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Using spline IK for arms w/o deformation

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i have a rather sensitive project. I have to animate a character (which you can see in the pictures) that heavily relies on facial expressions and arm movements. His arms however, are not human-like arms but simple cylinders. When i rigged them by parenting the joints to the spline using IK spline tag with handles, it worked well, but when i started animating the hands, i got some weird geometry deformation (as seen on  the second pictrue) at sharper angles. I need the hands to preserve their thickness the same at all times. 


There is also a second problem and that is the mouth. I tried to make it using pose morph, but the texture gets stretched alot, beacuse the mouth needs to be able to get to the very edge aswell (basically under the eyes). Any idea how to do it? I tried using displacement with spline effect in the material, but my c4d crashes after every few changes in the effect. (probably does not like the octane material too much). So im getting kinda stuck here.



Any advice is strongly appreciated!



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@DrDisco  perhaps we'll be able to help a little better if you could upload pictures of the topology and the strecthing of a checkerboard texture.


To be able to stretch the mouth all the way to the eyes means your topology must be allowing that degree of deformation. 


Also do you have a reference vid what you're trying to achieve? As I understand it you want a bendy rig, but you chose to use 2 lateral spines for the bending instead of a typical  centered one. Is there a particular reason for the lateral splines?

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Thanks for answering. I added more pictures of the base topology with and without SS. Also the texture stretching with the mouth movement and only reference picture that has the most complicated movement. Sorry, no video. As for the splines. The rig is unfortunately not mine, but the guy who gave me that said that he wanted the arm to not affect the outer geometry so he just made 2 separate joint chains to make sure the geometry around  the armpits does not move . I dont really think there is much of the difference, because the body is not supposted to bend since its a playing figure. 


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Ok, so as for the mouth deformation, what you would need to do is (after unbinding the skin from the mesh) adding a number of loops around the lips starting perhaps a little higher than the nose in this character's case. Then you could sculpt the pose morph (while keeping the same poly count) with the mouth at the desirable position. Try to have as little poly stretching as possible in your final position and that will give the texture a much easier time too. The unfortunate part is you'll have to reskin.


As for the arms bending what exactly is the problem? (Make sure your splines 100% in alignment with each joint chain, that they're set in bezier mode and they only have 2 handles) . If you want softer deformations hand paint joint weight extensions with the weight smoothing brush (auto normalize on), but given that all weights are balanced in your arms chains, overall thickness should remain the same when using the handles.


Just like the mouth there's a topological weak point (lack of loops) at the shoulder that may give you problems. You can rectify that (and you should) along with the mouth and reskin or

resort to the power of smoothing deformers after you have created your stifness maps. In your case you'd have less stifness around the shoulders.


Let us know how it goes




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you don't really need to reskin if you add in a couple of loops around the mouth. one or two extra loops should be enough. all you'd have to do is do a little weighting correction on the newly created vertices and redo your pose morphs if you already did some. also adjust your uvs to get rid of some remaining stretching.


as for the deformation problem: i assume you want the arm not to be that flush with the body and have it first stick out more at the shoulder? if so, you can play with the depth settings of your first ik handle (select your spline ik tag, and in the attributes window click on the handles tab, there's a depth setting to each handle which defines the length of the (unvisible) bezier handles). up that number for the first one a little, it should make the arm stick out a little more. if that's not enough you can also play with the weights a little. if that's still not sufficient, you'd have to reposition your joints a little.



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Ok  so now that i got all things solved, i would like to thank you for the suggestions that you provided me. I actually used after effects img seqence that i put as a displacement for the mouth which actually got me pretty good results. And solved the arms easily by re-rigging it. 


Thanks for all of your responses, really appreciate it!

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