View Full Version : IK: Unaffected by IK of Ancestors?

05-04-2006, 03:08 PM
I've made a simple human skeleton, with the root at the base of the abdomen. From there I've got three chains, two legs and a spinal cord.

Further up the spinal cord, it branches into a neck and two arms, and I think this is where the problem is. I'm using IK and controlling it all with goal nulls-> two feet nulls, two arm nulls, a body null and a head null.

The problem is that when the Body null is moved, the left arm goes haywire. The right arm simply goes along with the body, and its goal can still be easily moved (I have 'Keep Goal Within Reach' checked). The left null leaps to unexpected positions and is completely undirectable.

I think this must be something to do with LW's IK mathematics getting confused... it seems to be trying to follow through the maths of the spine into the left arm, while ignoring the head and right arm. I want it to follow through into the head, yet there's no way to tell the left and right shoulders not to be affected by the IK of their parent bones (these bones need IK as they are part of the arms, and sticking non-IK bones inbetween doesn't help.)

I can of course simply create new bones from the shoulders to the root, but I can't believe that Lightwave offers no provisions for determining which path an IK chain follows, and I can foresee the problem recurring in future situations where this fix won't be realistic.

Is there anything basic I've missed (am relatively new to LW) and how can I get greater control over the IK, when dealing with several interconnecting chains?

Any help would be greatly appreciated.

05-04-2006, 04:19 PM
Couple of things. Are you making sure that both the left and right arms have uneffected by Ik of descendants enabled at the base, probably the collar bones would be best? Also the way you've described your set up it sounds like you have IK for the head and spinal cord, have I got you right? If so unless you need a sort of rag doll effect I'd say go with FK for most of the body, really IK is mainly used for legs and sometimes arms.

I'm not an expert in this area by any means so someone else will probably give you a lot better soloution but hopefully it's of some help. Spline God knows a lot and will probably be along in a bit and be a lot more helpful :)

Pete B

05-04-2006, 11:09 PM
if the right arms is working and the left arm isn't
go over all your right arm setting and make sure you have the same on the left arm

OR delete the left arm uses mirrorbones command to copy the right arm over

OR post the scene file without the model and we can check it out ;)

05-05-2006, 02:33 PM
I think, although am not sure, that it's not anything to do with the bones themselves, but Lightwave's IK mathematics selecting the wrong heirachy of bones from a choice of three. Although, having just started using the things, I've no experience with how the system actually works and may be wrong.

Apart from not actually knowing what FK is, I've got no problems (aside from the reason I made this thread) with the body - I'm using Stiffness to reduce the ragdoll so it's more realistic.

File included... I didn't get rid of the model as that's where all the nulls are parented. (btw, does anyone know how to merge layers in Layout? So you can have a bone layer and a modelling layer in Modeler?)

05-07-2006, 10:27 PM
Bones and the mesh cannot be separated. If you need to replace the model the bones are attached to, you need to select the mesh, go to 'Items'>REplace>Replace with object

IK should be stopped at the base of limbs, Heirachy is this: Root>Thigh>calf; select the thigh bone and press 'm', under IK and modifiers, activate 'Unaffected by IK of descendants'. This is the IK stop you are looking for. The above heirachy is very simplistic and not reflective of the knowledge needed.

Rigging is a complex thing and not learned in an hour or a day. Try this tutorial:


Or buy Timothy Albee's Character Animation book. His rig provided with the book works very well for me. All the characters in this pic was posed using this rig. It also explains the rig and how it was constructed quite well. Some parts are non-intuitive but overall you should be able to learn what and how to rig a character.

05-07-2006, 11:37 PM
What did you use as the basis to set this up?
I hate to tell you this but the rig has a lot of problems.
1. First of all I wouldnt try and control every single part of the rig using IK. Legs definately, arms maybe but not the spine and head especially if this is supposed to be a basic general purpose rig.
2. You have IK turned on for every channel of every one fo your IK Bones. This is incorrect. You only need to control certain channels using IK and others need to be controlled with FK so you can at least orient the knees and elbows. Knees and elbows can only bend along one plane so you only need one channel to have IK.
3. Youll never need to have IK applied to the bank channel.
4. I would avoid using Keep Goals Within Reach...it just makes the rig wobbly.
5. The joints on your rig flip and pop all over the place. This is for two reasons, because of having iK turned on for every channel and you never want the root , middle and end of your IK chain to be in straight lines. This doesnt indicate to the IK solver how the joint should bend. You can fix that by prebending the joints.
6. Youve used contraints. Usually people use this when the joints are misbehaving. I would turn them off and fix the flipping problem as I mentioned previously.

Rigging is more than simply turning on IK. You have to have an understanding of what makes a good rig, what it needs to be able to do. It must be stable, stable in between poses and easy to pose. Rigging isnt that hard if you have a good basic understanding of what a rig needs to do to make it good.

Heres a couple of videos off my new rigging a quadruped CD. This rig is very stable, poseable and easy to use.

05-08-2006, 02:56 AM
I agree with some of what Larry said

But to help you out
check this rig out, the main thing is just pull it apart,
check the IK setting and see what you like and dislike
the joystick face controls are something you don't need work out just yet
there just a fancy thing I do for Character animation workflow :D

If your into rigging you should be able to fine where you went wrong and build a good one next time :thumbsup:
You already know where the controls are, you just need to know what to turn and and why.
the best rigging reference for Lightwave that I've seen is this one
a must buy if your really interested in the rigging side of things
get Both but the Vol 2 really gets in deep, which is my Fav volume:thumbsup:

05-08-2006, 04:43 AM
Or if you are adventurous, enable IK booster for your bones. Here's how:

Select your MESH.

Go to Modify tab, click on 'IK Boost Tool'

Right click on the pivot gizmo of your mesh and 'Apply IK Booster'.

Circles will appear at all your bone joints. If you grab any bone, it will be like a rag doll; not what you want? Then select your bicep bone, right click on the circle and choose IK stop. Voila. Instant IK stopped chain. Now when you grab say the hand bone, the arm will move as you would expect, with the rest of the body staying put, just like your original expectation.

If you click on the coloured H, P, B alphabets and drag you can rotate it interactively. Brilliant.

Do it for all your joints where you want independent IK solving, e.g, base of all fingers, thigh bone, foot bone.

Experiment with it. It's good enough to animate for fun. Spend enough time with it and get your heirachy right, you CAN get serious work done with it.

If you need to solve a whipping tail of a lizard, IK Booster will get it done where IK will just collapse in confusion. This is because IK chains are only reliable with no more than 2 bones in the chain and pre-bent as SplineGod mentioned.

05-08-2006, 05:17 AM
IKB has got pretty Bad Character animation workflow
Yeah you can uses it
But standard IK with a Good character rig is so much faster, easier to edit and more solid Ca workflow.

but that just my opinion
best try all the options and see what's best for you ;)

05-08-2006, 05:47 AM
I agree Vincent. More and more people are starting to figure out that IKBoost is a powerful addition to their workflow. You can produce nice, stable rigs with it and its got some nice features such as the ability to change your rig on the fly, pose saving/reusing, motion saving, reusing, dynamics etc.
Its part of LW so you get it for free. May as well learn to use it. :)

05-08-2006, 08:39 AM
Just found this link to IKBoost video tutorials, looks like its right up your alley: