View Full Version : IK in Softimage XSI

05 May 2002, 07:18 PM
Basically, my question is quite simple but i am not yet acustomed in the bones system of xsi (used maya so far for animation) so this might also sound a little stupid as a question... I create, for example, a leg and I put a 3d bone chain in it, then i create the envelope, paint the weights but I have a small grouping problem... I want to make a null object the parent of the effector at the end of the bone and the bone just before that one and this just doesn't want to be done with the parent option.... do you have any suggestions??

Thanx in advance everybody.....

05 May 2002, 10:50 PM
woohoo! first post :)


I assume you're including the null as part of a control rig? If so, I would normally just position constrain the effector to the null (or any other object you choose as a controller).

I believe the reason the method your using wont work (and I'm sure someone will correct me if I'm wrong) is that the the last bone is the parent of the effector, therefore another object cannot be the parent of both.

Is there any particular reason you're using a 3D chain? Normally, for a leg I'd use a 2D chain and an up-vector constraint to control the resolution plane.

05 May 2002, 02:33 AM
brad is can't have a null be a parent of both the effector and the bone. I would suggest using a cube or other simple control object with the effector constrained to the cube. Just turn off the 'render visibility' of the cube. I say this because I find using nulls difficult as control objects since they can be more difficult to pinpoint if you have a more complex skeleton or mesh....just my $.02.

05 May 2002, 10:38 AM
True. Also, if you put all control objects on one layer, you can just turn off that layers render visibility.

05 May 2002, 03:15 PM
First of all, thnx for the replies... I am a bit of a dummy i think in IK in softimage because i just got started on them but.... Is the effector at the end of the 2d/3d chain a parent to all of the previous bones??? Because I am trying to parent the cube at the right of the photo attached to the bone cyrcled... I select the bone, click parent, middle click the box to set it as parent but it does nothing... The oposite works just fine (if i set the bone as a parent to the cube)... Please help me if you can with any alternatives because i am desperate... I have been using Maya so far for character animation and I am used to thinking in the way i worked with maya.... There I created a compact chain and not fragments combined in one so i am having a hard time adjusting to the new enviroment... But i really like xsi and i am really getting into it at least as far as the modelling and texturing is concearned (IK needs lots of work, just got started)....

05 May 2002, 03:33 PM
Trust me. Once you've got a handle on this stuff in XSi, you'll be glad you did.

OK. We start off with the chain root. The first bone is a child of this, the next bone a child of the first bone, and so on until we reach the end effector, which is a child of the last bone.

This is where you encounter your problem. Since, in this case, the parent of the last bone is the middle bone, you cant assign the cube as it's parent (because it already has a parent).

This is what I would do (which is by no means the only way).

Create a 2 bone chain for the leg. Create an object to use as an 'ankle' controller. Constrain the position of the end effector of the leg chain to the 'ankle' controller object.

Now create a 1 bone chain for the foot. Create another object to use as the 'toe' controller, and position constrain the root of the toe chain to the 'ankle' controller and the effector of the toe chain to the 'toe' controller.

If this is a little unclear, I'll be happy to elaborate on any part of it. :)

05 May 2002, 05:50 PM
First of all thnx to both of you... Especially Bradf0rd010.... you have been of great help.... I solved it.... It didn't quite work the way you told me so i started experimenting with the two bones setup you suggested.... And I found a way to do it and it works like a love.... I selected the end effector of the two bones chain and then i used constrain/position with the box and the start effector of the other bone... now it all moves lovely.... Thanx again guys...

05 May 2002, 06:26 PM
Happy my first posts are of some use :D

anything else i can help with, i'd be happy to.

05 May 2002, 12:25 AM
@@and so on until we reach the end effector, which is a child of the last bone. @@

the effector is local child of the root of the skeleton and not of the last bone..this is one difference in bones between XSI and the previous versions of softimage3d.
this allows u to cut the child connection between the root and the effector and use another object as the effector's parent.(an alternative to constraining and sometimes more flexible).

05 May 2002, 08:12 AM
I stand corrected :)

05 May 2002, 07:30 AM
i do the same thing... but when i manipulate my "control-boxes"-- which btw are best made with implicit objects, because they never render-- insted of the translation propogating down the chain when the chain reaches its max rotation... the pieces come apart.

now i know that i could just set my rotational-limits, but then i couldnt pull his head up and have his shoulders, then spine , then hips etc. follow....

any ideas?... this seems to be a bright group

05 May 2002, 09:50 AM
You mean the problem when you move the control object beyond the limits of the chain and it 'points' toward the controller?

like from this

to this

05 May 2002, 10:03 AM
except my issue concerns the nexxus of two chains, because thats where i put my control objects.
if your foot had another chain that was its toe, and you slid it foward to the point where it is in your picture, the toes would follow the control object. and the foot would continue to point in its direction.

id like the foot to continue to follow the control object, and then the hip. so i could position my character like a marionette, by translating IK control objects that would propogate up or down the entire rig... is it crazy, impossible, or user error?

thanks i really appreciate you looking into in Bradf0rd010.

