2015 new bend model/twist tracking?

#1

I don’t understand what this actually does, compared to the old twist and bend settings:

nHair Bend Model
A new Bend Model attribute lets you choose between a bend solving method for long curly hair and one for generic hair styles. Use Twist Tracking to maintain twist in long curly hair and for when the base twist of hairs is already groomed at the start frame. A new Bend Anisotropy attribute for this bend method lets you control the relative bend resistance in different directions around the hair. Use Simple for the default nHair bending behavior used in previous versions of Maya

Is seems the main effect is for neighboring hairs with similar shape to stay parallel as a group. Is this the main reason it was introduced?
Where are the disadvantages for using it on straight hair (slower)?

Thanks.

-oddf

#2

If your hair rest shape is straight then it won’t make much difference. However if you have curly or shaped hair it will preserve the shape much better than the old bend. The old bend model had a significant problem in that its twist resistance computation became degenerate when the angle between segments was near zero. Thus curly hairs that had either had some straight sections, or simply had lots of cvs( the more cvs the smaller the angle between segments ) would twist and the “groom”, or rest shape, would not be held. Note that to preserve the bend with a curved rest shape one needs both bend and twist resistance.

It is a surprisingly difficult problem and took some research to solve.

#3

Interesting, thanks. I do have shapey hair with dense CVs so I’m going to use this setting. You did/have a great job :thumbsup:

#4

Some tips/workarounds I found that might help anyone working with twist tracking

[ul]
[li]like some other nHair features, twist tracking needs to be activated by going to frame 1 from another frame (rewind button from frame 1 itself isn’t enough)[/li][li]when you’ve been on frame 1 before in the same session, maya might hang when you want to go to frame 1 again[/li][li]to work around this, save your scene with twist-tracking off and while not being on frame 1[/li][li]after reloading the scene, turn twist-tracking back on, then rewind to the first frame or regenerate the cache (which also goes to frame 1 and can cause hanging)[/li][/ul]

#5

some more tips:

[ul]
[li]twist tracking doesn’t seem to work with rest curves, there’s some additional wobbling but they come out more or less straight.[/li][li]twist tracking can sometimes becomes unstable when an angle between two segments is too high (best stay < 90°)[/li][/ul]

#6

edited some of the above post so the thread makes some sense