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DB Hansen
08-29-2013, 08:43 AM
Using Bullet, is it possible to create a softbody cylinder that doesn't have its cross sectional geometry crushed upon collision? I'm creating a slow-mo animation that includes a string (or a rope, if you like) hitting the ground, and the results range from "eh" to comical.

Standard geometry was a mess, so I've set up a subpatch strand (extruded disc with multiple segments) with subdiv order set to last. The two main problems:

1) Shape retention. I cannot figure out how to keep the strand from flattening randomly into a ribbon. Various recipes of shape retention, stiffness and volume conservation help somewhat, but the deforming action of a string should only collapse lengthwise. I need to retain the cross section volume across the length of the object. Can this be achieved with a weight map? Is my geometry incorrect for this type of action? Is scale a factor?

2) Self-intersection. The collided string, aside from flattening, has terrible self-intersecting geometry. Tweaking the collision margin doesn't help. Turning on self-collision results in gothic-looking twisting patterns and/or keeps the animation bouncing and jittering far after the physics should end. Damping only seems to slow down the overall animation, and doesn't affect this odd jittering.

I've also tried using ClothFX, parenting and metalinking the same subpatch to a 2-point poly chain. This yields minimally better results, and there are more dials to tweak, but variations of the above problems follow me there as well (flattening/intersecting/jittering). I'm just not finding a usable solution.

I'm a dynamics noob, so any help is welcome.

prometheus
08-29-2013, 07:58 PM
You might want to check this free dynamic and detail video by Greg Sullivan.
https://www.youtube.com/watch?v=-xTKd7pA3rU

he goes in to creating cable wires that are falling and reacting to dynamics with cloth fx.
some tricks with stetch values and sub structures...check somewhere at 0.46 in the video, but you should follow it through earlier since he sets the cables up with two point polychains in combination with clothfx.
Old stuff but should work.

DB Hansen
08-29-2013, 08:58 PM
Sweet. Nice and dense (over 80 min.). I will check it out.

I had a feeling ClothFX was going to be the way through, as Bullet doesn't have quite the same nuances.

Cheers and thanks.

jasonwestmas
08-29-2013, 09:50 PM
Boy NT sure wasted their time with bullet soft bodies if cloth FX is still the answer. I wouldn't overlook bullet so easily but I guess I can understand if you can't find enough info on it.

prometheus
08-30-2013, 05:26 AM
Boy NT sure wasted their time with bullet soft bodies if cloth FX is still the answer. I wouldn't overlook bullet so easily but I guess I can understand if you can't find enough info on it.

wouldnīt say wasted time, keep in mind that the soft body dynamics wasnīt actually in the planned course for lw.11 I think they said so, since they didnīt had it quite ready, If it isnīt there already
to do such things as mentioned, it probably will be in the updates of the softbody system.
I suspect there arenīt that many of us that knows the inside out of the bullet softbody system yet, and I suspect you could do it with that too...if you know how:)

Cloth fx and using polychains is somewhat proven with reference to the video training from sullivean, but then again it will probably need the old hardfx too depending on what you need to do of course.

Michael

Ztreem
08-30-2013, 08:28 AM
In both Cloth fx and Bullet you need some reinforcement geometry to keep the cables not to flatten, like this model.
116706
It's the only solution I found that works, besides metalink a two-point-poly-chain. But with metalink it's hard to get the cables to collide with each other.

jwiede
08-31-2013, 01:57 AM
In both Cloth fx and Bullet you need some reinforcement geometry to keep the cables not to flatten, like this model.
116706
It's the only solution I found that works, besides metalink a two-point-poly-chain. But with metalink it's hard to get the cables to collide with each other.

I wonder if giving actual thickness to the tube "walls" would help prevent collapse/kinking as well. As it stands, you've got very, very thin walls relative to the diameter of the tube (basically Bullet's min evaluation distance, if you don't explicitly provide thickness by geometry) -- empty, they would normally be prone to kinking and collapse unless of ridiculously stiff material. If ZTreems (easier) approach doesn't yield the behavior you want, perhaps try thickening the tube walls?

prometheus
08-31-2013, 02:20 PM
heres a simple rope and collision object, not sure if thereīs anything in there that you would need or so.
clothfx is applied on polychain only and the rope object itself is metalinked to that dynamic simuation.

