Mayanclothadvancedtechniques2010

nCloth Advanced
Techniques 6

127

Overview

The nCloth Advanced Techniques tutorial demonstrates a workflow for
simulating high resolution clothing mesh using nCloth. In this tutorial, two
lower resolution versions of a high resolution mesh clothing are converted to
nCloth and then simulated. Using a wrap deformer, the nCloth objects become
influence objects for the high resolution mesh clothing, which then behaves
the same as the simulated nCloth.Using this workflow, you can quickly create
animated behaviors unique to simulated cloth using high resolution meshes.
This tutorial describes methods for identifying problem areas in your nCloth
simulations and explains how to resolve simulation issues, such as inaccurate
collisions and self collisions, which often lead to interpenetrations between
colliding objects. Inaccurate collisions are at play when the components
participating in collisions do not accurately detect the presence of other
participating components. This can happen when simulating models with
complex geometry. It also explains the importance of setting key Nucleus

128 | Chapter 6 nCloth Advanced Techniques

solver attributes early in the simulation process to ensure better results in
more complex nCloth simulations. When simulating complex models, the
default solver settings often need to be adjusted to optimize your nCloth.
Use of any data from The Spine in this tutorial is courtesy of the National Film
Board of Canada.

When is this workflow useful?
This workflow is useful for, but not limited to the following scenarios:

■ Simulating highly detailed polygon meshes.

■ Simulating clothing from various mesh resolutions, which can then be
used for characters in the background, mid ground, and foreground of
your scene.

■ Simulating the same clothing for multiple characters in a scene that have
similar scale and shape.

Why is this workflow useful?
■ Reduces the amount of simulation time and computing resources, allowing
you to spend more time testing and adjusting nCloth attribute settings.

■ Shorter simulation and caching durations provide quicker feedback on
how specific attribute adjustments affect nCloth behavior.

■ Lets you develop a better understanding of how your nCloth is behaving
and responding to your nCloth attribute settings.

■ Lets you troubleshoot simulation problems using a low resolution proxy
mesh, and then apply the solutions to high resolution mesh nCloth objects.

The nCloth Advanced Techniques tutorials assume that you have a familiarity
with nCloth in your Maya scenes, and have completed the nCloth tutorials
from the Getting Started with Maya sections of Maya Help. This chapter includes
the following lessons:

■ Lesson 1: Converting the pants to nCloth and setting up the simulation
on page 133

■ Lesson 2: Simulating the pants in a static stance on page 140

■ Lesson 3: Wrapping the high resolution mesh and simulating the pants
during animation on page 145

■ Lesson 4: Simulating the low resolution shirt on page 164

Overview | 129

■ Lesson 5: Simulating medium resolution meshes using custom nCloth
attribute presets on page 175

Preparing for the lessons
To ensure the lessons work as described, do these steps before beginning.

1 Make sure you understand the basic concepts of polygon modeling,
animation, and nCloth.

2 If you have not already done so, copy the nCloth Advanced Techniques
folder from the following location:
http://www.autodesk.com/maya-advancedtechniques. Then, set the
AdvancedTechniques directory as your Maya project.

3 Select the nDynamics menu set. Unless otherwise noted, the instructions
in this chapter assume you’ve already selected the nDynamics menu set.

About the tutorial assets
Throughout this tutorial, you use four variations of the character’s polygon
mesh clothing: a low resolution, medium resolution, high resolution, and
reference character version. To view the tutorial assets, open
Character_Multiple_Resolution.mb in Maya. The following sections describe
each asset.
Use of any data from The Spine in this tutorial is courtesy of the National Film
Board of Canada.


From left to right, the assets are, the medium resolution shirt
and pants, low resolution shirt and pants, high resolution
shirt and pants, and reference character.

Low resolution polygon meshes
The low resolution shirt (Shirt_LowRes) and pants (Pants_LowRes) objects are
the low resolution meshes that you convert to nCloth and simulate. Most
lessons in this tutorial focus on optimizing the simulation of these low
resolution objects. The low resolution shirt and pants are used as the wrap
influence objects for the wrap deformer. The Shirt_LowRes object consists of
180 faces and the Pants_LowRes object consists of 244 faces.

High resolution polygon meshes
The high resolution shirt (Shirt_HighRes) and pants (Pants_HighRes) objects
are the high resolution meshes used as the deform objects for the wrap
deformer. At various stages of this tutorial, you show these meshes in the
scene view to observe how the wrap deformer meshes behave when influenced
by the low resolution nCloth shirt and pants. The Shirt_HighRes object consists
of 6510 faces and the Pants_HighRes object consists of 5712 faces.

Medium resolution polygon meshes
In the final lesson of the tutorial, you apply a custom nCloth attribute preset
to the medium shirt (Shirt_MedRes) and pants (Pants_MedRes) objects. The
nCloth preset is created from the attribute values used for the low resolution
nCloth simulation. Like the low resolution shirt and pants, the medium
resolution objects are also used as the wrap influence objects for the wrap
deformer.

About the tutorial assets | 131

Reference Character
The reference character (Reference_Character) is a high resolution version of
the character and clothing (shirt and pants), which is animated using a cached
simulation. It represents the final high resolution mesh that you want to
emulate when simulating the low resolution meshes with the wrap deformer.
At various stages of the tutorial, you can display the reference character object
in the scene view to compare with your current low resolution nCloth
simulation.
In the tutorial scene files, the reference character appears hidden in the Layer
editor. To display the object, turn on Visible in the Layer Display editor.
For information about the Layer Editor, see Display Layer editor in the Basics
guide.

