Re: "link with" expressions in rigging - good or not?

["Bradley Gabe" <withanar(at)gmail.com>]

Hi Bernard. "Link with" operations are entirely stable and reliable, and I have used them many times in many productions without incident.

The thing that is potentially unstable are the local Euler results you get from a pose or orientation constraint. Constraints are applied at the global level and use quaternion math for the calculations. As the results are calculated and applied back to the local rotations, they may shift between quadrants. When you have a local rotation that relies on a consistent range of values, say between -180 and 180, this can destroy the relationship you thought you were getting, since local 360 is as good as 0 to the global value. This applies to normal expressions as well as "link with" operations.

The solution is, any nodes you are using for expressions or "link with" ops, make sure to set animation keyframes on the local rotation at 0,0,0. This will force the global constraints to always resolve in the same local quadrant, and thus you can rely on your relationships always performing properly.

FYI, I have notified Softimage about this issue many times. I have logged it as a bug and also via escalation. I don't know if it has a UDEV number or is being tracked. It may be one of those issues that's too deep to easily solve, and there is a known workaround, so it's low priority.

-Brad

On 9/25/07, Bernard Lebel <3dbernard(at)gmail.com> wrote:

Hello,

Not really a support question, but rather getting a feel people's
experience in general compared to my own.


We have a bunch of characters, that I have not rigged.
The modeler created some shapes.
The rigger created a cpset to control the shape weights, with nice sliders.
The thing is that the relationship between the shape weights in the
mixer and the cpset parameters are not (=) expressions, but "link
with" expressions.

Now, the problem we're continually having is that somehow the
relationship between the shape weights and the sliders break. It
doesn't look like it's broken (I've checked 100 times, every element
of the expressions is there and valid), but moving the sliders has no
effect anymore on the shape weights.

In time I have written lengthy scripts to workaround this problem.
It's ugly but it works. However, even though it's no longer a
show-stopper, animators are keep reporting that they have to use my
scripts to repair the shapes. The typical case happens when the
animator exports the mixer and imports it back onto the model.

We're not using reference models.

Perhaps there is something wrong we did in the first place, I don't
know. I don't know all the details of the rig (it's extremely
elaborate). However nothing seems non-legit to me.

Obviously, my faith in "link with" _expression_ has taken quite a toll
as a result. I could have fired this to support but we're not on
maintenance.



So my question is this: how much use of "link with" expressions do you
make in rigging? Is my experience unique? What are the things to do
and not do when using those?



Thanks!
Bernard
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