Long Stop at the Cloth Department

While trying to get everything in place for the entry scene I also allocated time for some serious cloth simulation.

Some may remember that I had several attempts to setup a workflow which would allow me to do fast animation prototyping and yet have all the links to the cloth simulation ready.

Well, that was my impression some days ago too. And then I started to actually apply the cloth simulation to one of the entry scenes. It failed miserably.

I had overlooked one little but significant detail. As suggested in several tutorials and also the blender wiki, I went for a setup with a combination of armature driven (torso and upper arms) and collision driven (skirt, sleeves) type of cloth simulation.

With such a setup it is important how you define your starting position. Usually I start in the rest position and slowly ( ~ 30 frames ) settle to the first pose before starting the actual animation to give the cloth simulation time to settle in. As parts of the cloth are only collision driven you have to find a way to fast position your cloth. My setup so far was to parent the dress to the main position bone of Ara’s armature. In each scene I only modified the position bone itself to move Ara ( and the dress around).

As soon as I actually enabled the cloth simulation and armature modifier on the real dress ( animation was done with the fake one) things got messy. Moving the position bone moved all the parts of the dress correctly which were only collision driven, but all the armature driven parts moved twice the way (of course) .

The correct setup would have been to position the armature object at the animation starting position and leave the position bone, which is not the usual way of doing it.

So now I had to manually reposition each animation to be based on the armature’s object position instead of the main position bone. A lot of tedious work. And this was now only for the first scene.

Anyway, I found a way and started work on the cloth simulation parameters. And that cost me ~30 hrs of work to arrive at a solution which is satisfying.

There are some (few) tutorials and threads on cloth simulation, and I also have Tony Mullens book Bounce, Tumble and Splash, but there is nowhere a real in depth discussion on how to best tackle cloth simulation. And as I see it, there isn’t only one type of cloth simulation but a whole spectrum full. With the sparse information you are more or less on your own and have to figure out how each parameter influences the simulation in this specific setup.

To have the turnaround cycles fast I lowered the simulation quality. This turned out to be a major mistake in my case. It seems if you have a very free moving cloth system where energy is easily added ( skirt collides with legs and gets accelerated or stopped ) the simulation tends to oscillate and isn’t able to converge on a good solution. What I observed was that as soon as I started to get good approximation of a dress fabric, the skirt started to get crazy after a few steps.

In a last attempt I increased the quality setting (from 5 to 20) and was amazed to witness a totally different ( but good) simulation). With this I could now really start to map out the best parameter settings. With 5 secs per frame and at least 100 frames before you have a good estimate on the type of simulation it was a long and tedious work.

Its amazing how many aspects contribute to a convincing cloth simulation. First you have to get the parameters right of course. Then the part which is armature driven has to be fine tuned, otherwise you get some strange artefacts. Stiffness and bending scaling was an important tool. I wished there was a mass scaling as well. And last but not least the shape and topology of the dress itself was equally important for nice and fluid movements. I got some hints from actual dress cuts of a fairy dress for my daughter.

What I think is not really achievable is a very stiff cloth skirt, which doesn’t bend easily and keeps its form or bends in sharp edges.

I have now a setup which is so far tested in each shot for the entry scene. The trickiest part is where Ara’s is walking and I am confident to have all aspects of the main scene covered as well. But that will be seen when I start work on that …

Oh, and yes, I still have to test it with some wind effectors as well …

After so much babbling see here shot 3 with the actual cloth simulation in action.

I wonder what surprises the hair simulation will have for me.

Author: loramel

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