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Cloth Simulation with SyFlex

By Mauro Frau
Web: Open Site
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Date Added: 9th December 2005
Software used:
Softimage

Damp

Damp is a viscous force that slows down the movement by dissipating energy. The intensity of the damp force is proportional to the speed. Damping prevents infinite oscillations (such as in a pendulum). On a cloth it is often better to lower the damp constant, in order to prevent dragging the cloth. High values (> 0.001) will slow down the simulation.

Air

This force is very similar to the previous force: it's also a viscous force. It takes into account the orientation of the cloth and its speed; the force will be maximum for the faces of the cloth moving in the direction of their normals.

Wind

The wind force is defined by the main direction of the wind, and a perturbation. The perturbation varies randomly, with a maximum given by the Perturb parameter.

Turbulence

The turbulence force is similar to the wind, but is not uniform both in space and time. Imagine a 3D grid, with a different wind force in each cell.

Spring

This force connects 2 vertices of the cloth with a spring.

Every single force has slightly different parameters but they're pretty understandable just doing some tests. If you need a more accurate description of dynamics forces, once more, you can do some research around the web; you should be confident with those theory stuffs if you want to have full control of realistic simulations for 3d animations.

Just a little note to remember that computer animation it is not reality, it just simulates reality. A physically prefect set of parameter alone do not assure a computationally perfect simulation.

We will now apply a gravity to our hat. To do this, select the object, then Forces > Add Gravity and the new stack-node will appear. You can set gravity for each local axis of the cloth. In case you forgot to freeze transforms before applying cloth, at this point you had to find the right normal direction to the ground.. but since we did freeze everything we just have to set it on a negative y. I used -0,05.

Look at the results, now the hat falls down following the pins on the head.
We now have an intersection problem, those kind of intersections happen quite frequently working with cloths, so don't worry.
 
1192_tid_tut04.jpg

Bad compenetrations

Select both the hat and the head, then Collide > Collide Mesh to set the head geometry as collision object. Bring up the damping to stabilize the collision and pull up the external envelope to 0,5 to avoid intersections with a little offset around the collide.

The other collision options are related to single selections of faces and vertices, with the possibility to reload them back and modify them anytime during your work: Set & Get Collide Polys, Set & Get Collide Points, Get Collide Mesh.

Now the problem has shifted, it's less evident but it's still there. If you look at the simulation you'll see there aren't intersections between head and hat anymore but there are a few between hat's polygons themselves.
 
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Self Intersections

To fix this, activate self collision. This is a good time to pull up the smoothing level of your original mesh (only if you're using subdivision surfaces of course) to have an higher definition of the problem. Collide > Self Collide will apply self collision, now the simulation should start to get a little slower, depending on your computer, but we are almost done. For self collision too we have a Get & Set command, referred to faces selections.

If you textured the hat with a single bitmap and UV editor you shouldn't be having any problem but if you created clusters for procedural materials, try to render your hat and you'll see you've lost your original colours. This happens because the self collide operator creates a cluster for collision faces into the object's stack called sySelfCollideCls that overrides the textures clusters. The trick here is done by moving the textures clusters after the self collide cluster into the stack. In XSI you have a Sort Order parameter into the cluster's options panel, bring it to a very higher number, such as 500 for wreath and 501 for hat's body. I don't use to apply textures after cloth operators, I generally prefear working on totally finished and freezed geometries to avoid creating instability into the scene.

Ok, now the simulation should work pretty well, if something does not satisfy you, try to manipulate syProperties parameters, uncache and play again and again the simulation to reach the desired result. Perhaps it's better mute the self collide operator while testing if you don't have a fast machine.

The State menu includes some operators that set and reset the initial state of the cloth, using its current shape and speed at a selected frame. You can call this function after running the simulation for a few frames. When coming back at the first frame, you will see that the initial shape has changed. The same happens for the velocity.
The Create Cache menu allows you to stock two different caches and blend their weight but sometimes it's a little tricky to obtain an easy-to-control result.

Any additional component of your cloth should be created and applied before working on the simulation, for example you may want to have a fur on the hat or other constrained objects. Try to plan it before applying Syflex operators.

Download movie to see some examples.


About the Author

Mauro Frau is a 3D artist and animator from Italy. He holds a BA in Computer Animation and has been working for television and videogames since 1999. Mauro is currently working as 3D generalist for Universal Video Corp. in Milan. You can find more about his art on www.maurofrau.com.



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