Yes, cycle checking is not always perfect.
For example, an operator (or expression) may look like it forms a cycle
based on its connections but obviously whether it actually does contain
a cycle depends on how it uses those inputs/outputs and this is
something that cycle checking cannot determine. (until the operators are
evaluated)
It is quite possible that we had some bad cycle warnings in the past but
that is not my main area of expertise. ;-)
--
Brent
-----Original Message-----
From: owner-xsi(at)Softimage.COM [mailto:owner-xsi(at)Softimage.COM] On Behalf
Of Andre DeAngelis
Sent: 28 February 2006 20:28
To: XSI(at)Softimage.COM
Subject: RE: rigging dilemma: driving a parent with attributes of the
child
Thanks Brent,
I wasn't disputing Kims logic (I know netter than to take on Kim), but
given Maya allows this to happen and that a simplistic test gave
promising results in XSI, I assumed that some cycles can be tolerated
more than others (as you suggest).
Pre 5.0, cycle errors would pop up for apprently no logic reason. i.e.
2 objects in the a scene with no dependencis whatsoever would sometimes
trigger this warning.
Andre
-----Original Message-----
From: owner-xsi(at)Softimage.COM [mailto:owner-xsi(at)Softimage.COM] On Behalf
Of Brent McPherson
Sent: Tuesday, February 28, 2006 3:09 PM
To: XSI(at)Softimage.COM
Subject: RE: rigging dilemma: driving a parent with attributes of the
child
Andre,
Kim is right and this is a cycle. (which can be verified by running
"cycleCheck -all -dag -l;" in Maya)
Now most cycles are bad because the result depends on the number of
times it is evaluated. In a worse case scenario the cycle can create a
feedback loop where the object flies off to infinity. (and beyond!)
In the child/parent example given the position of the child is dependent
on the number of evaluations. (so you might get different results
playing back every other frame vs. playing back every frame etc.)
However, XSI (and Maya) do allow cycles to be created because it is
possible to create "stable" cycles that evaluate consistently or you
might be able to plot the results for rendering etc. In general, though
it is not desirable to have evaluation cycles in your rigs if you can
help it.
--
Brent
-----Original Message-----
From: owner-xsi(at)Softimage.COM [mailto:owner-xsi(at)Softimage.COM] On Behalf
Of Andre DeAngelis
Sent: 28 February 2006 19:37
To: XSI(at)Softimage.COM
Subject: RE: rigging dilemma: driving a parent with attributes of the
child
Yes I relise that Kim,
But having tried it and seeing hat it works (at least in a simple test),
I am wondering what makes this colution less than desirable.
Andre
-----Original Message-----
From: owner-xsi(at)Softimage.COM [mailto:owner-xsi(at)Softimage.COM] On Behalf
Of kim aldis
Sent: Tuesday, February 28, 2006 2:29 PM
To: XSI(at)Softimage.COM
Subject: RE: rigging dilemma: driving a parent with attributes of the
child
Because the rotation of the parent is driven by the child, but the
rotation of the parent affects the position of the child, which then
affects the direction of the parent, which affects the rotation of the
child .....
> -----Original Message-----
> From: owner-xsi(at)Softimage.COM
> [mailto:owner-xsi(at)Softimage.COM] On Behalf Of Andre DeAngelis
> Sent: 28-February-2006 19:04
> To: XSI(at)Softimage.COM
> Subject: RE: rigging dilemma: driving a parent with attributes of the
> child
>
> There is only one parameter dependency here.
>
> If you parent one null to another and then apply a direction
> constraint to the parent (to point to the child) it workd just dandy.
>
> In fact, you can continur to rotate the parent in branch mode, to
> offset the whole set-up.
>
> Am I missing something here?
>
> -----Original Message-----
> From: owner-xsi(at)Softimage.COM
> [mailto:owner-xsi(at)Softimage.COM] On Behalf Of kim aldis
> Sent: Tuesday, February 28, 2006 1:48 PM
> To: XSI(at)Softimage.COM
> Subject: RE: rigging dilemma: driving a parent with attributes of the
> child
>
> I'd be intrigued to see how, as rob says, the child is dependant
upon
> the parent which is dependant upon the child, which is dependant upon
> the parent which is dependant upon the child which ...... And the
> whole thing rapidly crawls up it's own arse. Whichever software you
> use.
> Would have been the same in SI3D too.
>
> > -----Original Message-----
> > From: owner-xsi(at)Softimage.COM
> > [mailto:owner-xsi(at)Softimage.COM] On Behalf Of David Gallagher
> > Sent: 28-February-2006 18:39
> > To: XSI(at)Softimage.COM
> > Subject: Re: rigging dilemma: driving a parent with
> attributes of the
> > child
> >
> >
> > Maya has its benefits. It handles this situation better
> unfortunately.
> >
> > > I don't see how you can avoid a cycle.
> > >
> > > As soon as you tranlsate the child in negative Y the
> parent rotates
> > > causing the child to move causing the parent to rotate
> causing the
> > > child to move...
> > >
> > > _rob
> >
> >
> >
> > --
> > David Gallagher
> > Animator, Blue Sky Studios
> >
> > ---
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> >
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