Enabling Collisions in the RBD Bullet Solver in Houdini

When working with simulations most likely you will need to have geometry collide with what you are simulating. Today we go over how to enable collisions in the RBD Bullet Solver in Houdini.
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[@xendoss8823]

Holy heck this is exactly what I needed! Nice video, explains everything in an easy to understand way. Probably coming back to this channel for more tips and tricks

[@InsideTheMindSpace]

Glad you found it useful! Feel free to join the discord and you can ask questions if you need help

[@skunchtv]

exactly what i needed Thank you dude

[@Navins3003]

Awesome tutorial! Thank you!!

[@flyingsquad2534]

if we have an animated character do we check the animated option in bullet data ?

[@LIMBICNATIONARTIST]

Great tutorial! Thanks!

[@InsideTheMindSpace]

Glad I could help

[@fakn9159]

Nice Video!
When I render my scene, I see the collision but I don't see the Collsion Object (the ball) in the renderings... Do you know why?
Thanks!

[@InsideTheMindSpace]

You will need to merge the collision geo back together after the simulation

[@davidgolan1087]

Thank you for this video! I'm applying this information to create my own collision for an assignment in my college. Part of my assignment asks to export the collision into Unreal to add simulated effects. If I created a collision using this method, is it possible to export it out to Unreal? If so, how would you go about doing that?

[@InsideTheMindSpace]

You can do the entire simulation inside of Houdini and export as alembic. That would be my recommendation. Not exactly sure if that's what you are asking. There is also a way to export certain data and use that to drive effects inside of Unreal but do not have much experience with that to guide you unfortunately

[@madmodder]

I would recommend exporting the sim as an fbx, by using a labs RBD to FBX node in the out graph

[@aamg1992]

is there any way to glue colliding rbd objects?

[@InsideTheMindSpace]

If there is it would be a complex process. I've never tried it so I couldn't say for sure

[@xauthority]

very interesting. One question. How can I add second material. For example my first one is glass wall, behind it I want either concrete or wood so when the ball goes through. It gives both material simulation.

[@InsideTheMindSpace]

Materials are just attributes on the geometry so you can create all the attributes at the settings you need and merge everything together before wiring it into the solver.

[@xauthority]

@@InsideTheMindSpace but the merge node has only one out put. Where as solver has multiple

[@InsideTheMindSpace]

You merge each individual stream together so merge each of the first outputs and then wire them into the solver then the second outputs and so on.

[@tejareddy5732]

Hi Buddy, after rbd bullet solver, how to get the collision sphere out to display in render view. thanks!

[@InsideTheMindSpace]

You should just be able to merge the object that you wire into the Collison input with the output of the simulation

[@tejareddy5732]

@@InsideTheMindSpace i tried that bro, actually, I gave the initial vel to the sphere, after merging the bullet solver and sphere (after Point vel Node) , it's not working.

[@InsideTheMindSpace]

Without seeing your setup it's hard to diagnose the issue. You should be able to merge the collision geo with the output of the situation. The other option is to merge the objects before the sim and only use the first input of the rbd solver. You could also bring both into a second geometry node and use that as your rendered object.

[@debashischandra6152]

@@InsideTheMindSpace Yes I put the merge node below the Bullet solver and connected the collision geo into the merge node as well as solver node. Its working and i rendered out perfectly.

[@skunchtv]

my only question is how to get the Collision geometry (The sphere) to render instead of using the visualization. like when i go one level up out of this network i can see the collision happen but I cant see the ball anymore. Any ideas?

[@InsideTheMindSpace]

All you need to do is merge the collision geometry with the result of the simulation or alternatively you could wire it into an out null and object merge that in a new geometry node to keep things separated

[@skunchtv]

@@InsideTheMindSpace i did figure it out. I think its probably easier to do this in a dop network rather than using the rbdbulletsolver sop. But if using the sop, you need to use a dop import. so if you create another geo node and then throw down a dop import-> type /obj/< your geo name>/rbdbulletsolver1/dopnet/ in dop network field. For object mask type collision* then change import style to Fetch gemoetry from dop network. Voila. Your collision object. Then just merge in all the rest of your nodes with object merges, turn off flags as necessary and voila. i have no idea how the point velocity would still work otherwise. i could not get a simple merge to work.

[@InsideTheMindSpace]

That way is vastly more complex and unnecessary. Simply merging the resulting simulation with whatever you pipe into the collision geo input works just fine or like I said if you want to keep them separated wiring the collision geo into a null and bringing that into a separate geometry node with an object merge

[@skunchtv]

@@InsideTheMindSpace How will the point velocity be respected??? i seriously tried this. I must be stupid

[@InsideTheMindSpace]

Perhaps I am misunderstanding your setup. Are you simulating the collision geo in order to have movement there as well?

[@somnathdas7023]

in render the collision object is now showing whats the problem?

[@InsideTheMindSpace]

If the bullet solver is the last node in your tree or is being displayed that might happen I believe

[@somnathdas7023]

@@InsideTheMindSpace yes that"s my last node so in order to show it in render what i have to do?

[@InsideTheMindSpace]

If you are trying to not show the collision object at render if I remember correctly adding another node after the solver will only bring the object being simulated so adding something like a null and setting the display to the null would work I believe

[@somnathdas7023]

@@InsideTheMindSpace thanks bruh

[@JohnSlaughterND]

What if I want the ball to also shatter? no one seems to cover that scenario.

[@InsideTheMindSpace]

Been awhile since I messed with rbd but if I remember right you can set up the fracture and all of the attributes like normal and then merge the geometries together before running them through the simulation.

[@JohnSlaughterND]

@@InsideTheMindSpace I tried that, but only one of them keeps their glue constraints and the other immediately crumbles.

[@InsideTheMindSpace]

If you are not you will need to merge every input of the bullet solver otherwise it won't have the information from both.

[@JohnSlaughterND]

@@InsideTheMindSpace I did that too. I even merged their glue constraints before I attached them in the same manner.
I’ll keep pecking at it. Thanks.

[@InsideTheMindSpace]

I'm not at my computer at the moment. I will have to revisit. Like I said it's been awhile since I did much with rbd. Feel free to join the discord and I will try to remember to take a look when I have a moment