8

Cloth Overview 553
need a cloth object, such as a tablecloth or a pair of
pants. N ext, you need something for the fabric to
interact with. This can be a collision object such
as a table top or character’s leg, or a force such as
wind or gravity.
Limitations
While Cloth is designed to help you create
clothingforyourmodels,youshouldbeaware
that, by its very nature, cloth simulation is only
an approximation of how real fabric would react
under certain circumstances; this system does
have some limitations.
One of the most important aspec ts of working
with Cloth is the amount of time it can take to
create a simulation. If you’re looking to create a
fully physically correct simulation, you might run
into problems. Even with a fast computer, cloth
dynamics at that level of accuracy (and geometric
detail) could take virtually forever. So you must
learn to scale your simulations back to a reasonable
level. This doesn’t mean you can’t get believable
clothing; it simply means that there are tradeoffs
you should be willing to make.
Tradeoffs
In order to create a believable simulation, you
need to balance time against quality and accuracy.
Themoretimeyouhave,themoreaccuracyand
quality your simulation c an have. There’s no
reason to ma ke a model with 10,000 polygons if
you can define the form equally well with 3,000.
Thesameruleappliestoclothsimulations.
Internal and Ex ter nal Forces
When simulating cloth, different forces come
into play. Some internal forces like bend, stretch,
and shear al low the fabric to deform in a realistic
manner. External forces such as g r avity, wind,
and collisions make the cloth interact with
its environment. To obtain a good-looking
simulation, most or a ll of these things need to
come into play. Without these forces, a piece of
cloth will remain a flat, lifeless plane.
Collision Detection
Whenputtingashirtorpairofpantsona
character, you don’t want any p art of the b ody
to protrude through the fabric. The desired
result is to have the garment deform around the
mesh (rather than through it) so there are no
intersections. This is done with
collision detection ;
w ith Cloth, you tell the simulation system which
objects w ill act as cloth, and w hich ones will act
as collision objects.
Basically, virtual feelers are sent out from the
vertices of the cloth objects to see if there are
any other objects that they m ight collide with.
When one of the feelers hits something, the
simulationknowsthatitmustdeformthefabric.It
is important to remember that a cloth mesh with
more vertices has more feelers and can do a b etter
job of collision detection. This is critical, because
if you are working with a high-poly character
(collision object), you will need to increase the
densityofyourcloth,orthehigh-polymeshwill
protrude through the lower-poly cloth object. The
reason is that there aren’t enough feelers to detect
all of the detail in the collision object.
The alternative to this is to add one or more
low-polygon proxy meshes for the character so
there doesn’t need to be such high density cloth
objects that wi l l slow down simulation. We’ll cover
the mesh density a bit more in the next section.
Lastly, if you are simulating with fast-moving cloth
objects, you mig ht need to increase the Density
value to give you the benefit of more feelers. You
also might adjust the Step size to check more often
for collision objects in the way.