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So I want to animate my character, and I'm having trouble figuring out how to handle the hair. At the moment, it looks like this in T-pose:

enter image description here

But when I rotate the head, for example, this happens:

enter image description here

Even worse when rotating in the other direction:

enter image description here

The hair should be glued to the body and not move into the body. My idea was, that the hair, which is not glued to the body, for example the part between head and breast, could be stretched. So the hair is kind of a stretchy string.

I tried the boolean modifier to glue the hair to the body, but the hair still behaves differently.

If I understood the boolean modifier with union correctly, the part where the hair and body overlap should be deleted and the hair and body are combined to one surface.
So the hair wouldn't be able to lift up from the chest, theoretically. But it has its own surface and the breast below has its on surface underneath the hair, otherwise the part where the hair lifts up would have to be a hole.

I tried splitting to loose parts and then remerge it with the boolean modifier, and it also didn't work.

What can I do, and how should I handle the hair?

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The boolean will not make your hair "lift up from the chest" nor glue it to the body, it will just make a simple X + Y operation from your intersecting meshes volumes. Also, using boolean operations in a rig sounds like a recipe for performance hell.

What is your end goal? Probably just get a good deformation of your hair, not make geometry. But what kind of deformations would look good here when your head is fully up? If it was me, I would NOT stretch, or at least keep that as a last option: hairs do not stretch in reality, seeing hairs stretch as soon as your character looks up will scream off weirdness.

The way I see it, you could use two approaches. I will only explain why and how to use them but I will not give the technical details of how to set them up, as it would be very long and all the things in mention can be easily found well detailed and explained online already.

Tweak the neck ⬌ head deformation

At the end of the day, what we see in your second and third screenshots is that your head bone controls mesh parts of the hair that are way below the head bone's rotation point.

enter image description here

You could switch to weight paint mode and modify the weights of the hair so that they deform about at the same place as the body parts do:

enter image description here

You could add some shapekeys driven by the head bone's relative rotation to fine tune how the hair deforms in different situations.

Rig the hair

Eventually you will reach a limit to what you can do with deforming the hair from the body's bones. Having the hair handled by individual bones can give better results, and there are many ways to do it.

The simplest would be to have a simple chain like such, and why not with an IK constrain, with its target bones parented to the torso. which makes the ends of the strands stay in place while the rest of the bone chain adapts to the distance between the head and the targets:

enter image description here

Now depending on the pose, it might look weird, but since your hair strands are rigged: you are in control, you can pose them as any other bone if you want :

enter image description here

You could also make use of a spline IK setup.

Probably both

Looking at your model, different strands are laid out differently on the character, so they could be handled differently depending on that and what you want to be able to do. At a glance, I would say most of it could theoretically be just deformed with the body's bones and still look good, so I would start by working on the weights as a "by default" situation. And for the hair strands that have some wiggle room and can visually untangle without stretching (I.E. the curly ones from the forehead), I would consider adding specific bones for these.

enter image description here

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    $\begingroup$ That's exactly what I was looking for. Big thanks for the effort and visualisation!! I'll try the combination of both and look up in detail how to do it properly. $\endgroup$
    – Aletheia -
    Nov 16, 2023 at 6:42

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