[Opensim-dev] Redesigning the Physics-Interface

Sun Nov 23 03:44:34 UTC 2008

Well, also consider, Gerhard that ODE has the notions of "spaces", which are collections of geom's and somewhat analagous to a linked set in OpenSim. We are already using the spaceCollide functions to collide spaces, i.e., collections of prims/geoms.

Charles

________________________________
From: Gerhard Dünnebeil <Gerhard.Duennebeil at chello.at>
To: opensim-dev at lists.berlios.de
Sent: Saturday, November 22, 2008 7:28:39 AM
Subject: Re: [Opensim-dev] Redesigning the Physics-Interface

Stefan,

sure, you are completely right.

But you should understand the ideas of the ODE. ODE knows about bodies 
and geometries.

A body is something that has a mass and can move over time.
A geometry describes the collision behaviour of a body but is a distinct 
concept and the coupling is loose (means you can have bodies without a 
geometry and geometries without a body).

An object without a geometry won't collide. If this is something 
meaningful is another discussion.

A geometry without an object won't move on a collision. This is a static 
object in SL speak.

It is possible to add more than one(!) geometry to one body thus 
composing an object with a complex geometry but behaving as one rigid 
entity.

Currently we create one geometry and one body for each prim. That is -- 
as far as I understand things -- part of the instability problems. 
Trying to fix that lead me into a situation where I have to reconstruct 
the parent/child relationship from a lot of calls to the API. This leads 
to situations where a child prim is created first -- with a geometry and 
a body -- then linked to a parent so the body is destroyed again, ......

This is not only inefficient but very complex and error prone.

Things would be much easier if linked objects would be given to the 
physics API as such. After all we want them to behave as exactly that: 
one object.

Look at the new approach, you'll find an Orwell situation "each thing is 
entity but some are more entity than others" as the current approach 
also knows a hierachical relation ship.
I just propose, this hierarchy is given to the phys engine in a 
convenient way as well.

So "Or, would IEntity have a 'composite mesh' that is a combination of 
all child meshes?" Not exactly but in a more lenient way of looking at 
it, you are correct.

Best regards
Gerhard

Stefan Andersson wrote:
> Errr... actually, aren't we going the exact opposite direction, with 
> the 'everything is IEntity' approach?
>
> Or, would IEntity have a 'composite mesh' that is a combination of all 
> child meshes?
>  
> I guess, to be able to collide with a child, you would still have to 
> retain it as a separate mesh entity?
>
> Best regards,
> Stefan Andersson
> Tribal Media AB
>  
> Join the 3d web revolution : http://tribalnet.se/
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>
>
> ------------------------------------------------------------------------
>
> Date: Sat, 22 Nov 2008 06:44:44 -0600
> From: james.stallings at gmail.com
> To: opensim-dev at lists.berlios.de
> Subject: Re: [Opensim-dev] Redesigning the Physics-Interface
>
> Hello Gerhard :D
>
> Unless I am sadly mistaken, something of the sort is currently in 
> progress.
>
> Cheers,
> James
>
>
> On Sat, Nov 22, 2008 at 6:37 AM, Gerhard Dünnebeil 
> <Gerhard.Duennebeil at chello.at <mailto:Gerhard.Duennebeil at chello.at>> 
> wrote:
>
>     Hi
>
>     this is the start of a discussion about the interface to the physics
>     plugins.
>
>     After having tried working on the ODE plugin, specially focussed on
>     linked objects I think the current interface has a severe short
>     coming.
>
>     The problem I see is, that it is prim orientated while an object
>     orientation would better suit the needs.
>
>     Why is it so?
>
>     Currently it seems to me, the information is given to the phys engine
>     prim by prim. Any object information is only given in fragments.
>     This leads to the situation that the object relations must be
>     reconstructed by the phys engine and it often leads to the situation
>     that individual prims get constructed and shortly after get destructed
>     again as it turns out that the respective prim is part of a larger
>     link set.
>
>     For that reason I ask your thoughts about a redesign of that
>     interface.
>
>     Best regards
>     Gerhard
>
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