# [ODE] Arbitrarily positioning bodies connected by joints

Shamyl Zakariya shamyl at zakariya.net
Mon Jul 30 14:47:39 MST 2007

```> In fact, resetting the anchors could
> induce/enhance joint drift since joints are usually not *perfectly*
> satisfied
> during any timestep..

Not only that, but if you're measuring the "value" of some joint
( say the compression on a slider which is acting as a spring )
resetting it on translation could ( and probably would ) change its
zero point.

I posted a few days ago that there's no trickery required. My
troubles were coming from my having made a mistake in my
transformation code, not ODE.

shamyl at zakariya.net
"Such a theory has to be bizarre and elaborate, as well as being
stupid"
-- Jim Loy, regarding a hollow earth

On Jul 30, 2007, at 7:21 AM, Deak Szabolcs wrote:

> "The anchor is specified in world coordinates" - means that *you*
> specify the
> anchor in world coordinates when you set it. It is then transformed
> to the
> body space of the bodies connected by the joints, and it is used in
> the
> relative coordinate form for all internal calculations.
> So both statements are true, and the relevant one in this case is
> that you
> *don't* have to reset anchors if you transform the connected bodies
> except for
> those anchors that are set to the world. In fact, resetting the
> anchors could
> induce/enhance joint drift since joints are usually not *perfectly*
> satisfied
> during any timestep..
>
> Beau Albiston wrote:
>>  From the ODE manual: "the anchor is specified in world coordinates."
>> If your body frame and world frame are aligned, then your
>> statement is true.
>>
>> -Beau
>>
>> dan miller wrote:
>>> I do this all the time, without resetting anchors.  Why would
>>> that be
>>> necessary?  It's relative to the bodies, not global coords
>>> (unless you're
>>> anchored to the world)
>>>
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