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Compound collision on parent and child softbody not working as expected

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Steve Bentley
Compound collision on parent and child softbody not working as expected
on Oct 24, 2018 at 8:18:08 am

I've got a very thin softbody object I'm dropping and then bouncing on a rigid body surface. Of course the soft tall thin object ends up toppling over - not what I was looking for - what's the point in playing god if you can't screw with physics? So I thought I might give the tall thin object a little invisible "foot" to help stabilize it as it settles from the bounce and keep it upright. The foot is a separate object and a child of the tall thin parent doing the bouncing (so I can turn the foot off in the render and it wont show) and the parent has the softbody tag with Compound collision feature turned on.
I thought Compound Collision option would make both objects act as one for collision purposes (without colliding with each other) and I though the soft body aspects would hold for both.
Not only do the two objects break apart when they collide as a unit with the surface but the foot is acting more like a rigid body in the way it bounces than the soft body tag the parent demands.
Are my expectations too high?
I'm ok if the foot is calculated as part of the collision/bounce and affects the rebound reaction as the parent and child (as one) collide with the surface. In other words, I realize the foot may change the bounce calculations and the look will be different from just the parent bouncing on its own.



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Brian Jones
Re: Compound collision on parent and child softbody not working as expected
on Oct 24, 2018 at 9:29:48 pm

without any testing, some thoughts
-regardless of shape or pieces if Individual Elements is off should it not act as one piece (don't know if I've done that with a soft body)
-it sounds like you want weight at the bottom - mass map?
-can you attach a spring to the top and keep it loose but tighten it when you need to?


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Steve Bentley
Re: Compound collision on parent and child softbody not working as expected
on Oct 24, 2018 at 10:38:00 pm

Thanks Brian.
I think something's up. I did try a Mass Map and that doesn't seem to be working at all (I was using it like a weeble). I even tried moving the mass center below the object.
But I'll give the top spring a try (like one of those Boston Dynamics robots!)
And yes I have the Individual Elements button set correctly (at least Manual-Correct for what that's worth)
Cheers



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Steve Bentley
Re: Compound collision on parent and child softbody not working as expected
on Oct 28, 2018 at 3:09:06 am

So Compound Collision works just fine for Rigid bodies - I can't find no indication in the manual that it won't work for for Soft Bodies, but try as I might (have tried tag on the parent and have tried tag on a null that is the parent of all nested objects) the objects break apart and go their own separate ways.



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Brian Jones
Re: Compound collision on parent and child softbody not working as expected
on Oct 28, 2018 at 5:10:03 am

can you use Follow Rotation in Force?
Changing C of G didn't do anything here either (that I could tell).
I was able to get a mass map to work but I had to subdivide the bottom part to get enough mass down there and that made the bottom more flexible which meant the map had to be used in stiffness which changed the overall flexibility when it landed. Maybe a some delicate settings work would get it but follow rotation at more than 0% seems a lot easier if you can use it.


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Steve Bentley
Re: Compound collision on parent and child softbody not working as expected
on Oct 28, 2018 at 5:34:02 am

I was running in to that too (mass map making a disproportionate jiggle). To add to the complexity I'm firing the object out of an emitter. I had done this with a dummy and got it working perfectly. Of course now that I have my real object the dummy is me (I hate that - test a simplified version to make sure you can say yes to the client and then have the problems start when the actual geometry gets involved). The increased complexity and number of polys seems to be having an effect on the tension within the softbody, so now even firing it out of the emitter with no air resistance imparts a wobble and up-scaling that I didn't have with my dummy object. (the dummy had less polys). I am animating the damping and the stiffness etc which works in the collisions but the emitter seems to add its own energy that isn't controlled by the softbody tag on the ejected object.

I think I have it licked though. I'm bringing in rigid body collision objects (invisible) that nudge the object a little to both keep it upright enough and to add a damping at just the right spots. A bit of back seat driving if you will.

The only way I was able to make the compound body work was with rigid bodies. With the softbodies I can get it pretty close but the thin object ends up curling and sliding down the face of the support object after impact, as if its had too much to drink.

Thanks for being a sanity check. Somehow the blood on the brick wall doesn't seem so bad when there are marks there from others who have tried.



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Steve Bentley
Re: Compound collision on parent and child softbody not working as expected
on Oct 28, 2018 at 6:10:15 am

Oh and the custom center of mass only works on rigid bodies too. I made a tear drop out of a cone to have a weeble to test with. Turns out, if you have self collision turned on the tip of the cone splays open as it collides with itself - kind of like that early CG film panspermia, or an Alien egg if you've got the friction turned up. So I guess that means cones aren't really closed. Will have to run a 3D print to see if they are "water tight".



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Brian Jones
Re: Compound collision on parent and child softbody not working as expected
on Oct 28, 2018 at 3:01:46 pm

don't know if cones are water tight, they are just a cylinder with one end closed, you can open the end, and if it's Top or Bottom radius of 0 and you make it editable there are 36 points (at default) at the point.
..and no surprise about C of G being rigid only in retrospect I guess, since the soft body can deform the C of G would be constantly changing which wouldn't be bad for a few but would be nasty for a bunch of clones.


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