Packed RBD Objects and Flip Explodes

Forums Technical Discussion Packed RBD Objects and Flip Explodes

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syntheticperson: Member; 170 posts; Joined: July 2010; Offline

Aug. 26, 2018 1:23 a.m.

I'm trying to create cheerios falling into milk.
I did a test with 2 toruses falling into a flip sim.

If the toruses are separate RBD hero objects, then the sim works fine.
https://drive.google.com/open?id=1Z98WekxuaIQ89vmxReqRHTgWjxgkuPUX [drive.google.com]

However, if I try using packed RBD objects, then the sim explodes.
https://drive.google.com/open?id=13U4Su0a3iVEpLx38lEuWxpORUdciKZCP [drive.google.com]

I tried reducing the flip solver interaction scale from 1 to 0.5, but that didn't help.

Do packed rbd objects interact correctly with flip correctly in houdini 16.5?

What am I missing?

thanks

Attachments:
Torus2PackedRBD_FlipExplodes.v011.hipnc (2.5 MB)


Enivob: Member; 2658 posts; Joined: June 2008; Offline

Aug. 26, 2018 9:38 a.m.

I think what you are seeing is the result of different collision types. The hero object uses Volume Based collision which creates a volume to represent the torus with a hole in the center. But the default collision type for RBD is bullet convex hull which makes a pill type shape, not a torus type shape. (i.e. the hole in the center is not detected). To fix this turn on Show Guide geometry on the RBD Packed Object and choose Concave for the geometry representation. This will give you a collision object that better represents the torus.

Using Houdini Indie 20.5
Windows 11 64GB Ryzen 16 core.
nVidia 3060RTX 12BG RAM.


syntheticperson: Member; 170 posts; Joined: July 2010; Offline

Aug. 26, 2018 2:16 p.m.

I'll give that a try. Thanks!


syntheticperson: Member; 170 posts; Joined: July 2010; Offline

Aug. 26, 2018 2:44 p.m.

That fixed the problem!
Much thanks!

https://drive.google.com/open?id=18nTq-8GFyKJA9IBRB2Rswbb6aZxw0AI_ [drive.google.com]


syntheticperson: Member; 170 posts; Joined: July 2010; Offline

Aug. 26, 2018 6:43 p.m.

Sorry, I celebrated too soon.

The previous test turned out to be 4 hero RBD objects with the friction and bounce fixed.
The 2 packed RDB objects still explode with the concave collision geometry.
Here is a flip book:
https://drive.google.com/open?id=1qylYEp4lXltHR_e7ef3nvvARQzUwadyn [drive.google.com]
One torus gets stuck in the collision walls of the fluid container.
The other torus shoots off frame over one frame.
I reduced their bounce and friction to 0.1
What am I missing?
thanks

Attachments:
Torus2.PackedRBD_StillExplodes.v014.hipnc (2.6 MB)


N-G: Member; 213 posts; Joined: March 2018; Offline

Aug. 28, 2018 3:20 a.m.

RBD Packed Objects have serious problem with FLIP in mutual relationship.
Use one way collision relationship or use unpack rigid body objects (like RBD Hero Object) if you want mutual relationship.
Hope this helps.


syntheticperson: Member; 170 posts; Joined: July 2010; Offline

Aug. 29, 2018 2:01 a.m.

Good to know. I do want mutual relationship between the RBDs and the flip.
Does this mean that packed RBDs are really not suited for this?
When you say “unpack rigid body objects”, do you mean there is a way to convert a packed RBD object into hero RBD objects procedurally?
Or do I need to rebuild my sim so I'm using hero RBDs only?
If that is the case, how do you manage many hero RBDs?
I want to be able to have potentially 100s of hero RBDs interacting with flip.
Is there a way to do this so that I don't have to set them all up manually?


N-G: Member; 213 posts; Joined: March 2018; Offline

Aug. 29, 2018 9:10 a.m.

If you want lot's of unpack rigid body objects, use RBD Fractured Object node in DOP to bring them from SOP to DOP at once.
I don't know why RBD Packed Objects are not suitable for mutual relationship and I can't get stable behaviour in any way.


Enivob: Member; 2658 posts; Joined: June 2008; Offline

Aug. 29, 2018 10:43 a.m.

Try turning on Bound and Friction for the RBD objects under the FLIP Solver. This links the Phyical TAB of the RBDs with FLIP. I see less explosion when this is on. Also turn off Remove FLIP outside boundary.

Using Houdini Indie 20.5
Windows 11 64GB Ryzen 16 core.
nVidia 3060RTX 12BG RAM.


syntheticperson: Member; 170 posts; Joined: July 2010; Offline

Aug. 29, 2018 3:47 p.m.

I'll give that a try.
thanks!


syntheticperson: Member; 170 posts; Joined: July 2010; Offline

Aug. 29, 2018 3:55 p.m.

I checked every tab and subtab for the flipsolver and the RBD solver, but I couldn't find the controls you mentioned.
Could you send me a screen grab?
thanks


N-G: Member; 213 posts; Joined: March 2018; Offline

Aug. 29, 2018 6:23 p.m.

HIP file attached.

Attachments:
RBD and FLIP.hip (2.1 MB)


syntheticperson: Member; 170 posts; Joined: July 2010; Offline

Sept. 5, 2018 4:08 p.m.

i've been told that packed RBD objects(bullet) don't support mutual interaction with flip fluids.
I'm going with fractured RBD shelf tool, to generate a large number of hero RBDs interacting with flip.
This issue is closed.
thanks all!


HarmonCreative: Member; 1 posts; Joined: March 2017; Offline

May 5, 2020 3:38 p.m.

Thanks for these posts. I'm stuck doing the same thing, and have arrived at the same place, good to hear it's just an inherent problem with Packed objects and not that I'm missing something or just don't know what I'm doing.

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