# Constrain rbd object to a curve?

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Hello,

Is it possible in Houdini to constrain an RBD object to a curve? So that gravity will still make the object fall, yet its position is always constrained to said path. I've been searching though both forums and in the help and googling but I can't find anything which suggests it's easily doable, nor anyone trying a similar effect. It would be like having beads on an abacus pretty much. If anybody is familiar with reactor, there is a PointPath consraint which does what I'm after.

If you constrain an object to a path normally but then make it an rbd object it ignores the path and just falls down. I was wondering whether some complicated mathematical expression would give the same effect, or using expressions to change the direction of gravity based on the point numbers of the curve, but that might be very difficult.

Thankyou

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Hello,

Is it possible in Houdini to constrain an RBD object to a curve? So that gravity will still make the object fall, yet its position is always constrained to said path. I've been searching though both forums and in the help and googling but I can't find anything which suggests it's easily doable, nor anyone trying a similar effect. It would be like having beads on an abacus pretty much. If anybody is familiar with reactor, there is a PointPath consraint which does what I'm after.

If you constrain an object to a path normally but then make it an rbd object it ignores the path and just falls down. I was wondering whether some complicated mathematical expression would give the same effect, or using expressions to change the direction of gravity based on the point numbers of the curve, but that might be very difficult.

Thankyou

hi there,

excuse me for asking , but what I gather from this question is something like this images?

"It would be like having beads on an abacus "

mangic

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Hi mangi!

Yes- that's the sort of thing I'm talking about- although the rails would need to be curved in the end (such as in the image below). Ultimately it will be for something more complex but using the abacus example is the easiest way to describe it and probably a good starting task anyway. Today I was looking into using wire dynamic objects- they will be useful to me later based on a test I did earlier- but still will be dependent on being constrained to a path. I've been able to make the dynamics bit using reactor and importing using fbx, but I'm trying to shake off my dependence on other software while I learn Houdini- but so far I'm struggling to find a way to do this task.

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Hi mangi!

Yes- that's the sort of thing I'm talking about- although the rails would need to be curved in the end (such as in the image below). Ultimately it will be for something more complex but using the abacus example is the easiest way to describe it and probably a good starting task anyway. Today I was looking into using wire dynamic objects- they will be useful to me later based on a test I did earlier- but still will be dependent on being constrained to a path. I've been able to make the dynamics bit using reactor and importing using fbx, but I'm trying to shake off my dependence on other software while I learn Houdini- but so far I'm struggling to find a way to do this task.

Well this is not intended to be nothing but a real basic way in what I think maybe a technique to do this.

Of course With some tweaking and the modeling of the curve.

hope it helps a bit.

Cheers

mangic

abacus.hipnc

visexample.mov

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Hi again,

Thanks- that has been my plan B all along and I have indeed been using that method for the past few weeks in a different program and have recently started to do it in houdini- but the results so far for me aren't as good as when I was doing it with reactor. Also, I'm trying to make the movement precise. I managed to smooth the bumping motion/rotation using chops (recently started on Andrew Lowells chops book) a little using filters and so on, but then that always causes the path to move slightly.

I've come across in the Constraint Tools Dops a couple of dops - such as AnchorObjectSpace that have under Constraint Type option- 'Constrained to Line'. This sounds promising but I'm still trying to figure out how it works and how you set it up. If it does what I think it might- then I'm wondering if subsequently I could reference something into the xyz fields, so the line is actually a tangent which constantly changes depending on how far the object travels along the path. Of course I may be thinking about this all wrong- even if I was right with that- I'm not sure I could pull it off as I'm still new to all this.

Thanks again-

EDIT: in fact the method I've been using up until now and that which you mentioned also has it's advantages, because it sort of helps with the next step in my procedure for my project, but again- only approximately- using the abacus example the beads rattle around- the hole in the middle of them needs to be big enough to allow bends in the path otherwise it gets stuck, but at the same time, it's more vulnerable to tiny movements (hence my filtering it with chops).

But because this weekend I started to sort out a workaround with respect to the rotation using attribute transfer- I'm hoping if I can get dops objects to follow a curve precisely directly in houdini it will make everything a lot more accurate. otherwise I will fall back to importing the dynamics using fbx and filtering it inside houdini, reexporting etc etc. My final goal would be to make a digitial asset with a 'Select your input curve' affair. Currently I'm not as comfortable with curve creation in houdini as the other package where I've been importing my rail geometry from. I've not mentioned what is I'm actually trying to do yet- as I would like to advance further first..

Edited by spf987

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Hi there ,can you make a small clip of what you are after, the way you made the sim in reactor,

I am having difficulties , in trying to see what you are after.

mangic

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Hopefully this is visible- The path I've just made renderable in viewport. The rotation you can't tell but at least the position is faithful to the curve and doesn't jump about.

abacus2.mov

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Hi there,

I saw your post when I was at work.

You said you made this with reactor, I guess in 3d max.

Well I am going to take a look at it, but at first hand I don't think there is an easy way to do this In DOPS directly.

Yo did say you can pump it out via FBX , this maybe you only way out of this,

At first hand it just looks like a ball with a path constraint , maybe there is a way to pass the animation controled by a dynamic object via chops. Just wild guessing.

mangic

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Hello- yes it was 3ds max. I think I will have to leave it with fbx as you say until I come up with a better solution. Sorry- the clip i made wasn't particularly descriptive- I should have put a couple of objects on the path like in yours. I will keep trying..

Thanks for your help and suggestions

okay

cheers

mangic

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Very happy After various weeks on and off trying different combinations and methods both in and outside of Houdini I've at last achieved the effect I need. I had to do the physics in reactor as I was mentioning (also only recently properly achieved) but have managed to use Houdini to solve all orientation problems (which resulted as a side effect of having position-perfect data). I never mentioned the specific orientation problems here as I knew it was something I should be able to solve myself, and a matter of spending time digging through forums, checking out help files and trying to adjust to my situation, and it has payed off. Basic but critical technical bits out of the way, I can get on to the interesting stuff now. I'll continue to use Houdini where I can with this learning project.

I'll come back to the curve constraint issue in dops at a later date, but for now though the major milestone is beaten down, it's nearly Friday and it's warm and sunny outside.

cheers mangic

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