How to put objects inside a URDF container (e.g. pot)?

[SuperFanBin]

Hi, I'm trying to simulate a container (like a pot or bowl) in SAPIEN where small objects (e.g. balls, food) can be placed inside it, similar to a real-world container.

I noticed that RoboFactory (a project built on SAPIEN) successfully made this — objects can fall into and be contained by pots or bowls.

I tried using the same asset files (.obj, .urdf) from RoboFactory, but in my simulation, small objects cannot enter the interior of the container — they behave as if the pot is a solid volume, as shown in the image below:

I suspect this is due to differences in how the URDF is loaded or parsed, possibly related to collision geometry handling. The RoboFactory loading code is quite complex, and I couldn't identify the key part that enables "hollow" collision behavior.

My questions are:

1. Is there a simple and standard way in SAPIEN to make a URDF link act as a hollow container?
2. Should I use a concave triangle mesh in the <collision> tag? Are there any special settings (e.g., normals, scale, collision groups) required?
3. Does SAPIEN automatically support concave mesh collision for containers, or do I need to enable it explicitly?

[fbxiang]

1. SAPIEN does not support non-convex shapes as URDF links by default. The best method is to decompose the original mesh in advance, and set load_multiple_collisions_from_file to true in URDF loader. If you want automatic convex decomposition, you can additionally set multiple_collisions_decomposition to "coacd" (make sure you have coacd installed from pip). The multiple_collisions_decomposition_params reflects decomposition parameters provided by coacd. However, if default settings do not work well, I recommend use your own procedure to do the decomposition in advance since it would be more flexible.
2. non-convex mesh is not supported. You can use non-convex triangle shape for static scenes or kinematic objects, but not for articulations.
3. See 1