View Full Version : Getting Bullet to behave...

04-21-2012, 05:11 AM
Here's what's happening, guys. I built a house of cards (see pic) on a "table" (cards object on layer 1, table on layer 2). I then added a sphere to layer 3 and brought everything into Layout.

I made the table a Static object, made the cards a "Parts" object, and the sphere a Kinematic object. See the attached video to see what happened as a result of this. Nothing worked as it should have. The cards fell through the table and those that didn't fall hung in space as though nothing had changed.

Any ideas as to why this might be happening or what I'm doing wrong?

04-21-2012, 05:15 AM
Oh, one more thing...

I parented the ball to a null to get the arc of the swing nice and smooth, but I don't know if Bullet plays well with nulls and kinematic objects. (Sure wish I had some documentation on all this!)

Thanks for any help in advance!

04-21-2012, 05:15 AM
What's your unit scale? Make sure it's not to big or to small. Are the card single sided geo?

04-21-2012, 05:20 AM
Cards object (the stack of them) is double-sided and about 4m tall.

04-21-2012, 05:24 AM
Want me to post the object so you can all take a shot at it?

04-21-2012, 05:48 AM
Increase the dynamics frame rate (default of 180fps isn't very accurate); try putting up pretty high if needed.

You could also increase the collision margin a little (default is 0m, try putting it to a few mm).


Just checked the object file, you really need to give the cards thickness or it just wont work.

04-21-2012, 05:55 AM
Ok I added thickness to the cards (made them 2mm thick) and increased the frame rate to 360fps; I didn't change the margins or anything and the cards simulate fine.

I'll render off a little sequence and post it in a mo.


added video

04-21-2012, 06:16 AM
OK. Thanks, Skonk...will remember to do that from now on.

04-22-2012, 02:24 AM
After uninstalling and reinstalling 11.01 a couple of times, and trying every setting I can in FX Tools to get this object to collapse properly, I'm about to give up. The cards - as a Parts Object - don't fall, they "clip out" and disappear, many of them hanging in their original position. I would've gotten a screen grab of this behavior, but after several attempts to scrub through the timeline to reproduce this, I get a critical error and Layout crashes.

I'm running Windows Vista 32 (Aero disabled) on an HP DV6000 laptop (Dual Core processor, 2GHz) with 3 GB RAM, and an Nvidia GM8400 mobile graphics card.

By comparison, 10.1 runs flawlessly on this machine. I will post an image of the issue if I can keep the effect from crashing the program long enough to do so.

04-22-2012, 02:41 AM
Please see attached series of pics.

The first image is rendered 1 frame before the collision.
The second image is rendered on the frame of the collision.
The third image is rendered 10 frames after the collision.

The last image is a screen cap of the Bullet settings for my cards object. Activation is set to "Start Sleeping."

04-22-2012, 07:08 AM
It seems to work here, just add some depth to your cards.



04-22-2012, 09:32 AM
I added thickness to the cards after the first fail. Close up image shows 1 cm thickness.

04-22-2012, 11:21 AM
I remade the stack of cards in a new object, adding a little more thickness to them and adding a very tiny gap between each edge. (I also made the stack a little smaller for rendering purposes.)

Yeah. It's fun blowing s*** up! :thumbsup:

04-22-2012, 11:40 AM
This may not have been a secret to anyone else, but I just found out nulls can act as kinematic bodies, too.


04-22-2012, 12:03 PM
Did you ever try changing the Dynamics Framerate?

If you have small/thin/fast moving objects then it's possible for objects to pass directly through each other in the space of 1 frame; thus not colliding. Increasing the Dynamics Framerate will make the simulation more accurate (but slower to calculate) and I suspect this was your issue.

This is why you should always make sure all your objects have at least a little thickness (even floors) to help prevent the issue of objects passing through each other between frames and allowing you to accurately simulate without having to increase the FPS so much that it takes an age to calculate.

In the render I did, the only thing I changed on your model was to extrude all the cards (using LWCad Mass Extrude) by 2mm; I didn't move or separate them or anything.

But I did change the Dynamics Framerate from the default 180fps, to 360fps and it calculated fine for me.

04-22-2012, 01:54 PM
I think I did change the frame rate on that last render I uploaded, too. But I just got back in from mowing the lawn and haven't cracked Layout open yet. I'll check later and get back to you.

Thanks for the tips, guys. I have a plan for the whole "house of cards" thing in mind and want to be able to use this for an effect in a future piece I may be working on.

I just checked the dynamics fps, Skonk: in the latest render I posted it was set at the default of 180 fps.

04-22-2012, 04:42 PM
Thanks so much for these files and info, the generosity of this forum knows no bounds.