[ODE] Still Spheres Vs Trimesh at Triangle Edges

Ettore Pancini ekt at libero.it
Mon May 1 08:28:06 MST 2006 

i still have problem mentione in the original post
(spheres rolling over a trimesh jumps on every triangle edge)

to me, turning off MERGECONTACTS gives worse results
don't know why is the opposite for Sinecky.

commenting all edge collisions in collision_trimesh_sphere.cpp
(except interior region) works, but then I get a bad collision
detection when the wheel goes over a sidewalk. this makes
sense, though it becomes a half-solution.

i don't have the knowledge to implement the preprocessed
stuff for mesh-sphere collision

what is my next best move?



----- Original Message ----- 
From: "Jaroslav Sinecky" <sinecky at telefonica.net>
To: "kurimail" <kurimail at arrakis.es>; <ode at q12.org>
Sent: Monday, April 03, 2006 18:53
Subject: Re: [ODE] Still Spheres Vs Trimesh at Triangle Edges


> There is really an issue with contact depth in trimesh_sphere. There was a
> change in depth calculation couple months ago. I didn't find it very good
> because it introduced discontinuities. Now I have tried the svn trunk
> version and found there was also a real bug introduced by this change 
> (quite
> difficutl to see at first glance but "side" value isn't actually distance 
> of
> sphere and triangle plane).
> So I changed it to following: depth is calculated as triangle-sphere
> penetration depth projected to triangle (=contact) normal. This is still
> just aproximation but I find it better than previous two implementations.
> Also reorganized a bit the code to pull out duplicate calculations. 
> Checked
> in svn trunk.
>
> Still, you will get some oscilations when running over mesh edges (I get
> them too, but they don't have noticable negative effect). This is because 
> of
> close contacts generated - one from each triangle, so implementing some
> contacts postprocessing as suggested by Geoff is a good idea.
>
> Also, if we implement the trimesh preprocessing for sphere collisions, it
> could help a lot because all concave edges or edges between co-planar
> triangles would be skipped from collisions (so for example rolling over
> planar mesh should be as smooth as rolling over plane).
>
> The last thing ... you could also play with #define MERGECONTACTS (at the
> beginning of collision_trimesh_sphere.cpp). For me it works better to have
> it turned off. In fact I  think it could be turned off by default because
> both our test cases (test_trimesh and test_moving_trimesh) and most of 
> real
> world cases work better like that. With this merge turned on, if you drop
> sphere in test_trimesh, it last ages to get still and also a sphere behind
> bunny's ears has hard time finding stable position with one contact only.
>
> Hope this helps,
> Jaroslav
>
>
>> -----Original Message-----
>> From: ode-bounces at q12.org [mailto:ode-bounces at q12.org]On Behalf
>> Of kurimail
>> Sent: Saturday, April 01, 2006 12:09 PM
>> To: ode at q12.org
>> Subject: Re: [ODE] Still Spheres Vs Trimesh at Triangle Edges
>>
>>
>> Mmm, the problem i am having is not about the normals, it looks like a
>> problem with depth. Although the normal is always (almost) vertical for
>> the wheels, the depth is higher on the triangle edges, so the wheel 
>> jumps.
>>
>> I am going to try to comment the edge detection as suggested...
>>
>>
>> > Hi,
>> > I was going to reply with a note about this problem in regard to 1.0, 
>> > so
>> > I'm glad the issue came up.  I found this is a terrible problem with 
>> > ODE
>> > and trimeshes, and in fact been a little puzzled why nobody else has
>> > ever seemed to complain about it.  I haven't had time to make up a
>> > simple test showing the problem, but I'm sure a flat trimesh and 
>> > rolling
>> > sphere would clearly show what the poster was talking about.  I've 
>> > found
>> > the contact normals vary from the correct 90 degrees to clearly wrong
>> > glancing angles that are almost horizontal.
>> >
>> > In my case, I changed the trimesh collisions to always return the
>> > triangle normal, rather than the "near to edge" direction.  In the 
>> > cases
>> > where the object should be pushed "out" at a non-normal direction, 
>> > there
>> > is a triangle neighbour which will do exactly that.  I didn't try
>> > commenting out the edge collisions as suggested below, but that seems 
>> > an
>> > even better way to do it.
>> >
>> > Ideally it needs a post-process of merging contacts, which I found
>> > helped correctness and stability (there is nothing like conflicting
>> > contacts to spin an object to infinity).  In my case it was done after
>> > all object-object collision routines, to deal with the case of merging
>> > contacts from different objects, such as from two trimeshes sitting 
>> > next
>> > to each other.  The merging in my case was to take all contacts at the
>> > same position and lerp the normals.  As such, a sphere that hits a
>> > trimesh with a box-edge feature would resolve from two contacts at 90
>> > degrees into one contact at 45 degrees.
>> >
>> > Geoff
>> >
>> >
>> > kurimail wrote:
>> >> Thanks for your answer!.
>> >>
>> >> The problem is that spheres rolling over a trimesh jumps on every
>> >> triangle
>> >> edge. I have a 4-hinged spheres car, and when a sphere rolls over 
>> >> every
>> >> triangle edge, a little jump occurs.
>> >>
>> >> As i sad, decreasing cfm solved a little the problem, but not all.
>> >>
>> >>
>> >>
>> >>> If you really think your problem would be solved if there were no
>> >>> sphere-edge collisions (I doubt it because then your spheres should
>> >>> sink
>> >>> on
>> >>> trimesh convex edges), you can eighter implement the preprocessed 
>> >>> mesh
>> >>> for
>> >>> sphere collisions and submit it to svn (the more difficult way ;) or
>> >>> take
>> >>> much easier and faster way: have a look on GetContactData in
>> >>> collision_trimesh_sphere.cpp and comment out all edge collisions (all
>> >>> branches except region 0 - interior).
>> >>>
>> >>> Anyway, as I say I doubt this would be a good solution. What
>> exactly is
>> >>> the
>> >>> problem you are experiencing?
>> >>>
>> >>>
>> >>>
>> >>>> -----Original Message-----
>> >>>> From: ode-bounces at q12.org [mailto:ode-bounces at q12.org]On Behalf
>> >>>> Of kurimail
>> >>>> Sent: Friday, March 31, 2006 4:01 PM
>> >>>> To: ode at q12.org
>> >>>> Subject: [ODE] Still Spheres Vs Trimesh at Triangle Edges
>> >>>>
>> >>>>
>> >>>> Hi again. I am still having troubles with spheres colliding at
>> >>>> triangle
>> >>>> edges. I don't know if the problem is the edge or the change from 
>> >>>> one
>> >>>> triangle to another.
>> >>>>
>> >>>> I looked at the preproccesing trimesh, but it is only for 
>> >>>> ccylinders.
>> >>>>
>> >>>> I tried changing erp and cfm values. This helped, but not too much.
>> >>>>
>> >>>> I tried different scales, sizes, masses, mass shapes, etc... and
>> >>>> nothing
>> >>>> helped too much.
>> >>>>
>> >>>> I can accept any trick to avoid such jumps at triangle edge.
>> >>>>
>> >>>> Any help, please?
>> >>>>
>> >>>> Thanks in advance!
>> >>>> _______________________________________________
>> >>>> ODE mailing list
>> >>>> ODE at q12.org
>> >>>> http://q12.org/mailman/listinfo/ode
>> >>>>
>> >>>
>> >>
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>> >
>>
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