General modeling considerations
In this tutorial, the terms low, medium, and high resolution are used to
distinguish each mesh. The terminology is not intended to define the
composition of a low, medium, or high resolution mesh, but rather to describe
the mesh’s general characteristics. When modeling geometry for nCloth,
consider the following guidelines:

■ High resolution meshes typically look better in a scene than low or medium
resolution meshes; however, they take considerably more time to simulate.
It is important to establish where in the scene the character wearing the
nCloth is to appear. The character’s placement helps determine the level
of detail for its clothing mesh.
For example, if the nCloth is used for a character in the foreground of your
scene you will want to use a high or medium resolution mesh. If the
character is a stand-in or in the background of the scene, lower resolution
meshes may be sufficient.

■ This tutorial assumes that no original low polygonal meshes were created,
as all models are derived from the high-resolution meshes. The low and
medium resolution meshes have been modeled based on the original high
resolution mesh by superimposing the lower resolution meshes on top of
the high resolution mesh. Modeling the lower resolution mesh on top of
the original high resolution mesh lets you use the high resolution mesh
to quickly identify problem areas in your simulation, such as the crotch
and knees in the nCloth pants.

■ You can use Smooth Polygon Preview mode to see what your low resolution
mesh looks like with a higher polygon count. Be aware that when the mesh


is converted to nCloth, it uses the base mesh and not the preview
subdivided mesh.

■ When modeling your low resolution meshes, try to superimpose them as
closely as possible to the high resolution mesh. Doing this minimizes
offsets between the meshes during the simulation, which can cause
interpenetrations between the character's body and the high resolution
mesh. These interpenetrations may not appear in the static frames (initial
set up) of your simulation, but may appear in frames in which the character
is animated. Be aware that interpenetrations are easier to resolve in the
initial frames of your simulation.

■ It is a best practice to model your mesh using equal sized quads, as this
directly impacts how your nCloth behaves in the simulation. nCloth
created from input meshes with similar sized polygons tend to produce
more accurate collisions than nCloth meshes with polygons of varied sizes.
Typically, meshes created from uniform quads provide better results in
other character creation tasks, such UV layout, skinning, and deformation.
For more information, see Optimizing geometry for nCloth conversion on
page 32.

Lesson 1: Converting the pants to nCloth and setting up
the simulation
Before setting up the simulation environment, you need to convert the low
resolution pants mesh (Pants_LowRes) to an nCloth object. When the nCloth
object is created, an nClothShape and a Nucleus node are created. You can
then set up the simulation environment.
In this lesson, you will:

■ Convert the Pants_LowRes object to an nCloth object.

■ Convert the Body and Shoe objects to passive collision objects.

■ Set Nucleus solver properties, such as Time Attributes and Scale Attributes.

Converting the pants to nCloth

To convert the pants to nCloth

1 Open Character_LowRes_1.mb in Maya.

Lesson 1: Converting the pants to nCloth and setting up the simulation | 133

2 Make sure that only the low resolution pants (Pants_LowRes), Body, and
Shoes are displayed in the scene view.
If other objects are displayed, such as the Shirt_LowRes object, hide them
by selecting each object in the Outliner and selecting Display > Hide >
Hide Selection.


3 Select the Pants_LowRes object and select nMesh > Create nCloth >
.
The Create nCloth Options window appears.


NOTE If the nCloth menu set appears in the Maya interface do not use it.
You must use the nDynamics menu set to create and control nCloth. See
nCloth menu set appears in Maya on page 379 for information about removing
the nCloth menu set from Maya.

4 Select Edit > Reset Settings.

5 Click Create Cloth.
The low resolution pants are converted to an nCloth object, and its Maya
Nucleus solver is created.

6 To ensure that all nCloth objects you create in this tutorial are easy to
identify in your scene, rename the pants nCloth object. To do this, in
the Outliner, double-click nCloth1, type nCloth_Pants, and press Enter.

7 Play back your simulation.
When you playback the simulation, you notice that the nCloth is not
responding to the Nucleus forces, meaning it is not being simulated. In
the next steps, you adjust the scene’s start frame so that your nCloth is
simulated.

Setting up the simulation
Before you begin simulating nCloth, it is important to adjust the Nucleus
solver properties to suit your simulation. The Maya Nucleus solver properties
control internal forces that affect all of the nodes that are members of a
particular solver system.
One of the most important Nucleus solver attributes to be aware of is Space
Scale. When Space Scale is set to its default value of 1, the Nucleus solver
applies Gravity and Wind to objects as if the objects were scaled in meters. In
some production pipelines, the geometry that is used in nCloth simulation
is modeled as life size or in large scale, and so Space Scale needs to be adjusted
to compensate for the difference in scale. For more information, see Space
Scale on page 359.


Setting Nucleus solver attributes
For this tutorial, the Time Slider settings specify the Playback time of the scene
to start at frame 1000 and end at frame 1150. By default, the Nucleus solver
starts simulations at frame 1. To begin simulating the scene, set the Start Frame
of the simulation to frame 1000.

To set the simulation Start Frame

1 In the Attribute Editor, select the nucleus1 tab.

2 In the Time Attributes section, set the Start Frame to 1000.

3 Go to frame 1 and play back your simulation.

When you play back the simulation, you notice that the nCloth pants are
falling under the influence of gravity generated by the Nucleus solver. However,
they fall too slowly to be realistic. This occurs when the scale of the objects
in your scene do not match the scale of the Nucleus solver system. Since the
Nucleus solver simulates as if centimeters were meters, you need to adjust the
Space Scale of the Maya Nucleus solver. In this scene, the character and
clothing meshes have been modeled using centimeters as the Maya scene
units. Therefore, to simulate your nCloth objects (which were modeled lifesize),
you need to set Space Scale to 0.01.

To edit Space Scale

1 In the Nucleus node Attribute Editor, in the Scale Attributes section, set
the Space Scale to 0.01.

When you reduce the Space Scale attribute, Maya evaluates the pants as
if they are a much smaller object. The result is a visibly increased
gravitational affect on the nCloth pants object.

