions in. This is on our roadmap, and I'll be sure to ping you when they're ready!