BUG: intermittent reproduceable failure in physics engine

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animats
Posts: 15
Joined: Fri Nov 08, 2002 8:18 pm

BUG: intermittent reproduceable failure in physics engine

Post by animats » Wed Nov 03, 2004 6:23 pm

Multiple collisions intermittently fail, resulting in objects falling through the "floor". Here's a very simple test case: http://www.overbot.com/public/vendors/blender.

The test case drops a cube into a 3-sided "funnel". Most of the time, it works right. Sometimes it fails, and the cube falls through the "bucket".

We've previously seen similar problems with "55wheels.blend" from the GameKit, where hitting the curb can result in falling through the ground. But now we've reproduced the problem in a very simple test scene.

animats
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Post by animats » Thu Nov 04, 2004 11:38 pm

We've now added screenshots of the physics engine failing to the defect description page at http://www.overbot.com/public/vendors/blender/, along with a link to the .blend file so that others can reproduce the bug.

malCanDo
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Location: Ireland
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Post by malCanDo » Fri Nov 05, 2004 1:32 pm

Hi animats,

The overbot project looks very cool!

If you are basing your project on Blender etc, and you are hiring interns, it could be well worth assigning one of them to work with Kester on the game engine ( it's currently only him ).

Your intern could be allocated the task of resolving physics problems, helping your team get an in-depth knowledge of Blender, and allowing you to fix issues directly.

Regards...
Mal

animats
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Post by animats » Fri Nov 05, 2004 6:56 pm

We don't have the resources to fix Blender; we have too much else to do between now and the Grand Challenge in October 2005. Blender almost works for us. If we can just get this "intermittent falling through the ground" problem fixed, we'll be in good shape.

If the engine can be made reliable enough for simple driving games, that's good enough. We don't need physics good enough for ragdolls.

It looks like the problem may relate to CPU load. It may be that if the physics engine takes too big a step, it can cause a fly-through. It may be sufficient to limit the size of a physics step, so that under load, Blender slows down but doesn't break.

malCanDo
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Post by malCanDo » Fri Nov 05, 2004 9:44 pm

I think I read that there was a change to the code, so that the physics engine ran at either 30 or 60 frames a second, independantly of how fast the game was updated. If there was an option to increase the number of steps that the engine used between each update, that would help increase the stability of your simulation.

> We don't have the resources to fix Blender; we have too much else to do between now and the Grand Challenge in October 2005. Blender almost works for us. If we can just get this "intermittent falling through the ground" problem fixed, we'll be in good shape.

It might be worth having someone look at just the physics code for a week or two... they might discover something that could improve the physics engine.

As it stands, only one person ( Kester ) deals with the whole real time engine side of the coding, which is a HUGE task... the physics is only one part of the game engine. If you're needing updates quickly ( and believe me, having worked with a few different physics engines, you'll be doing a lot of tweaking ), having the expertise on your side will be helpful for you in the long run.

Mal

animats
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Joined: Fri Nov 08, 2002 8:18 pm

Physics engine problems in 2.35a

Post by animats » Mon Nov 22, 2004 7:06 pm

Try running "55wheels.blend" from the GameKit in Blender 2.35a. Movement is much more sluggish. And the car still falls through the ground now and then, just like 2.34.

Our simple test case, http://www.overbot.com/public/vendors/b ... index.html runs far slower than it did in 2.34. Try it in both. It's just a falling block that falls into a funnel. The stats don't show it as compute-bound; physics is using under 0.5%, and frame rate is steady at 60fps.

Our simple test case at first seemed to work better in Blender 2.35a, but that's an artifact of the sluggishness. Try dropping the block from higher up and it will fall through the funnel.

Metallica
Posts: 6
Joined: Thu Oct 17, 2002 12:46 am

Post by Metallica » Mon Nov 22, 2004 11:49 pm

Please submit bugs to the bug tracker. It will be helpful to tell the developers. Make sure you make this one in the game engine catigory and high priority.

http://projects.blender.org/tracker/?fu ... 9&atid=125

animats
Posts: 15
Joined: Fri Nov 08, 2002 8:18 pm

Post by animats » Tue Nov 23, 2004 2:41 am

Metallica wrote:Please submit bugs to the bug tracker. It will be helpful to tell the developers. Make sure you make this one in the game engine catigory and high priority.

http://projects.blender.org/tracker/?fu ... 9&atid=125
Yeah, right. "ERROR Artifact: This ArtifactType Does Not Allow Anonymous Submissions. Please Login."

OK, I just registered for my fourth Blender-related ID.

jesterKing
Site Admin
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Location: Finland

Post by jesterKing » Tue Nov 23, 2004 7:29 pm

animats wrote:
Metallica wrote:Please submit bugs to the bug tracker. It will be helpful to tell the developers. Make sure you make this one in the game engine catigory and high priority.

http://projects.blender.org/tracker/?fu ... 9&atid=125
Yeah, right. "ERROR Artifact: This ArtifactType Does Not Allow Anonymous Submissions. Please Login."

OK, I just registered for my fourth Blender-related ID.
Contact me at projects@blender.org with the username you registered with, and we can work this out.

/Nathan

animats
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Post by animats » Tue Nov 23, 2004 7:55 pm

I was finally able to register for the bug tracker. So far I've had to register separately for blender.org, elysium.org, gameblender.org, and the bug tracker. (I'm not sure where blender3d.org fits into this.)

Every time I report a problem, someone on some forum says it should be reported on some other forum. Are all these web sites really necessary?

Saluk
Posts: 110
Joined: Wed Oct 16, 2002 6:52 am

Post by Saluk » Wed Nov 24, 2004 6:19 pm

They all serve their own purpose. Bug tracker for a direct line of bugs for developers (crawling through many forum posts to find real bugs would be a nightmare.) Blender.org for chatting about blender features and development in a more casual setting. Elysiun for the user community. Blender3d.org is blender's official prescense to the net. Gameblender.org, um, I've never been there so I don't know. Probably a central place to find people's blender games.

Again, searching the forum for current links to people's games is not super easy. The main purpose of forums is for chatting, not organizing information.

I don't see the falling through problem very often, but it still happens once out of every 25 or so test runs.

animats
Posts: 15
Joined: Fri Nov 08, 2002 8:18 pm

Post by animats » Wed Nov 24, 2004 7:09 pm

Saluk wrote:I don't see the falling through problem very often, but it still happens once out of every 25 or so test runs.
Good; it's now a reproduced bug.

I seem to get a bit more than that. It may be CPU speed dependent.

It makes driving games like 55wheels unplayable. You fall through the track every minute or two.

(I should note, though, that 55wheels, from the GameKit, has some geometry bugs. There are a few "holes" in the track, where collision isn't enabled for a face. But even after fixing those, there are still fall-through bugs.)

Sutabi
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Post by Sutabi » Thu Nov 25, 2004 7:08 am

Never fell though for me... Although I put bounds [box] and there was no jitter.

celeron 1.8 geforce2 mx400 32 megs

Metallica
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Post by Metallica » Thu Nov 25, 2004 8:32 pm

heh sorry about that error, thought you would have a username :/

i tried the test on my better computer which can run blender lol. i get the jittering and stuff, i think thats because its automaticly a sphere? if you don't set the bounds. if i set the bounds to box it doesn't jitter. but on sphere it does. doesn't fall through though, specs are blender v2.35, p-4 with h-t 2.87 ghz and 512 ram... also the ati 9200. think this might be fixed in 2.35. dunno what version you were using?

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