2 Play back your simulation.


When you playback the simulation, you notice that the nCloth pants now
fall from the start position at a realistic speed, however they are falling out of
the scene. To stop the pants from falling out of the scene view, turn on the
Nucleus Ground Plane.

To turn on the Nucleus Ground Plane

1 In the Nucleus node Attribute Editor, in the Ground Plane section, turn
on Use Plane.

2 Play back the simulation.
When you play back the simulation, you notice that the pants are not
colliding with the body and shoe objects. This is because the body and
shoe meshes are not yet part of the nCloth’s Nucleus system. In the next
step, you convert these objects to passive collision objects.


Converting the body and shoes to passive collision objects
The last step in setting up the simulation is to convert the body and shoe
meshes to passive collision objects. As passive collision objects, the body and
shoe meshes will collide with the nCloth pants object.

To convert the body and shoe meshes to collision objects

1 In the scene view, Shift-select the body and shoe meshes, and select
nMesh > Create Passive Collider > .
The Make Collide Options window appears.


2 From the Solver drop-down list, select nucleus1.

3 Click Make Collide.
Two nRigidShape nodes (nRigidShape1 and nRigidShape2) appear in the
Outliner.

4 To ensure the body and shoe objects are easy to identify in your scene,
rename the nRidgidShape nodes in the Outliner. To do this, double-click
nRigidShape1, type nRigid_Body, and press Enter. Do the same for the shoes
object by renaming nRigidShape2 to nRigid_Shoes.

Lesson 2: Simulating the pants in a static stance
Before simulating nCloth on an animated character, it is important to ensure
that your nCloth simulates correctly while the character is at rest or in a static
stance. By allowing the nCloth to settle, it is easier to identify problems in


the simulation and apply the proper adjustments before your simulation
becomes too complex and hard to diagnose.

■ Create a Point to Surface constraint to constrain the pants to the
nRigid_Body object.

■ Edit Stretch Resistance and Compression Resistance.

■ Edit Damp to decrease the amount of time required for the simulated
nCloth to reach its rest state.

Lesson setup
To ensure the lesson works as described, do these steps before beginning:

1 If you have not already done so, copy the nCloth Advanced Tutorial
http://www.autodesk.com/maya-advancedtechniques. Then, set the nCloth

2 Open the scene file named Character_LowRes_2.mb.
This file can be found in the AdvancedTechniques directory that you set
as your Maya project.

Constrain the pants to the body
When you play back the simulation, you notice that the nCloth pants are
now colliding with the body and shoe objects, but they are falling off the
character’s body. To keep the pants from falling, create a Point to Surface
constraint to constrain the nCloth pants object to the nRigid_Body object’s
waist area. For more information about nConstraints, see Constraining Nucleus
objects on page 12.

Lesson 2: Simulating the pants in a static stance | 141

To constrain the pants to the character’s body

1 In the scene view, select the nCloth pants object.

2 Right-click the nCloth and select Vertex from the marking menu that
appears.

3 Select the first two rows of vertices at the top of the nCloth pants.
Dolly and tumble the camera to ensure that only those vertices in the
first two rows are selected.


4 n the scene view, Shift-select the nRigid_Body object and select
nConstraint > Point to Surface.
For this lesson, leave the dynamicConstraintShape attributes at their
default values.

Edit the pants' Dynamic Properties attributes
When you play back the simulation, you notice the following:

■ The nCloth pants simulate using the default Dynamic Properties settings.
This behavior might not resemble the type of material suitable for the
character’s clothing.

Lesson 2: Simulating the pants in a static stance | 143

■ The nCloth pants come to rest state after 40 frames of the simulation
(frame 1040).

When simulating nCloth on a character that is standing still, edits to the
nCloth’s Dynamic Properties attributes may only produce subtle changes to
the cloth’s behavior since gravity and wind are the only forces working on
the nCloth. When the nCloth is simulated on an animated character, changes
in the cloth’s behavior will be more noticeable as these forces begin to generate
velocity, collisions, and self collisions, causing your nCloth to deform in
different areas and in different directions. Therefore, to simplify your
simulation set up, you can give your nCloth some of its unique characteristics
by editing some Dynamic Properties attributes using a static simulation.
Stretch Resistance and Compression Resistance are important attributes in
determining the nature of your cloth. These properties iterate resistance on
the links and cross links on each quad of the nCloth mesh. Stretch Resistance
works to stabilize the pulling or stretching effect which results from the Nucleus
forces, such as gravity and wind. Compression Resistance works to stabilize
the compressing or squeezing effects on the links as a result of Nucleus forces.
For example, to create nCloth that behaves like heavy denim, burlap, or thick
leather, higher Stretch Resistance values are required, while lighter materials,
such as silk or cotton, use lower Stretch Resistance.
The higher you set these two attributes, the more the nCloth resists
deformation, although they tend to increase simulation time. For more
information about Stretch Resistance and Compression Resistance, see Stretch
Resistance on page 326 and Compression Resistance on page 326.
Be aware that adjusting one Dynamic Properties attribute can affect other
attributes. For example, increasing your nCloth's Mass makes it heavier, which
may require an increase in Stretch Resistance to compensate. A useful method
for editing your nCloth's Dynamic Properties attributes is to set one attribute
at a time, and then playback the simulation to observe how the adjustment
affects your nCloth.

To edit Stretch Resistance and Compression Resistance

1 In the Attribute Editor, select the nCloth_PantsShape tab.

2 In the Dynamic Properties section, set Stretch Resistance to 70.

3 Set Compression Resistance to 5.



When you play back the simulation, notice that the nCloth pants still come
to a rest state after 40 frames of simulation. Increasing Damp can bring your
simulated nCloth to rest sooner as it dissipates the energy generated from
your nCloth Stretch Resistance working against the pull of the Nucleus Gravity.
You can also use Stretch Damp to dissipate this energy.

