In the RBD Bullet Solver node's Solver tab - Collision tab, when I set Geometry Representation to a shape like Sphere and then visualize the blue collision hull by checking Show Geometry Representation in Visualize tab, it remains as a Convex Hull, not a Sphere shape.

It works if I use a DOP Network method instead. I am using Houdini 18.5, is this working in later versions, or am I missing something?

This it Houdini 20.0 (don't have 18.5)

Thanks, I have v18.5 and there is no Show Collision Shape option, only Show Geometry Representation, which only shows the Convex Hull type whatever you set Geometry Representation to. Perhaps it wasn't implemented until later versions?

The RBD Bullet Solver's Collision tab is to control the collision geo plugged into the 4th input.
The RBDs (plugged into the 1st and/or 3rd input) come in as Convex Hulls by default. To change that, use a RBD Configure SOP to change the bullet_georep.

Thanks, so is the Geometry Representation menu the Collisions tab related just to the collision object fed to input 4? It shows convex hull whatever shape I select from the menu.

On the RBD Bullet Solver, "RBDs" or "Geometry" refers to the geometry plugged into the 1st or 3rd inputs. "Collision" refers to the geometry plugged into the 4th input.

That is correct, the Geometry Representation menu in the Collisions tab refers to the collision geo plugged into input 4.

The visualization tab has options for both the Geometry/RBDs (plugged in 1st or 3rd inputs) and the Collision (plugged in 4th input)

Displaying the Visualization > Geometry > Collision Shape shows the collision shape for the RBD geo.
Displaying the Visualization > Collision Geometry > Collision Shape shows the collision shape for the Collision geo.

This visualization geo is extracted directly from DOPs. If you aren't seeing what you expect, it'll be because you aren't setting the geo representation as you think.

The settings on the RBD Bullet Solver SOP are all overridden by attributes set on the input geo, either set manually or via a RBD Config SOP.

You cannot change the Geometry/RBD collision shape (geometry representation) on the RBD Bullet Solver node itself, you need to change it by setting the "bullet_georep" string point attribute on the packed fragments. The RBD Config SOP has options for setting this.

Here's an example file with 3 different setups. The default (everything comes in as convex hulls), the RBD Bullet Solver's Collision Geometry Representation changed to Sphere, and finally overrides using the RBD Config SOP.

Although the RBD Bullet Solver's UI has changed significantly since 18.5, the parms and the way this works hasn't changed, so you should see the same results (despite some warnings when you open the file).

Thanks for the file, npetit, and the explanation, that makes it very clear.

One more question, I see the Collisions Padding in the Collisions tab affects the collider object plugged in to input 4, but where is this parameter for the geo/RBD plugged in to input 1?

litote
but where is this parameter for the geo/RBD plugged in to input 1?

on RBD Configure SOP

Thanks again Thomas!