When you are designing your dynamic hair, you need to put special consideration in if you want to drape it or run physics simulations on it. If you just plan on positioning the guide hairs where you want them manually and letting the hair remain static then things are relatively simple because you have a great amount of control. Once, you start running simulations, however, you are loosing control over where the hairs will go to some extent. Therefore, you must set up your hair in such a way as it will behave the way you want when you run that simulation.
The first thing to keep in mind is the placement of your guide hairs. You must place guide hairs all along the edges of your growth region to ensure that anything the hair drapes over will collide with the guide hair before passing through any generated hairs. My previous article on this subject covered the topic, so I will not repeat it all here. The previous article also talked about the
importance of setting up hair groups or using auto-grouping. You should read that article first to get those important concepts.
Another important thing to know is concerning a peculiarity with how Carrara simulates hair. All of the hair on a single hair object has the same base length, which you can see in your hair parameters. Even if there are multiple hair groups on that object, there is only one base length for the whole thing. You can use the length and scissors tools to alter the length of certain guide hairs and thus the generated hairs around to those guides. You may have done this to cut bangs out of the front of a character’s hair. This, however, can cause trouble when draping or running a simulation.
For whatever reason, if you have significantly altered the length of some of the guides within your hair object, those guides may not simulate correctly. I discovered this when I attempted to run a simulation on a hair object where I had cut bangs out of the front using the scissors tool. In my experience, the shortened hairs bounced wildly while the unaltered long hairs simulated as expected. I was very frustrated by this until I discovered the solution. If you must have a section of hair that is significantly shorter than another section, then you must use different hair objects for those two sections. Different hair groups is not enough. I suspect that the reason is because different groups on the same object would still share the same base length and that wide variation in length between hairs with the same base length throws off the simulation.
There is no problem having two totally different hair objects associated with the same source object, however. Each hair object operates independently, and simulates as you would expect. You can just use a secondary hair object the same way you would use a secondary growth region. It can be a little tricky defining the growth region, however, since the other object’s region is not visible while you are defining it. If you were defining two hair groups on the same hair object, then you could see both growth regions at the same time. That would definitely be easier.
So, the new rule of thumb that I am giving you in this article is that when you need one section of a character’s hair to be significantly longer or shorter than another section, use two totally different hair objects. If you follow all of the guidelines that I have set forth in both this and my previous article, then you should be on your way to designing hair that simulates smoothly and without moving through objects. If you follow my advice, I would love to see your work. You can send me a message on my YouTube account animatigital. Have fun!