To edit Damp


2 In the Dynamic Properties section, set Damp to 1.


Other attribute settings to consider
Before proceeding with your simulation, consider adjusting the collision
Thickness attribute for each nCloth and passive collision object to optimize
the collision detection between them. When using a workflow that includes
a wrap deformer, you will want to ensure that the nCloth collision volume
(Thickness) allows room for the wrapped mesh without causing
interpenetrations with the nRigid_Body and nRigid_Shoes objects.

Lesson 3: Wrapping the high resolution mesh and
simulating the pants during animation
In this lesson you will:

■ Create a wrap deformer to influence the behavior of the high resolution
mesh (Pants_HighRes).

Lesson 3: Wrapping the high resolution mesh and simulating the pants during animation | 145

■ Import geometry caches to animate the body (nRigid_Body) and shoe
(nRigid_Shoes) meshes.

■ Create an nCache for the simulated nCloth pants object.

■ Identify problem areas in the simulation, such as poorly deformed polygons
and interpenetrations caused by inaccurate collision detection between
the nCloth and passive collision objects or self collisions within the nCloth
itself.

■ Edit Nucleus solver Substeps to improve the overall quality of the
simulation.

■ Set keyframes to increase Substeps at specific problem frames in the
simulation.

■ Resolve interpenetrations.

Lesson setup

http://www.autodesk.com/maya-advancedtechniques. Then, set the nCloth
For this lesson, you need to access Maya geometry cache files in addition
to the lesson scene file.


Wrapping the high resolution mesh
In this lesson, you use a wrap deformer to create an influence object that
causes the high resolution mesh to behave like the simulated low resolution
mesh. For more information about the wrap deformer, see What are deformers?
in the Rigging guide.

To wrap the high resolution mesh

1 In the Outliner, expand Character_HighRes, and show the Pants_HighRes
object by selecting it, and then selecting Display > Show > Show Selection.


2 Shift-select the nCloth pants.

3 From the Animation menu set, select Create Deformers > Wrap.
A wrap node appears in the Attribute Editor, and in the Outliner, you
can see that the Pants_LowResBase object has been added to the
Character_LowRes group.

NOTE You may need to expand Character_LowRes to see the Pant_LowResBase
object in the Outliner.


4 In the Attribute Editor, click the wrap1 tab.

5 In the Wrap Attributes section, turn on Exclusive Bind.

With Exclusive Bind turned on, each surface point (vertex) on the
Pants_HighRes object is constrained or affected by the Pants_LowRes
object vertices. This ensures that the Pants_HighRes object inherits the
properties of the simulated nCloth pants.

6 Hide the Pants_LowRes object, and play back the simulation.



■ The Pants_HighRes object appears to be simulating as if it were an nCloth
object.

■ The speed of the resolution is not reduced or affected by the Pants_HighRes
object.

With the high polygon count of the Pants_HighRes object, one would expect
to see the simulation slow down. However, this is not the case, since the
Nucleus solver is not simulating any data associated with the Pants_HighRes
object. All Nucleus calculations involved in the simulation are still performed
on the low resolution nCloth pants. The behavior of the Pants_HighRes object
is being influenced by a new instance of the simulated nCloth pants, the
Pants_LowResBase object.
The Pants_LowResBase object is the base shape for the deformation of the
Pants_HighRes object, which is the wrap influence object. Like all wrap
deformers, any difference in position, orientation, or shape between the base
shape and the wrap influence object results in a deformation of the surface
being influenced by the wrap deformer.


Simulating the nCloth pants on an animated character
In this section, you import geometry caches for the nRigid body and nRigid
shoe objects and begin simulating the nCloth pants object while the body
and shoes are animated.

Lesson setup
To ensure that this section of the lesson works as described, do these steps
before beginning:

1 Hide the Pants_HighRes object by selecting Pants_HighRes in the Outliner,
and selecting Display > Hide > Hide Selection.

2 Show the Pants_LowRes object by selecting Pants_LowRes in the Outliner,
and selecting Display > Show > Show Selection.

3 If you have not already done so, set the nCloth AdvancedTechniques
directory as your Maya project.

Importing geometry caches
Before simulating the nCloth pants on the animated character, you need to
import the geometry caches which animate the character’s body and shoes.

To import the geometry caches to animate the body and shoe meshes

1 In the scene view, select the nRigid_Body object.

2 From the Animation menu set, select Geometry Cache > Import Cache.

3 In the Import window that appears, select Body_Animation.mc and click
Import.

4 In the scene view, select the nRigid_Shoes object.

5 From the Animation menu set, select Geometry Cache > Import Cache.

6 In the Import window that appears, select Shoes_Animation.mc and click
Import.

You can play back the simulation to see how your nCloth pants behave when
they are simulated with the animated character. However, it is difficult to
closely inspect the simulation without being able to scrub the Time Slider. To
make it easier to identify problems in the simulation, create a cache for the
nCloth pants object. For information about nCaches, see nCaching overview
on page 245.


To cache the nCloth pants simulation

1 In the scene view, select the nCloth pants.

2 From the nDynamics menu set, select nCache > Create New Cache >
.
The Create nCache Options window appears.

3 In the Create nCache Options window, do the following:
■ Set the Cache directory to the folder where you want your caches
saved.

■ Set the Cache Name to nCloth_PantsShape.

■ Beside File distribution, select One File.

■ Click Create.


Identifying problem areas in the simulated nCloth
When you play back the simulation, you notice the following problems with
the nCloth pants object:

■ Between frames 1025 and 1035, the polygon faces around the waist and
crotch area of the pants are exhibiting poor self collisions, resulting in
interpenetrations. Also, the polygon faces in the front and back of the
knees are poorly formed, with some polygons protruding outward making
the cloth look heavy and stiff.


■ At frame 1037, the polygon faces around the ankle area are folding over
each other and are becoming trapped.

