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Chipmunk Physics • View topic - Objects constantly move against each other

Objects constantly move against each other

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Objects constantly move against each other

Postby ragamuffin » Mon Aug 25, 2008 7:48 am

Hi there. I'm just starting out with Chipmunk, and I'm writing my code using the Python wrapper, Pymunk. So far the library looks amazing and is exactly what I was looking for.

I have a certain problem I'd be very grateful if anyone would help me solve.

Consider the following scenario: http://i110.photobucket.com/albums/n110 ... nshot2.jpg

All of the shapes are perfect rectangles. The problem is that they never completely stand still. They seem as if to always move when in contact with one another (the bottom columns seem to always be motionless...), so if I leave that tower alone for a few minutes, most of the shapes (the columns) seem to drift towards the right and finally the whole structure collapses. :o

The infinite mass and inertia segment that holds the entire structure above it has a friction of 1.5 and an elasticity of 0. The rectangle shapes have the same attributes. I build the tower by adding objects to the space one by one, while the program is running. (For instance, I create a block above another one and it falls down on top of it.)

What am I doing wrong? I'd hate to be known as a George Costanza of the architect world. ;)
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Re: Objects constantly move against each other

Postby maximile » Mon Aug 25, 2008 10:13 am

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Re: Objects constantly move against each other

Postby viblo » Mon Aug 25, 2008 2:31 pm

Sleeping bodies would also help. Someone just have to add them to chipmunk. :) rje has done it in his actionscript library based on chipmunk: viewtopic.php?f=1&t=221 so that would be a good staring point for an experiment like that.
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Re: Objects constantly move against each other

Postby ragamuffin » Tue Aug 26, 2008 8:13 am

Ah, I was afraid this would be something hard to solve. The doc tips only serve to delay everything, the objects still move and the tower will collapse eventually, given enough time. Also, I'll look into that actionscript library. Flash hasn't been one of my strongpoints, but I've never actually given it enough time.

Thanks for the help! :geek:
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Re: Objects constantly move against each other

Postby slembcke » Tue Aug 26, 2008 12:19 pm

One of my demos (#5) does a very similar pyramid of blocks. I have no issues with stability in that one. It's rock solid stable. I've ran it to thousands of time-steps and the blocks don't shift at all.

As long as you are using fixed size time-steps, the stability should only increase over time as long as you are using enough iterations. If Chipmunk supported sleeping the bodies, it wouldn't help much in the case where the objects never stop moving. Sleeping isn't meant to increase stability, it's to increase speed in sparse or largely static simulations.
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Re: Objects constantly move against each other

Postby ragamuffin » Fri Aug 29, 2008 5:09 am

It looks like I really didn't use enough iterations. The demos cleared some things up. Things are much better now, although the tower does collapse if I build it too high. However, I suspect it's because I'm shaking it up when I slam blocks to the top of it and don't wait, so it becomes more and more unstable. So I really am the George Costanza of the architect world after all. :roll:

Again, thanks for the help!
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Re: Objects constantly move against each other

Postby slembcke » Fri Aug 29, 2008 11:10 am

Yeah, basically the more layers you add between any one object and a solid object like the ground, the more computationally expensive it becomes and you have to add more iterations to compensate.
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Re: Objects constantly move against each other

Postby viblo » Sun Aug 31, 2008 11:52 am

http://www.pymunk.org - A python library built on top of Chipmunk to let you easily get cool 2d physics in your python game/app
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Re: Objects constantly move against each other

Postby slembcke » Mon Sep 01, 2008 10:46 pm

Well sort of...

Every paper that I've read physical simulation that mentioned sleeping has only referred to it as an optimization and not a way to increase stability.

Maybe our ideas of "stable" are different as well. I'm thinking about this from the idea of numerical stability of the simulation. Sleeping does not solve that. Once an object gets woken up again, it will be just as unstable as before if it was creeping for some reason. I'd argue that it can increase the realism of an unstable simulation by reducing the artifacts.
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