Bug in box-sphere collider

Intro

MuJoCo dev. Original bug report in #2199

My setup

n/a

What's happening? What did you expect?

The box-sphere collider returns correct contact positions, but the depth (contact.dist) is wrong after full penetration of the sphere into the box.

Steps for reproduction

Run the code below, get this plot

Minimal model for reproduction

See below.

Code required for reproduction

xml = """
<mujoco>
  <worldbody>
    <geom name="plane" pos="0 0 -.005" type="plane" size="0.5 0.5 0.01"/>-->
    <geom name="box"   type="box" pos = "0 0 -0.255" size="0.5 0.5 .25"  />
    <body>
      <freejoint/>
      <geom name="sphere" type="sphere" mass="1" size="0.005"/>
    </body>
  </worldbody>
</mujoco>
"""
model = mujoco.MjModel.from_xml_string(xml)
data = mujoco.MjData(model)

depth = np.linspace(-.01, -1e-6, 200)
contact = [[], []]
for z in depth:
  data.qpos[2] = z
  mujoco.mj_forward(model, data)
  for i in range(2):
    c = data.contact[i]
    condata = np.array((c.pos[2], c.dist))
    contact[i].append(condata)

plt.plot(depth, contact[0], label=['plane contact pos', 'plane contact depth']);
plt.plot(depth, contact[1], '--', label=['box contact pos', 'box contact depth']);
plt.xlabel('Penetration (m)');
plt.legend()

Confirmations

[X] I searched the latest documentation thoroughly before posting.
[X] I searched previous Issues and Discussions, I am certain this has not been raised before.

google-deepmind / mujoco

Bug in box-sphere collider #2206