■ Between frames 1040 and 1045, self collision issues continue at the waist,
knees, and cuff of the pants.


Dolly and tumble the scene while scrubbing the Time Slider so that you can
closely observe each problem area.

Editing Substeps
A useful first step to fixing simulation issues is to increase the number of
Nucleus solver Substeps and Max Collision Iterations. Substeps specify the
number of times per frame that the Maya Nucleus solver calculates everything
involved in the simulation, from the collision detection between nCloth and
passive collision objects, to the effects of Dynamic Properties attributes and
the dynamic constraints. Adjusting Substeps allows you to control how the
simulation time is broken up into calculation segments. Simulation quality
and collision accuracy generally improve with increasing substeps values. Max
Collision Iterations specify the maximum number of collision iterations per


frame for this Maya Nucleus solver. A high number of Substeps or Max
Collision Iterations may result in slower solving.
Depending on the complexity of your scene, the default values of 3 Substeps
and 4 Max Collision Iterations may work well when simulating a static scene
in which only gravity and wind are affecting the nCloth objects. When the
nCloth begins to move and self collide, higher Substeps values are required
to ensure accurate collision detection, especially on fast-moving objects.
Increasing Max Collision Iterations becomes important when the number of
nCloth collisions increases, and when dynamic constraints are added to your
object. For more information, see Substeps on page 357 and Max Collision
Iterations on page 358.

To edit Substeps and Max Collision Iterations

1 In the Attribute Editor, click the nucleus1 tab.

2 In the Solver Attributes do the following:
■ Set Substeps to 7.

■ Set Max Collision Iterations to 8.

3 Before playing back the simulation to see the results of these new settings,
you need to replace the simulation's previous nCache or create a new
nCache. Otherwise, your changes will not appear in the simulation.
Replacing a cache file overwrites the existing nCache with a new file.
Since you will cache your simulation a number of times in the lesson,
using Replace Cache is the best way to reduce the number of cache files
saved to your hard disk. To replace the previous nCache, select nCache
> Replace Cache.

4 When the Create Cache Warning appears, click Replace Existing.


When you play back the simulation, you notice that starting at frame 1030,
the polygon faces in the area of the knees look more realistic. The cloth
deformation is now spread over more faces, making it look more realistic.


However, not all of the simulation problems were resolved. Higher Substeps
and Max Collision Iterations may be required.

Because increasing Substeps and Max Collision Iterations cause the Nucleus
solver to perform more calculations each simulated step, it slows down the
simulation. To develop a balance between simulation quality and performance,
it is useful to identify the specific areas or frames in the simulation that require
higher Substeps and Max Collision Iterations. You can then animate the
Substeps and Max Collision Iteration values over the duration of the simulation
by setting keyframes.

To set keyframes for Substeps and Max Collision Iterations

1 In the Time Slider, go to frame 1000.

2 In the Attribute Editor, select the nucleus1 tab.

3 In the Solver Attributes section, do the following:
■ Set Substeps to 5.

■ Right-click the Substeps attribute field, and select Set Key from the
pop-up menu.


■ Set Max Collision Iterations to 6

■ Right-click the Max Collision Iterations attribute field, and select Set
Key from the pop-up menu.

4 Continue setting keyframes in your simulation using values from the
following table:
Frame Substeps Max Collision Itera-
tions

1019 5 6

1020 7 8

1022 9 12

1025 15 20

1060 9 12

5 Cache your simulation (nCache > Replace Cache) and play it back.

Fine tuning nCloth attributes
When you play back the cache, you notice that the overall quality of the
simulation has improved. However, there are still problem areas in the waist
and in the crotch. For example, at frame 1030, poor self collisions in the area
of the crotch are resulting in undesirable polygon deformations. To improve
the quality your simulation, you can start to fine tune the Dynamic Properties
attributes.


When fine adjusting nCloth attributes, such as the Dynamic Properties
attributes, it is useful to create a duplicate of the nCloth mesh at different
frames of your simulation. When you play back the simulation, you can then
compare your currently simulated nCloth against the duplicate mesh and
evaluate:

■ If you have correctly identified the attribute that requires adjustment.

■ How the adjustment affects the nCloth behavior.

To help solve the problems in the crotch area, you can increase Bend Resistance
so that the edges of the polygons better retain their shape. For more
information, see Bend Resistance on page 326.

To create a duplicate mesh and edit Bend Resistance


2 In the scene view, select the nCloth pants and select Edit > Duplicate.


3 In the Outliner, select Pants_LowRes1 and in the scene view, move the
mesh object to the right of your character.

4 In scene view, select the original nCloth pants object, and in the Attribute
Editor, click the nCloth_PantsShape tab.

5 In the Dynamics Properties section, set Bend Resistance to 5.

6 Cache the simulation (nCache > Replace Cache) and play it back.

When you play back the simulation, you notice that the cloth is bending more
realistically in the crotch area. In the Time Slider go to frame 1030, and
compare your simulated nCloth to the duplicate mesh. Dolly and tumble the
scene so that you can closely observe the improved area.


Editing Quality Settings and resolving interpenetrations
When you play back the simulation and compare the problem areas with
duplicate meshes, you notice that the problem areas between frame 1030 and
1035 still persist.

Another way to improve simulation quality is to edit the Self Collision Flag
and Quality Settings attributes, which affect the nCloth component collisions
and self collisions. By default, Self Collision Flag is set to VertexFace, which
means that the nCloth object's vertices and faces collide with each other.


Setting Self Collision Flag to Full Surface sets all of the object components
(vertices, edges, and faces) to participate in self collisions.
When simulating a low resolution mesh, increasing the number of potential
self collision areas of the nCloth object is important because of the mesh’s
low vertex, edge, and face count. Setting Self Collision Flag to Full Surface on
high resolution nCloth meshes increases simulation time. This is less of an
issue with low resolution meshes, which makes this alternative much more
appealing for iterative simulation tests. For more information, see Collisions
on page 319.