Michael

DB Hansen
08-31-2013, 04:19 PM
Yeah, I don't know. Thanks heaps, Michael, for the files. The set up is ideal for that motion. Looks good here. Sadly, anything more complicated than horizontal collision seems a little too wonky for the dynamics engine. The ClothFX settings are touchy and highly iterative, that's for sure. So maybe I'm still not finding the secret sauce.

Polygonal cross-sections, as Ztreem suggested seem to work the best for falling/vertical collision, but I still get intersections no matter what I try. Perhaps a thicker geometry would help, as j suggested, but even reworking Michael's rope (which has internal geometry) didn't yield much better results.

The best I've come up with is attached. Ultimately I may have to find a workaround. In the end it'll be a slow-motion animation, so I may just replicate the falling action with some bones. There is shattering glass too, so maybe I can hide the rope action in all that chaos.

Too bad Bullet is even harder to get clean geometry. The physics are much prettier there.

DB

jwiede
09-01-2013, 01:04 AM
Too bad Bullet is even harder to get clean geometry. The physics are much prettier there.

Try Chronosculpt?

erikals
09-01-2013, 03:57 AM
Bullet is harder to get clean geometry?

haven't used it much, but i thought Bullet could do it...

prometheus
09-01-2013, 07:24 AM
Bullet is harder to get clean geometry?

haven't used it much, but i thought Bullet could do it...

neither clothfx or bullet can retain itīs volume shape properly if used on the rope geometry itself it seems, I tried some hour yesterday with bullet softbody, testing a lot of changings
and also ran a weight map on linear centerpoints to use as gradients for shape retension and other settings, but nada.
you could get the rope to retain itīs shape but it will be rubbery and will not bend like a rope, just bounce like a thick rubber stick or wooden stick, if you try and get the ropey look, you
end up with stretched and deformed geometry as with cloth fx.

However, instead of my sample with clothfx on a polystring that is metalinked the rope geometry, you could also run bullet softbody on a polystring and metalink that the same way, since
bullet gives nicer physics than cloth fx, thatīs probably the best to do.

Michael

erikals
09-01-2013, 08:09 AM
but if you run Bullet on a polychain, wouldn't you in the end get the same rubbery movement.. ?

you might wanna check out this old thread at cgtalk >
http://forums.cgsociety.org/showthread.php?p=2743317#post2743317

you'll have to add a metalink object though...

simulations will be so-so of course, due ClothFX...

prometheus
09-01-2013, 08:35 AM
but if you run Bullet on a polychain, wouldn't you in the end get the same rubbery movement.. ?

you might wanna check out this old thread at cgtalk >
http://forums.cgsociety.org/showthread.php?p=2743317#post2743317

you'll have to add a metalink object though...

simulations will be so-so of course, due ClothFX...

No..Im not talking about dynamic movement on polychain that looks rubbery, bullet on a polychain seem to yield nice results for its dynamic motion, itīs when you apply bullet on the geometry rope directly it behaves rubbery.

Im not sure bullet on polychain is the way to go though, I thought I could metalink the geometry as I did with cloth fx, but appaerently I have run in to some issues here, it isnīt working right now? tried mdd baking the polychain and
reload the motion and metaling the geometry ..but nada, either it doesnīt work or I have missed something.

