Autodesk maya; Stretchy Ik on/off switch

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  08 August 2012
Autodesk maya; Stretchy Ik on/off switch

Hey i'm new to the site and am rigging a character using the three joint chain system. I'm semi experienced with rigging but wanted to rig something of my own. I did a tutorial on how to create a stretchy ik chain using expressions but realized there was no way to turn it on and off but with trial and error i figured i could do it or "fake it" using constraints. similar to weighting orient constraints in a ik/fk switch 3 joint chain system. I wrote down what i did so i don't forget and just wanted to "double check" my thinking.

create joint chain


create ik

Go to create/measuring tools/distane

then (holding v key) select the "arm" joint and "wrist" joint

parent locator to "wrist" joint

select ik handle and move until joint chain is completely

write down distance given

then go to window/animation editors/expression editor

name your expression (Example Stretchy joint chain)

enter expression: (**This expression was provided from the book
"The Art of rigging"** which is an amazing book that is
uber helpful!! Numbers given are for example purposes)

if (distanceDimensionShape1.distance >= 12.9 )

arm.scaleX = (distanceDimensionShape1.distance / 12.9) ;
elbow.scaleX = (distanceDimensionShape1.distance / 12.9) ;

} else
//Do not Stretch
arm.scaleX = 1 ;
elbow.scaleX = 1 ;

**to add other joints you would just add, underneath elbow.scalex
ex// forearm.scalex = (distanceDimensionShape1.distance / 12.9);

If there are no errors then move joint chain and joints should stretch
the wrist won't stretch unless you specify otherwise.
reason why the scale is set to X is because that is the axis that
,by default, follows the joint chain.

I realized theres no real way to turn the stretch on and off
without knowing some kind of expression or something so i
figured constraints could work. This is what I came up with

bring ik handle to original location

create a control curve/object, give it a name ex//Ik Wrist cntrl

In the channel box, add a float attribute named ex//IK Stretch
minimum 0 max 1 default 0
and point constrain the distance end locator to the curve/object.

create another locator and v snap it to the distance start
locator, give it a name ex// distance helper

Freeze transformations

Point constrain the "distance helper" to the " IK Wrist cntrl"

in outliner or hypergraph, select the distance end's point
constraint. This should have two values just below the offset.
Distance Helper1W0 = 1
Wrist CNTRL1W1 = 1

go to window/animation editors/set driven key/set

select the "wrist cntrl" as the driver and the point constraint
as the driven. change the value of the "wrist cntrl" ikstretch
to 0

select the point constraint then theand the distance helper to 0 leaving the wrist cntrls value
at 1.

select the point constraint then the Distance Helper1W0 = 1
Wrist CNTRL1W1 = 1

change the distance helper to weight to 0 leave the wrist cntrl's
weight at 1

in the driver window select the controller, then the attribute
in the driven window select the point constraint and the values

press key, select the controller in the
driver window, change the attribute to 1

select the point constraint in the driven window and change the
Distance Helper1W0 =1 Wrist CNTRL1W1 = 0 and then press key.

this should have created the setup for an on and off switch for
the stretching of the joint chain.

like i said, i figured this out through trying to think of the
best way to do this kind of a setup through simple weighting of
constraints. Kind of like how you would do a ik/fk setup using
3 joint chains. I just wanted to make sure that this process would
be ok if i were to add this to a rig and then animate with said rig
any criticism or additional input is greatly appreciated.
  08 August 2012
It's an interesting solution. I admit, I'm curious to see how it performs. Generally I've always found that expressions take much longer to evaluate only behind the dreaded wrap deformer and Cloth simulation in resources demanded.

From my research stretch oftentimes seems to be implemented via:

SDK via mel to make the translate X of the elbow and wrist keep uniform distance. Just have the SDK in the graph editor set to linear on post infinity. The advantage to this method is that using a blendAttr node you can greate a really awesome elbow locking system for pinning a elbow or knee in place.

To turn the stretchy system on or off since we're having the final chain follow the stretchy using translation I just key the weight of the point constraint to determine whether the final chain follows the Ik or not. Technically the Ik arm is always stretchy but since it's using utility nodes or a sdk and not expressions the evaluation doesn't affect the mesh verts.

Or: via utility nodes: 2 multiply /divide and a condition node. and another for normalizing scale OR using zeth willie's awesome measure joint chain that allows for proper scaling without plugging the master's scale directly into a multiply divide node's input 1x.

again to turn it on or off you key the scale constraint you use to make the final arm follow the stretch. I also want my FK arm to have stretching ability so this requires a bit more of a work around using a stretch grp that's controlled via FK stretch activated by the IK 2 FK switch option.

I hope that might help you in your studies. If anyone more knowledgeable seems something off with my method please let me know.
  08 August 2012
+1 to the utility nodes suggestion.
That rarely (if ever) fails me and keeps things fairly quick.
  08 August 2012
Wow great feedback, @gagex2z, I completely didn't even think about using the graph editor. Both methods make sense, its just I'm not to keen on utility nodes although I'm told there easier to manage but that's something I probably need to get over.I will definitely try that out.

@ldunham1, Thanks for the comment, I just wanted to let you know I used one of your scripts to fix the deformation order on my rig since there seems to be a bug with trying to do it in maya. Scripting is one thing that scares me and you make it look easy

Very useful and appreciated feedback guys, Thanks!
  08 August 2012
Whoa man, stop thinking right now. Let me enlighten you ")

First, utility nodes to handle the stretch is the only viable solution. But there are easier ways of toggling the stretch on and off.

Add an attribute on your ik controller called "allow stretch" and also "force stretch"

Allow stretch [0-1, default 1]
Force stretch [0-10, default 1]

Translate this into utility nodes

D = distance from root to controller (the stretch to distance)
B = original total chain distance (both bones unstretched)
J = original joint length (lets say this is the upArm joint for this example)

Condition: If D is > B

True: D - B / 2 * allow stretch + J / J * force stretch

False: 1 * force stretch

Keep in mind the order in which these need to be evaluated. I typed them left to right, in the order they should be evaluated. Running theoretical numbers through this will show why the eval order is crucial.

Adjust for global scaling. By dividing this by whatever node your scaling

And also, just a small little hint. You can reverse the effect of a toggle/switch by: *-1+1
Keep it simple

Last edited by everyFrame : 08 August 2012 at 05:24 PM.
  08 August 2012
Sorry for the long response time. Thats also a really good idea, but like I said prior I'm kinda fresh to utility nodes. So trying in trying to understand what you said, essentially I'd have a multiple/divide node a condition node and the joints I want to stretch hook up the joint scale x to these nodes and walaa? Putting it simply
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