To edit the Self Collision Flag attribute


2 In the scene view, select the nCloth pants and select Edit > Duplicate.

3 In the Outliner, select Pants_LowRes2 and in the scene view, move the
mesh object to the left of your character.

4 In scene view, select the original nCloth pants object, and in the Attribute
Editor, click the nCloth_PantsShape tab.

5 In the Collisions section, set Self Collision Flag to Full Surface.


After initial fine adjustments to the Self Collision attribute, you edit the Quality
Settings to improve collisions and self collisions in the simulation. This will
resolve any remaining interpenetrations between the nCloth pants object and
the nRigid shoes. You can also increase Thickness or create a Thickness Map
to improve collision detection.

To edit the Quality Settings


2 In the Quality Settings section, do the following:
■ Make sure Max Iterations is set to 10000.

■ Turn on Trapped Check.

■ Turn on Self Trapped Check.

3 Cache the simulation and then play it back.

In the Time Slider, go to frame 1033, and compare your simulated nCloth to
the duplicate mesh. Dolly and tumble the scene so that you can closely observe
the problem area. When you are satisfied with your observations, delete the
Pants_LowRes1 and Pants_LowRes2 duplicate meshes.


Comparing the wrapped nCloth simulation with the simulated high resolution
mesh
To complete the attribute adjustments of the nCloth pants object, you compare
the wrapped nCloth simulation to the reference character object (the original
simulated high resolution mesh).

To compare the low and high resolution nCloth meshes

1 Hide the Pants_LowRes object, selecting it in the Outliner, and then
selecting Display > Hide > Hide Selection.

2 Show the Pants_HighRes object by selecting it in the Outliner, and then
selecting Display > Show > Show Selection.

3 Using the Display Layer editor, turn on Visible to display the
Reference_Character object in the scene view.

4 Hide the Reference_Shirt object, selecting it in the Outliner, and then
selecting Display > Hide > Hide Selection.

5 In the Shading menu, turn off Wireframe on Shaded.

6 Scrub through the Time Slider to compare how the nCloth meshes behave
in the animation. Dolly and tumble the scene so that you can closely
observe the problem area.


NOTE If the Reference_Character is not animated in the scene view when you
play back or scrub the simulation, you may need to reconnect the reference
character’s caches to their respective objects. See Reconnecting objects to
existing caches on page 163.

Reconnecting objects to existing caches
1 In the Outliner, select the object whose cache you want to reconnect.

2 In Attribute Editor, select the object’s cache node.
For example, if you are reconnecting the cache for the character’s body,
select the Body_AnimationCache1 tab.


3 In the Cache File section, make sure that Base Directory is set to the

AdvancedTechniques project data folder by clicking the icon.

4 In the Browse for Folder window, click OK.

Observations
When you play back or scrub the simulation, you notice the following:

■ Overall, the wrapped nCloth pants and reference character objects behave
in the same way.

■ Between frames 1040 and 1047, the pant legs on nCloth pants object are
falling a little faster than the pant legs on the reference character object.

TIP To force the pant legs of the nCloth pants to simulate in the same way as the
pant legs on the reference character object, you can adjust the Friction attribute.

Lesson 4: Simulating the low resolution shirt
In this lesson, you simulate the character’s shirt using the workflow described
in Lesson 3. Although there are some variations in attributes and values, the
principles are the same.
In this lesson you will:

■ Convert the Shirt_LowRes object to nCloth.

■ Create a wrap deformer to influence the behavior of the high resolution
mesh (Shirt_HighRes).

■ Apply the nCloth tshirt preset to the nCloth shirt.

■ Edit the nCloth shirt Collisions and Dynamic Properties attributes.

■ Create an nCache for the nCloth shirt.

■ Identify problem areas in the simulation, such as poorly deformed polygons
or interpenetrations.

■ Edit the nCloth shirt Quality Settings.

■ Resolve interpenetrations.


Lesson Setup

www.autodesk.com/maya-advancedtechniques. Then, set the nCloth


3 Import the geometry caches which animate the character’s body and
shoes. See Importing geometry caches on page 150.

4 To speed up the simulation of the nCloth shirt, either create a new cache
for the nCloth pants, or use the pants nCache from the previous lesson.
To create a new nCache, in the Outliner select nCloth_Pants, and select
nCache > Create New Cache.
To use the nCloth pants nCache from the previous lesson, you may need
to reconnect the cache to the nCloth object. See Reconnecting objects
to existing caches on page 163.

If you use a Maya scene file from a previous lesson, ensure that you:

■ Turn off the visibility for the Reference_Character object in the Display
Layer editor.

■ Hide the Pants_HighRes object.

Converting the shirt to nCloth

To convert the shirt to nCloth

1 Show the Shirt_LowRes object by selecting it in the Outliner, and then

2 Select the low resolution shirt object (Shirt_LowRes) and select nMesh >
Create nCloth > .
The Create nCloth Options window appears.

Lesson 4: Simulating the low resolution shirt | 165

NOTE If the nCloth menu set appears in the Maya interface do not use it.
You must use the nDynamics menu set to create and control nCloth. See
nCloth menu set appears in Maya on page 379 for information about removing
the nCloth menu set from Maya.

3 From the Solver drop down list, select nucleus1.

4 Click Create Cloth.
The low resolution shirt is converted to an nCloth object, and is added
to the existing Maya Nucleus solver.

5 To ensure that the low resolution nCloth shirt is easy to identify in your
scene, rename it. To do this, in the Outliner, double-click nCloth1, type
nCloth_Shirt, then press Enter.

6 Cache the nCloth shirt simulation by selecting nCloth_Shirt in the Outliner
and selecting nCache > Create New Cache.


Applying an nCloth preset to the shirt

■ Around frame 1033, the shirt collar is stretching too much to resemble the
fabric it's trying to emulate.

