[Opensim-users] Changed default av_capsule_tilted to false inOpenSim master

[Justin Clark-Casey]

Zane Legion wrote:
> Thanks for the explanation nlin. For me (on 0.6.7-post-fixes), tilted=false 
> solved the bendy knee issue and the feet partially underground issue, 
> although everyone now seems to float slightly above the ground.

Yes, thanks very much for the insight, Norman.

> 
> I'll keep a lookout for the other two issues you mention.
> 
> Zane
> --------------------------------------------------
> From: "nlin" <nlin.message at gmail.com>
> Sent: Tuesday, November 10, 2009 12:37 PM
> To: <opensim-users at lists.berlios.de>
> Cc: <opensim-dev at lists.berlios.de>
> Subject: Re: [Opensim-users] Changed default av_capsule_tilted to false 
> inOpenSim master
> 
>> Hi,
>>
>> 2009/11/10, Justin Clark-Casey <jjustincc at googlemail.com>:
>>> There was a bit of discussion last week in the OpenSim dev's office hour 
>>> and on
>>> irc that changing av_capsule_tilted = false in OpenSim.ini.example would 
>>> be a
>>> good idea in the master branch for avatar related physics for ODE.
>> The av_capsule_tilted=false should generally be safe. Please let me
>> mention some general things to watch out for. These shouldn't be a
>> problem now, but were problems that I encountered and hopefully fixed:
>> 1) Avatar falling through terrain
>> 2) Extremely rapid changes of direction and movement could conceivably
>> cause the avatar capsule to become unstable.
>>
>> Note that these items are refer to poor behavior in terms of the
>> physics engine itself (visualized with drawstuff), as opposed to ways
>> that poor behavior might manifest itself in the viewer (such as
>> bendy-knees).
>>
>> Regarding 1, terrain collisions, reducing the avatar tilt seems to
>> increase the tendency of the avatar to fall through the terrain - this
>> has something to do with the ODE terrain collider, which I wasn't able
>> to fully debug - anyone wanting to give it a shot, let me know. As a
>> workaround, currently a small, experimentally-determined tilt is
>> maintained (even with tilt=false) to prevent falling through the
>> terrain. This isn't ideal, but seems to work well enough.
>>
>> Regarding 2, rapid changing of avatar direction: The tilt rotates with
>> the avatar, snapped in 90-degree increments to one of  four directions
>> (NE,NW,SE,SW), so that the direction of the tilt always is opposite
>> the direction of movement. In other words, if your avatar is moving
>> forwards, the avatar is leaning slightly backwards. The reason the
>> tilt is rotated with the avatar is to reduce/remove
>> direction-dependent behavior, such as being only able to climb over
>> prims from a certain direction. The reason the tilt rotation is
>> snapped to 90-degree increments, instead of smoothly rotating with the
>> avatar, is to prevent extremely rapid changes in tilt orientation,
>> which is done by setting ODE motor stops. If you change ODE motor
>> stops too rapidly, the avatar capsule may "explode" and go flying
>> several hundred meters into the air. Conceivably, if your avatar
>> walked directly straddling a line facing northeast and constantly did
>> a zig-zag motion above and below the NE line, this could cause the
>> avatar orientation to change rapidly enough to cause avatar
>> instability. In practice I doubt that this will be an issue. (Still,
>> would be interesting to write a bot to try to provoke this behavior.)
>>
>> More generally, however, this dynamically rotating tilt is not ideal.
>> I think there are 2 avatar physics issues that need further
>> investigation:
>>
>> 1. Removing the tilt completely instead of just reducing it. Currently
>> not workable because the terrain collider then allows the avatar
>> capsule to fall through sometimes. Fixing this would allow complete
>> consistency (direction-independence) in avatar physics behavior, close
>> avatar interactions like hugging, and no need for changing the motor
>> stops while the avatar is moving, reducing risk of explosion.
>>
>> 2. Allowing flexible height of steps that the avatar can step over.
>> Currently the bottom-most hemisphere of the avatar capsule always
>> drags against the terrain or any objects it collides with. This has
>> the limitation that the height of obstacles that can successfully be
>> stepped over cannot be directly tuned, as it depends indirectly on the
>> size of the hemisphere and the specifics of collision handling (things
>> like ERP, CFM, bounce). Ideally we would want to be able to directly
>> tune the height of "steppable" objects in OpenSim.ini, or perhaps have
>> this be different per avatar (dependent e.g. on the avatar height).
>> But it looks like we would probably have to implement a new avatar
>> physics mechanism to handle this flexibly: something like a floating
>> capsule, capsule+sphere, or using mesh collisions instead of primitive
>> capsules.
>>
>> -nlin
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-- 
justincc
Justin Clark-Casey
http://justincc.org