How to represent geometry correctly

[lyd405121]

Intro

Hi!

I am an graphic engineer, I use MuJoCo for developing an Embedded AI simulator.

My setup

Latest Mujoco 3.2.6
using pure C++
windows 10

My question

Background

• Using mujoco to pick & place basic geometry or the combination of them is tested for my case
• Which is deployed in our factory for cmos sensor pick and scan.
  
• Then my ambition turns to coffee shop for picking paper coffee cup which is treated as rigidbody while it is not
• Unfortunately, it failed。
  
• Because I use mesh type for geometry collision test, the same mesh for rendering， which is easy but not work

 <geom mesh="coffee_cup0" material="coffee_cup0" class="coffee_cup0_visual"/>
 <geom name="coffee_cup0_collision1" class="coffee_cup0_collision" type="mesh" mesh="coffee_cup0"/>

• Then I change the collision type to cylinder which is not as accurate as the reality
• But works!

<geom mesh="coffee_cup0" material="coffee_cup0" class="coffee_cup0_visual"/>
<geom name="coffee_cup0_collision1" class="coffee_cup0_collision" type="cylinder" size="0.035 0.07" pos="0.0 0.07 0.0" quat="1 1 0 0"/>

How to balance

• If I want both accurate and bettter friction to simulate collision for frustum of a cone
• There are three ways after racking my brains:
• ① fisrt，make many small cylinder to represent frustum of a cone which is like the process of differential
• ② second，generate sdf for a frustum of cone ，and using the extension of mujoco，which need lots of work to develop
• ③ third，modify the basic geometry for mujoco，make my own version of frustum of cone collision test
• Or there is a method for frition setting that out of my knowledge
• ** Really Need Help**, cause there are too many geometry shape in our world to conclude into basic shape of mujoco

Minimal model and/or code that explain my question

If you encountered the issue in a complex model, please simplify it as much as possible (while still reproducing the issue).

Model:

minimal XML

<mujoco>
  <worldbody>
    <light pos="0 0 1"/>
    <geom type="sphere" size="1" rgba="1 0 0 1"/>
  </worldbody>
</mujoco>

Code:

import mujoco
import mediapy as media
model = mujoco.MjModel.from_xml_string(xml)
data = mujoco.MjData(model)

with mujoco.Renderer(model) as renderer:
  mujoco.mj_forward(model, data)
  renderer.update_scene(data)

  media.show_image(renderer.render())

Confirmations

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

[lyd405121]

• Find a trick，but not physicslly right
• Set an option more than 1.0 like the document said below
  
• But get an artifact which is frition make every contact too strong!
  

[Balint-H]

Hello,

You correctly identified most of the approaches (Decomposition, SDF, Custom primitive, solver configurations). The main limitation of mesh-mesh contacts is that a single contact point is calculated. (Compared to e.g. box on a plane where 4 contacts are active). Here are a couple more considerations:

• Decomp with primitives: You may want to take a look at the mug example model from the release folder, decomposition with boxes might work better than stacked cylinders (although there will be some gaps since the coffee cup walls are not parallel)

• SDF: As far as SDFs go, this a reasonable shape to approximate. If the approach interests you, it would be a good choice as there are truncated cone SDF examples online, and it is more straightforward than writing your own custom primitive and building the engine.

• Flexes: This is the only main missing approach you have not mentioned, and quite flexible. If appropriately set up, you can use flex bodies to represent rigid meshes, while able to produce more contact points than regular mesh geoms (thank you for clarifying this difference @emotodorov and @quagla ).
  https://mujoco.readthedocs.io/en/stable/XMLreference.html#body-flexcomp

Do also check out the multiccd flag, as enabling that can stabilize a lot of gripping situations, and may be the simplest solution of them all:
https://mujoco.readthedocs.io/en/stable/XMLreference.html#option-flag-multiccd

[lyd405121]

• Learn a lot 😏. Thanks.
• mujoco is really awsome framework, so many feature I have not explored!
• I will try what you have said and maybe post some pictures after that :)