■ At frame 1033, some of the shirt’s vertices are self trapped within the object,
and interpenetrations between the shirt and pants appear around the
character’s waist. This is caused by inaccurate collisions between the nCloth
shirt and nCloth pants objects.


■ At frame 1037, the shirt is riding too high on the character’s body and the
polygon faces in the front of the shirt are deforming poorly.

■ Between frames 1040 and 1098, some of the shirt’s vertices are trapped
within the object, and there are interpenetrations.


A useful way to solve a number of these simulation problems simultaneously
is to apply an nCloth attribute preset to the nCloth shirt object.

To apply an nCloth preset to the shirt

1 In the scene view, select the nCloth shirt.

2 In the Attribute Editor, select the nCloth_ShirtShape tab.

3 Click-hold the Presets button.

4 Select the tshirt preset, and then Replace (tshirt > Replace).

Some important attribute adjustments made by the t-shirt preset are the
following:

■ Stretch Resistance is adjusted from 20 to 35.

■ Bend Angle Dropoff is adjusted from 0 to 0.4.
Bend Angle Dropoff adds resistance to bending forces by responding to
the angle and strength of the force applied to the nCloth. For example, a
high Bend Angle Dropoff causes an nCloth to resist bending at higher
angles more than at lower angles. When you simulate the shirt, you will
see Bend Angle Dropoff reducing the nCloth's tendency to bend when the
it is beginning to bundle up at frame 1032.

■ Mass is adjusted from 1.0 to 0.6.
The reduction in Mass reflects the light-to medium-weight material of a
cotton t-shirt.

■ Damp is adjusted from 0.0 to 0.8.

Cache the simulation by selecting the nCloth shirt and selecting nCach >
Replace Cache and then play it back to observe the results.

Wrapping the high resolution mesh
In this section of the lesson, you use a wrap deformer to create an influence
object for the high resolution shirt.

To wrap the high resolution mesh

1 Show the Shirt_HighRes object by selecting it in the Outliner, and then

2 In the scene view, Shift-select the nCloth shirt object.


3 From the Animation menu set, select Create Deformers > Wrap> .

4 In the Create Wrap Options window, turn on Exclusive Bind.

5 Click Create.
A wrap node appears in the Attribute Editor, and a Shirt_LowResBase
object appears in the Outliner.

6 Hide the Shirt_HighRes object.

Identifying problem areas in the simulated nCloth

■ At frame 1032, the excessive stretching around the shirt's collar is reduced.
However, this area of the nCloth is still stretching too much.

■ At frame 1033, in the waist area, some previous instances of self trapped
vertices have been resolved, however some interpenetrations still remain.


■ Between frames 1039 and 1049, in the chest area, the shirt deformations
have improved, but the polygon faces still protrude too far outward to
resemble the realistic behavior of a t-shirt.

The nCloth t-shirt preset has improved many of the simulation issues, however
some Dynamic Properties attributes need further adjustment.
When editing the Dynamic Properties attributes, cache your simulation and
then play it back between attribute adjustments to see how your new settings
affect the nCloth shirt's behavior. For comparison purposes, you can also
display the reference character in the scene. To do this, in the Display Layer
Editor, turn on Visible.

To edit the shirt's Dynamic Properties

1 In the Attribute Editor, click the nCloth_ShirtShape tab.

2 In the Dynamic Properties section, make the following settings:
■ Stretch Resistance: 55

■ Compression Resistance: 3

■ Bend Resistance: 0.3


3 Cache your simulation, and play it back.

Editing the Quality Settings and resolving interpenetrations

■ Between frames 1028 and 1034, around the character's waist, there are
areas where the pants and shirt no longer overlap. This exposes the
nRigid_Body object, which might be a desired result.

The offset between the nCloth object occurs because the collision Thickness
is currently set to accommodate the high resolution wrap deformer.
Reducing Thickness may solve this issue, but be aware that
interpenetrations can result from such an adjustment. For the purposes of
this tutorial, Thickness is left at its present value.


■ At frame 1042, there are still too many quads bunching up in the shirt at
the character’s chest, causing interpenetrations. Although this behavior
can be considered realistic, reducing interpenetrations in this area would
make the shirt’s behavior more realistic.

To smooth the mesh and reduce instances of interpenetrations, edit the
Collisions and Quality Settings.

To edit the Collisions and Quality Settings

1 In the Attribute Editor, select the nCloth_ShirtShape tab.

2 In the Collisions section, set the Self Collision Flag to Full Surface.

3 In the Quality Settings section, set the following:
■ Max Iterations: 12000.

■ Max Self Collision Iterations: 8.

■ Turn on Trapped Check.

■ Turn on Self Trapped Check.


4 Cache the simulation (nCache > Replace Cache) and play it back.

Fine tuning the simulation
To further improve the quality of the simulation and reduce the instances of
interpenetration in problem areas, such as the bunching of the shirt in the
chest area (see frame 1040) you can set Push Out and Push Out Radius. Try
setting the attributes values between the following ranges:

■ Push Out: 0.1—20

■ Push Out Radius: 10—50

Experiment with your settings by creating duplicates of the meshes at specific
frames of the simulation. For example, create mesh duplicates at frames 1040
and 1043. Cache the simulation, and then scrub through the Time Slider and
compare your simulation with your duplicate mesh to see the results of your
attribute edits.

Comparing the final low resolution simulation with the simulated reference
character mesh
In this lesson, you compare the final simulation result of the nCloth shirt and
nCloth pants objects with the simulated reference character object (the original
simulated high resolution mesh).

To compare the simulations

1 Hide the Shirt_LowRes and Pants_LowRes objects, selecting by selecting
them in the Outliner, and then selecting Display > Hide > Hide Selection.

2 Show the Shirt_HighRes and the Pants_HighRes objects by selecting them
in the Outliner, and then selecting Display > Show > Show Selection.


