Cloth simulation
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30 September 2008

So, all the movements of the three-dimensional character are ready, but still something is missing… We lack gentle summer breeze, which flaps the curtains of our wagon. And Character’s clothing is repeating his movements too much, making a false impression of “second skin.” The torch on the wagon is rather permanently welded, than hanged…
Though it is an animated and not a real world, it is still a copy of our reality, where, as is well known, the laws of physics were not annulled by anyone. So, we breathed life into the character, now it is time to breathe life in the world, where our Character lives. The next stage of animation – is addition of secondary movements.
Let us start with probably the easiest one – torch, which is hanged in front of the wagon.

If to look more closely, the problem reduces to the fact that the torch is attached to the wagon in one point. At each move of the wagon, the torch has to tweak too, trying to go back to its primary position. In order to achieve this effect, we should only use ‘hair system’ (as strange as it sounds). But why not? Because in general, the scheme of working with hair is the same: we can attach it to whatever we want, we can settle its rigidity, can attach gravity to it, at that simulating the presence of weight by the object. After the adjustment of hair system in the right way, we should make it conduct the bones to which our torch is attached. Voila! Without extra installations of keys and so on, we have the completely different picture: the torch is dangling on the wagon, and the animator, after he had done his job, smokes coolly on the balcony :).

Now it is the turn of a far more complicated task – simulation of fabrics. These are the clothing of heroes, the fabric on the wagon, curtains, mat and different paraphernalia, hanged here and there. The problem is that we should make all this stuff behave adequately, interact with one another, not allowing it to “fall down” in one another, which means that we should consider friction among them. At that, our animated fabric should act as a real one: form wrinkles, respond to wind, stretch and contract and we also should consider the possibilities of computers (Render is not a trifle – it can take a looot of time). That is why, almost always the work adds up to “diplomatic efforts” in the search of a compromise between speed and quality of simulation.
First we should indicate which object is “fabric” and which is not. Then, with the help of a brush we draw the sections, which will distort in greater or lesser degree on this fabric and set the forces, with which they will react (wind, gravity, etc.) and how they will do it. This stage constitutes probably the 90% of all the work, because we should indicate the force of gravity, degree of influence of the wind, grade of flexibility of fabric, where it is needed, because one and the same shirt can have several of them. All this is done to speed up the estimation: in the places where the fabric is strained or is not seen at all, why should we calculate the dynamics, if it can be just turned off, at the same time preserving visual quality? 😉

Different kinds of constraints are also frequently used: it is a sort of stopper, a pin, with the help of which we can attach our fabric to some surface.
After many hours of torments with adjustments we get the result, with which everybody are satisfied. But even at this stage it is not always possible to evade defects. In this case we have to correct them manually, using tools from 3D package. And already after this it is really HOORAY! Everything is done; we wait for a new scene from the animator.

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