Using custom geometry with rigid body as particles

[jakob-wagner]

Hi

I'm trying to create a specific setup with the particles system in C4D:

1. A particle system creates a bunch of particles with a velocity.
2. The particles shape is one or more different custom objects/geometries. Not spheres.
3. The objects will inherit the velocity of the particle, but then physics simulation takes over.
4. The objects falls and lands and comes to rest. The collision cage is not a sphere, but fits the custom geometry.

I can't find a setup that will simulate correctly. I have tried to create a particle system which is then being used by a cloner, but I get the following problems:

1. The objects penetrate the colliders and the other objects. They suddenly jumps (like popcorn) at random times. Probably because the intersect with the collider geometry.
2. The seem to keep sliding. Perhaps following the original particle a little bit?

I've attached the following files:
test particles.c4d - a test setup of the described attempt.
bunch of eggs.c4d/bunch of spheres.c4d - a test with a lot of eggs or spheres, but no particles.

The "bunch of eggs.c4d" file is to test the collisions. Although I don't think that is the only problem, a lot of my problems have to do with that. In these two simple files I get a lot of penetration between the colliders and the eggs. I don't get the same problem with the file with the spheres.
I have tried to give it a lot more substeps/iteration/passes but no luck. I have also tried to set "Collision Shapes" to a higher value. No change. That brings me to another question; is it possible to see the autogenerated collision cage generated?

Sorry for the long post, any help is greatly appreciated.

test particles.c4d
bunch of spheres.c4d

Thanks,
Jakob

[jakob-wagner]

For some reason I can't upload the bunch of eggs scene, but it was exactly like the bunch of spheres file, but with eggs instead.

[Dr. Sassi]

Hi jakob-wagner,

Thanks for the files!

The eggs grow based on your particle setup and the scale slider inside the MoGraph Cloner.
When they land on the Stage, the growth might be faster than the calculation, and they go through the Stage.

Things look better if you set the Scale slider to zero in the MoGraph Cloner.

However, if that scaling is wanted, here is an alternative that ensures that the eggs reach full size before the "land.

CV4_2025_drs_25_MGeg_01.c4d
I have those "remeshed", and you get the shape back with an SSD.

The Collision shape was set to Auto, Complex Hull should work or Triangle mesh. Check in Attribute Manager> Mode> Scene> Simulation> Simulation (Tab)> Draw, what works best.

I also set the Scene to 20cm, which was on Default (100cm), and increased the Steps.

Enjoy your weekend

[jakob-wagner]

@Dr-Sassi Thank you for the quick reply.
The tip on showing the collision cage is exactly what I was looking for. Without that it was difficult to demystify.

It's a great idea to force the eggs to finish scaling when they hit the ground.

My problem is still the collision mesh. Isn't it possible to get a more dense collision mesh? It doesn't really change even when adjusting the "Geometry Accuracy". The current will lead to the eggs pocking through the ground to much, which will be visible in the final scene. Also the egg rolling will be really choppy.

The image shows the collision mesh is landing on the floor correctly, but the since it doesn't fit the egg shape well enough the egg it self is going through the floor.

[Dr. Sassi]

Hi jakop-wagner, (no capital letters as per avatar)

Please use any mesh density you like. To get any performance on my little four year old laptop, I reduced the mesh to not wait for another test to finish.

Your observation is spot on, and I love that you use the tools to clarify that. I think this is a great contribution to this thread, and I hope to motivate people to use those (I often feel I suggest those, and there is a low motivation to put the legwork into it). So, thank you for that.

Since I write in a forum, here is an overview of the four "cage" options
https://projectfiles.maxon.net/Cineversity_Forum_Support/2025_PROJECTS_DRS/20250312_CV4_2025_drs_25_SIcc_01.c4d.zip

All the best

[jakob-wagner]

@Dr-Sassi Thank you. Much appreciated.
I'm not explaining myself correctly. I'm not talking about the mesh density of the original mesh. But the mesh density of the collision shape mesh (collision cage or what actually is the right term for it ). I can go with triangular collision shape which will be really detailed. Or I can go with Auto or Convex Hulls, which is to simplified. What I really want is something in between.
In the docs it says that you can use the Geometry Accuracy to make it closer assembly the original mesh. But it doesn't really do much. In this image I have tried a geometry accuracy of 0, 5 and 100. The collision mesh changes a bit, but it doesn't get to a closer proximation of the original mesh. Do you have any idea why?

Thanks,
Jakob

[Dr. Sassi]

Hi Jakob,

I would say that the Egg is close to the perfect shape of the Sphere and, even in the lowest setting, finds good coverage. Since this might be a point of debate, I would like to point to a case that shows how it works with visible changes.

https://projectfiles.maxon.net/Cineversity_Forum_Support/2025_PROJECTS_DRS/20250315_CV4_2025_drs_25_SIch_01.c4d.zip

Now, the Documentation needs to be quoted to make the point:
"If set to 0, recessed indentations or recesses or holes will not be included in the collision calculation, but only a single "rubber skin" will be placed around the object…"

The Convex Hull is unlike a Tesselation algorithm that changes with the surface angle, like Adaptive in Spline interpolation.

For example, if you have a hole in the Egg, changes will appear.

https://projectfiles.maxon.net/Cineversity_Forum_Support/2025_PROJECTS_DRS/20250315_CV4_2025_drs_25_SIch_11.c4d.zip

Have a great weekend