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572 Chapter 8: Modifiers
that, by its very nature, cloth simulation is only
an approximation of how real fabric would react
under cer tain circumstances; this system does
have some lim itations.
One of the most import ant aspects 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 b elievable
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 can have. T here’s no
reason to make a model with 10,000 polygons if
you can define the form equally well with 3,000.
Thesameruleappliestoclothsimulations.
Internal and External Forces
When simulat ing cloth, different forces come
into play. Some internal forces lik e bend, stretch,
and shear allow the fabric to deform in a realistic
manner. External forces such as gravit y, wind,
and collisions make the cloth interact w ith
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 Detec tio n
Whenputtingashirtorpairofpantsona
character, you don’t want any par t of the body
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 which ones w ill act
as collision objects.
Basically, vir tual feelers are sent out from the
vertices of the cloth objects to see if there are
any other objects that they might collide with.
When one of the feelers hits something, the
simulationknowsthatitmustdeformthefabric.It
is important to remember that a cloth mesh with
more ver tices has more feelers and can do a better
job of collision detection. This is critical, because
if you are working w ith 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 will 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 m ight need to increase the Densit y
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.
Clothing and Pattern D e sign Overview
Traditionally, sewing patterns are cut from flat
pieces of cloth and stitched together. The place
where one piece of cloth is sewn to another is called
a seam. Patterns are generally symmetr ical, w here
theleftsideofthegarmentmatchestheright.
Ski rt
The simplest is a skir t pattern with two pieces, with
a similar shape for the front and back. The back