05 May 2002, 11:33 AM
Could you zip an example scene of the leg steup you're currently using, and mail it to me? I'd like to take a look.

05 May 2002, 12:09 PM
@@id like the foot to continue to follow the control object, and then the hip. so i could position my character like a marionette, by translating IK control objects that would propogate up or down the entire rig... is it crazy, impossible, or user error? @@

hi sean_g
don't know if i understood what u want ,but if this is to have some automation ,(e.g move the ankles and the hips move accordingly), then u can use expressions to control the motion of an object through other ones.

For example u can use average expressions to control the local position in x and z of an implicit hips cube using as the control objects the 2 implicit ankle cubes, condition expressions for the hips rotation (with ankles again controlling the motion) ect…

The problem is that this might result in a “automated” look (don't forget the secondary motion: anticipation,overlap, follow through ect), something u can always solve with “layered” animation, but again in my opinion for more natural motion it’s better to manually animate very basic control objects that require IK, and use expressions in motion that requires FK..(e.g animate the fingers of a hand through sliders which control with expressions the rotation of the finger bones).

In my opinion when a rig full of expressions is tested in simple motions can make u say “wow”, but when comes the time to create a shot this can be a nightmare. It’s always good to have a plan when creating your rig in order to decide where to use expressions and where not.
best regards

05 May 2002, 12:25 PM
George, i appreciate your sincere attempts at an intelligent overview of animation techniques. . .however, im not sure you understood me clearly. i dont want a newtonian-automaton-frankenstien of a rig. i just want a fully IK enabled rig, with control objets at the joints that propogate translation-values up and down the adjacent IK chains. in order to manipulate it much like an action figure.

and Bradford: yes i would do that, and i really appreciate your help, perhaps you could PersonalMessage me through CGchannel if you dont want to disclose your E-mail address in public forum.
and Gee wilikers, i sure am glad you started posting here. your comments have already helped. PS. if you help get my character going... you can use him, for non-profit reasons only of course, unless you hook me up. but id gladly send you the full-monty for your efforts-- including SSSskinshader,PaintedTex,SoftBody,and custom morph clips. ive got most of this worked out, i figure if i could just get to animating him itll be just like a toy...only more fun

Thank you both...

05 May 2002, 03:52 PM
geoath is right, expressions can prove very useful in character rigging. On the other hand sometimes i find them a hindrance. It comes down to what the characters going to be doing, and what the animators preference is.

05 May 2002, 10:54 PM
@@i just want a fully IK enabled rig, with control objets at the joints that propogate translation-values up and down the adjacent IK chains. in order to manipulate it much like an action figure.@@

In my opinion a fully IK enabled rig wouldn't simple work since human motions r based on rotations..IK is a solution for easier animating but u can't make all motions with IK only, u need FK too..(try to make the motion of a dog tail with IK only, but still creating the drag effect in motion and u'll understand what i mean)

IK ,as it's name says, drives the motion from the end effector and as it is "built in" in the bones it works only for a chain..from the moment u have 2 different chains connected via constraints or parenting, each chain has it's own IK solutions. U can't have control objects on the joints and work with IK..One solution for similar results is to use multiple single chains parented together and use expresions to control the motion..(maybe u can also make it with a single chain and use pseudo-root, haven't tried it yet)
A simple example on what i wrote above is the human spine where u want to use at the same moment IK and FK as the spine can bend (something u could make with IK) but it can also shift ..wouldn't it be easier to build it in a way that u can make it bend with FK and shift with IK than be restricted with IK only?

what Bradf0rd010 wrote before is the most important thing for me when creating a rig : it's what the character going to be doing...!! :)
best regards

05 May 2002, 06:28 AM
im not in disagree-ment with you george.
however, the character im working on right now isnt a human.
eventually he will have expressions, when his rig works properly in its own right. like one to rotate its hips in an alternate direction when his shoulders are rotated...this expression hold true for bipeds and character is a quadra-ped.

but why cant you use control objects at the joints in an IK RIG... is that why you get bones that pull but cant push back-ward without sticking? - well for a dogs tail you could animate it with and IK chain along a spline couldnt you?-

05 May 2002, 09:39 AM
Sean, did you get my message with my email?

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