Michael

erikals
09-01-2013, 09:12 AM
you're thinking metalinking the Bullet sim..? not sure, haven't tried it yet. maybe you'd have to MDD bake it... :/

XswampyX
09-01-2013, 09:23 AM
2 point poly chain -> negative edge rendering?


http://youtu.be/IEuxwj2ETF8

erikals
09-01-2013, 09:35 AM
They're Alive! http://erikalstad.com/backup/misc.php_files/aiwebs_001.gif

i guess MDD baking could do the trick, for then to use the DPont MDD node, i think it can fade out the MDD, so they stop moving...
(not 100% sure though...)

http://dpont.pagesperso-orange.fr/plugins/nodes/nodes/MDD_Pointer.html

prometheus
09-01-2013, 01:23 PM
you're thinking metalinking the Bullet sim..? not sure, haven't tried it yet. maybe you'd have to MDD bake it... :/

yepp...I did that and tried metalink directly, but didnīt get it to work, then I did mdd bake it out with the native lw mdd baker and reloaded it, deleted the bullet sim completly then tested metalink, but I just couldnīt
get the geometry to metalink and deform strangely, have to check somethings if I missed anythign or so.

Xswampy...yes I though of negative line too, but I would rather use real geometry and smoth subpatched ones.

Michael

prometheus
09-01-2013, 01:49 PM
Ahh...sort of solved it, Instead of running mdd baker, after the bullet sim on the polystring as softbody, I added cloth fx to the polystring, but I didnīt calculate, here we just need to scan the motion, then the metalinking is working with that
bullet softbody simulation

So it was a an old cloth fx scan motion, with bullet softbody and metalinking that works, thereīs nothing stopping geometry to overlapping and intersect since theres no dynamics applied to it, maybe trying with old softbody
on top of all this, and for the initial sim ..one would probably set a collision offset matching the rope geometry diameter.
http://www.youtube.com/watch?v=yAP5sbBC3qU

erikals
09-01-2013, 02:49 PM
Nice! http://erikalstad.com/backup/misc.php_files/smile.gif so... >Bullet >ClothFX >Metalink

prometheus
09-01-2013, 03:01 PM
Nice! http://erikalstad.com/backup/misc.php_files/smile.gif so... >Bullet >ClothFX >Metalink

yepp..and clothfx only to scan motion (add clothfx on the polystring previously simulated with bullet, dont deactivate bullet, and go the clothfx file tab/scan motion) after that just metalink
the rope geometry, this gives the best dynamic behavior of the rope..except that the interaction, the self collision of the rope geometry surface itself isnīt there.
So that is an issue..I tried adding soft fx, and bullet again on the rope geometry that still is metalinked, but it wonīt do anything on the rope geometry since the mdd loaded file is locking that up.
116755

the other way to use bullet only on the rope, I canīt figure out...since the rope will be stretched and not retaining itīs rope geometry, using bullet on the geometry directly will let it avoid geometry intersection and have self collision, but
as mentioned..it will look like rubber or slime mostly:)


Michael

prometheus
09-01-2013, 05:07 PM
Chronosculpt might be something to use to correct intersecting geometry, maybe I should post the scenefile for those of you that have chronosculpt and test if it would be easy to fix.

DB Hansen
09-01-2013, 06:28 PM
You guys are great. Thanks for digging in further to point things in the right direction. I don't have chronosculpt for final cleanup, but am eager to see what kind of results can be achieved using these two dynamics options together.

erikals
09-01-2013, 11:12 PM
...using bullet on the geometry directly will let it avoid geometry intersection and have self collision

looking at XswampyX's 2-point-poly Bullet example though, it does look to have both...?

prometheus
09-02-2013, 04:51 AM
looking at XswampyX's 2-point-poly Bullet example though, it does look to have both...?

Thatīs probably just a general good simulation setting regarding collision offset, or self collision, I donīt think he has real geometry there since itīs only negative line trick rendering and the only thing that can interact
is the collisiom offset from the polystring itself, but it looks decent regarding that, depends also on the radius of the geometry your simulation if using geometry that is.
Maybe the dynamic FPS is set much higher to give more accurate results or something.

Michael

erikals
09-02-2013, 05:28 AM
yep, XswampyX's note from YT >


A quick test of using a 2 point poly chain and the bullet physics system in lightwave 3d to simulate a piece of string. I got carried away and added many strings and ended up with noodles.