View Full Version : Weird...

09 September 2004, 03:33 PM
I come up with an unsual XSI behaviour:

- Make a 2 bone chain

- Link the Child rotation (any channel) to the Father rotation (any channel), so that when u rotate the Child, the father rotates also.

- Divide the expression by 2, so that the Father only rotates half of what the Child does, for example: bone1.kine.local.roty / 2

-Rotate the Child and the Father rotates half like expected

-Undo the rotation, the Child and Father "jumps" to an oposite rotation...............

I can put it all like they where if i ZERO out the rotations of the Child in the numeric fields, but, does anyone knows why Undo F**** up???????? Bug?

09 September 2004, 05:15 PM
I re-did the setup but this time i used 2 Cubes. Moved their pivots and parented Cube2 to Cube1 so they behave like a regular Bone Chain but without IK, same thing happens... :cry:

09 September 2004, 08:02 PM
I dont know, Ive had simular problems with undoing bone rotations aswell, cant for the life of me figure it out either. Oh well, computer is screwed up right now so...I gotta get fixing it :sad:

So, your not the only one with this problem, sorry I cant help much with solution tho.

09 September 2004, 09:59 PM
If I understand fully what you are setting up here, it may be a cyclic dependency loop.

The effector controls the parent, but the effector is a child, so the parent controls it. This is a loop. XSI does it's best to process the situation and seems to work while you interact with the effector. However, when you undo, XSI can't know which operator is supposed to take precedence. If it undoes the child's transform first, that results in a parent transform which then has to transform the child again.

When you get to a point where order of operation matters regarding animation control, you run into problems with repeatability, plotting, saving, etc.

If there is no loop here, ignore me. :-)

09 September 2004, 01:19 PM
XSI doesnt give me any Loop error message and i use this same procedure in at least 2 other 3d softwares without a single itch (Messiah and LW), in fact i really need to come up with a solution to this, itīs part of my workflow on the rigging process.

As soon as i find a viable solution iīll post it so others with the same or similar problem can maybe take advantage of it, coming from LW im not scared of (and very used to) workarounds :)

09 September 2004, 01:57 PM
I have a somewhat similar situation with a rig i'm working on. I following along with a rig tutorial from I created rib defomers and have them constrained the a cuvre spine. I then have an expression linking the rotation of the upper control object to the rib deformers. everything is fine in X but I get counter-rotation on Y & Z when i spin the upper control object. Has me scratching my head wondering what i'm doing wrong...really frustrating since i'm following along with the tutorial but not getting the same result.

For my case I think i will look in to what Withanar said about cyclic dependency loop. There's lots of constraining and parenting going on that i think the programs gets all mixed up.

I'm on my last day of the Foundation evaluation so the answer might not matter to me unless I buy Foundation. I really, really love XSI tho... I'm a lightwave user and like Sil3, i'm used to trying to come up with workarounds.

09 September 2004, 06:25 PM
yea it happens but you can easily get them back to their neutral position by zeroing out all the transforms.

They had the same problem in 3.5 also I dont know why its not fixed yet.

09 September 2004, 01:25 PM
Fixed it hehehe :bounce:

What i was looking after in the first place was to have a Spine bone chain and get it rotate using a CTRL object, why didnīt i use an IK Spine? because i prefer the manual control than the more automated one with Spine IK.

I switched to the Old Si3D way of making skeletons, made 3 chains for the Spine instead of one, aligned all axis rotations to be the same (both the nulls and the bones), Set Neutral Pose for ALL of them.

Then with some creative Constraints i managed to have an Implicit Cube drive the Chain Rotations (even with expressions dividing the Rotations values), pose like i want, undo and it goes back the the original Position, no more screwy bend Bones :scream:

The trick was to make the CTRL object stick with the Spine, so i could rotate it and it followed the tip of the Spine chain, so that i could have a better visual feedback, instead of having the CTRL floating in space.

Iīll try to make a basic html tut to explain it in more detail, but itīs possible :buttrock:

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