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Bullet vs Houdini RBD


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I am not familiar with the RBD systems, apart from reactor in Max, but from the tests I have seen Bullet seems pretty fast, whereas Houdini's RBD seems slow on even simple sims. I notice it easily even when simulating dozens of boxes free falling via gravity.

I am not writing this to criticize but to ask "why the gap?". Is it because Bullet is using GPU but Houdini's RBD uses CPU only? (I don't know if this is true)

Thanks.

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Bullet is a RBD system designed for video games. It's not intended to be accurate.

Houdini's RBD supports things like dynamic friction, multiple methods for building the collision volumes, substep processing, etc.. etc..

Isn't Bullet suppose to be in H12?

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Bullet barely uses the GPU for RBD. Think the Bullet Cloth solver is GPU accelerated but can't remember at the moment and too lazy to google it.

And yes Bullet will be in H12 according to the presentation from SIGGRAPH.

I think he's confusing that with PhysX, which is nVidia GPU based engine for games. It's also very fast.

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My understanding is that Bullet uses crude bounding boxes to determine collisions, whereas the native Houdini solution uses a much more accurate bounding solution(such as volumes if you want). This results in more accurate simulations, but they are a lot more computationally expensive and neither one is really using the GPU. Hopefully that will change in H12.

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Thanks guys, I wasn't aware. If it's mainly for games though why are FX folks getting excited for this to be integrated into their app (Max folks for instance).

I believe rayfire also uses Physx.

What's the motivation of Houdini getting Bullet? Also why not Physx if that use GPU, thus faster?

I didn't know Houdini's RBD support different collision geometry. Do you know where can I specify this? Also is there a "Use Convex Hull/Geometry" option? Because that would work for most cases and speed up the sims big time. Right now I am making some proxy objects but this step is semi-procedural and time-consuming. Max reactor also has this option:

ill_floater.gif

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I only had the chance to test the bullet physics engine with the Dynamica plugin in Maya. It really runs very fast, and does a pretty good job with massive simulations. The rigid body objects can be set to collide by: box, sphere, convex-hull or geometry being the last one the slowest. Convex hull works very good. One thing bullet is missing (at least i couldn´t get it working) is as hopbin9 says, dynamic friction while working with sliding situations.

Edited by NicoZ
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Thanks guys, I wasn't aware. If it's mainly for games though why are FX folks getting excited for this to be integrated into their app (Max folks for instance).

I believe rayfire also uses Physx.

What's the motivation of Houdini getting Bullet? Also why not Physx if that use GPU, thus faster?

I didn't know Houdini's RBD support different collision geometry. Do you know where can I specify this? Also is there a "Use Convex Hull/Geometry" option? Because that would work for most cases and speed up the sims big time. Right now I am making some proxy objects but this step is semi-procedural and time-consuming. Max reactor also has this option:

You can specify what collision geometry to use on the Collision tab of the RBD Object. It can also be written out etc.

Physx can only run on Nvidia cards hence it's not a very awesome solution. Bullet is working on a OpenCL implementation that will run on the GPU and can be used on both nvidia and ati cards.

Convex-hull i doubt would speed up Houdini's RBD solver much as it uses volumes for collisions. It would not make doing the lookup in the volume any faster (I think).

Anyways, Bullet is exiting because you can simulate shitloads of pieces in a reasonable time. If you only got some "Hero" pieces you are probably better off simulating them with Houdinis own solver to get a more correct simulation.

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Physx can only run on Nvidia cards hence it's not a very awesome solution. Bullet is working on a OpenCL implementation that will run on the GPU and can be used on both nvidia and ati cards.

You mean there are still a few people using ATI cards?

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You mean there are still a few people using ATI cards?

Hehe, they are not such terrible buys. The 6950 is about same price and performance as a 560Ti. Not saying I would buy one tho but I am still strongly against code that only runs on specific systems unless it's very specific.

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Hehe, they are not such terrible buys. The 6950 is about same price and performance as a 560Ti. Not saying I would buy one tho but I am still strongly against code that only runs on specific systems unless it's very specific.

We've had some ATI cards do some funky things with Houdini. Freezing, crashing, etc.. etc..

The drivers for the nVidia 560->590 cards have been very stable. We have three here working fine.

Anyway, don't want to redirect the discussion into yet another video card war.

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@Erik, now that I thought about it, are you sure convex hull option for rbd sims wouldn't speed up? When I make a hand made convex hull for the same shape with much low res of course, it sims so much faster. Probably 20-50x at least. I think this option would be useful.

But I didn't see any operator to procedurally generate convex hulls. Is there one? (Maybe a VEX SOP?)

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Don't know if this will help or not, but Digital Tutors has some nice basic tutorials on sims and how to use custom boundary objects for both the native Houdini sim, and for ODE (which is the Bullet solver, right?). At least I found it helpful.

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Don't know if this will help or not, but Digital Tutors has some nice basic tutorials on sims and how to use custom boundary objects for both the native Houdini sim, and for ODE (which is the Bullet solver, right?). At least I found it helpful.

That's how I learned Houdini. On DT for a few months. They're stuff is nicely organized. To bad it's dated and not kept up to date.

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Thanks guys, I have DT, and only watched their Houdini intro. I am just trying to keep it at the same level for all training vids I have. Then slowly go into more specialized topics.

I want something like this...

Accept we replace Kung Fu with Houdini.

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