3 Using the Display Layer editor, turn on Visible to display the
Reference_Character object.

4 In the Shading menu, turn off Wireframe on Shaded.

5 Scrub through the Timeline, to compare how the nCloth meshes behave
observe the problem areas.

character’s caches to their respective objects. SeeReconnecting objects to


There are subtle differences in the way the high resolution and reference
character objects simulate, but overall the nCloth behaves in similar ways.
Considering the amount of time saved by repeatedly working through the
simulation workflow of caching the simulation and adjusting attributes on
the low resolution mesh, this workflow provides good results. Although the
reference character object can be improved by optimizing the simulation, the
higher resolution mesh slows down caching and simulation time and might
require more attribute adjustments.

Lesson 5: Simulating medium resolution meshes using
custom nCloth attribute presets
Now that you have used the workflow in Lessons 3 and 4 to realistically solve
the low resolution nCloth meshes, you can use them to quickly prepare a
medium resolution version of the nCloth pants and shirt for simulation.

■ Save the nCloth attributes for the Shirt_LowRes and Pants_LowRes objects
as custom nCloth presets.

■ Change the Scaling Relation attribute and observe its impact on the nCloth
simulated meshes.

Lesson 5: Simulating medium resolution meshes using custom nCloth attribute presets | 175

Lesson Setup
www.autodesk.com/maya-advancedtechniques. Then, set the nCloth

2 Open the scene file named Character_MedRes.mb.
The medium resolution shirt and pant meshes have already been
converted to nCloth objects and named nCloth_Shirt_MedResShape and
nCloth_Pants_MedResShape.
A Point to Surface constraint has been applied to the pants to constrain
them to the character's waist. The medium resolution shirt and pants
have already been wrapped (using a wrap deformer) with a new, duplicated
high resolution shirt and pants mesh. The low resolution shirt and pants,
and the nCloth shirt and nCloth pants objects have been cached and are
positioned next to the medium resolution meshes for reference.

3 Import the geometry caches which animate the character’s body and
shoes for the low resolution and medium resolution characters. See
Importing geometry caches on page 150.


Save the nCloth attributes as custom presets
By the end of Lesson 3 and 4, you generated optimized simulations of your
character’s shirt and pants. A number of steps were involved in obtaining the
attribute values that drive the behavior of these nCloth objects, and so you
may want to save these optimized attribute values.
Using Maya attribute presets, you can save the attribute values of the low
resolution nCloth shirt (nCloth_ShirtShape) and nCloth pants
(nCloth_PantsShape) objects as custom nCloth presets. Your new nCloth
presets can then be applied to other versions of the meshes, including the
medium resolution shirt and pants. This workflow allows you to quickly
prepare other simulations with geometry that has similar topology to your
original objects.

To save the nCloth shirt object's attributes as a custom nCloth preset

1 In the scene view, select the low resolution nCloth shirt.


3 Click-hold the Presets button.
The nCloth attribute Presets pop-up menu appears.

4 Select Save nCloth Preset.
The Save Attribute Preset window appears.

5 In the Preset name field type nCloth_ShirtShape for the name of the custom
attribute preset you want to create, and then click Save Attribute Preset.

6 Repeat steps 1 to 5 to save the nCloth pants as a custom nCloth attribute
preset nCloth_PantsShape.

Since you no longer need to simulate the low resolution nCloth shirt and
pants, you can disable them from the Nucleus solver. Doing this increases the
simulation speed of your medium resolution nCloth shirt and pants.


To disable the low resolution nCloth shirt and pants

1 In the scene view, select the low resolution nCloth shirt.


3 Turn off Enable.

4 Repeat steps 1 to 3 for the nCloth pants object.

5 Hide the low resolution character and clothing by selecting
Character_LowRes in the Outliner and then selecting Display > Hide > Hide
Selection.

Applying the custom nCloth attribute preset to the medium resolution shirt
and pants
In this lesson, you apply your custom nCloth attribute presets to the medium
resolution versions of the nCloth shirt and pants (nCloth_Shirt_MedResShape
and nCloth_Pants_MedResShape). After the attribute presets are applied, you
can fine tune the simulation by adjusting the Collisions, Dynamic Properties,
and Quality Setting attributes.

To apply your custom nCloth attribute preset to the medium resolution nCloth

1 In the scene view, select the nCloth medium resolution shirt.

2 In the Attribute Editor, click the nCloth_Shirt_MedResShape tab.

3 Click-hold the Presets button, select the nCloth_ShirtShape preset, and
select Replace.

4 In the scene view, select the medium resolution nCloth pants.

5 In the Attribute Editor, click the nCloth_Pants_MedResShape tab select
the nCloth_PantsShape preset, and select Replace.

6 In the scene view, Shift-select the medium resolution nCloth shirt and
pants, and create a new cache for by selecting nCache > Create New
Cache.
The nCloth shirt and pants simulations are saved in one nCache file.



Comparing the final simulations
In the final sections of this lesson, you compare your final nCloth simulations.
First, you compare the behavior of your medium resolution nCloth shirt and
pant objects with your final low resolution shirt and pants simulations. This
comparison allows you to see differences in the way the high resolution mesh
behaves when it is wrapped on low and medium resolution meshes. In the
last section, you compare all three simulation versions of nCloth (the original
high resolution, medium resolution, and low resolution meshes).

To compare your medium and low resolution simulations

1 Hide the Shirt_MedRes, Pants_MedRes, Shirt_LowRes, and Pants_LowRes objects
by selecting them in the Outliner, and then selecting Display >Hide >
Hide Selection.

2 Show all the objects in the Character_HighRes group by selecting
Pants_HighRes and Shirt_HighRes in the Outliner, and then selecting Display
> Show > Show Selection.
The medium resolution and low resolution nCloth meshes, each wrapped
by the same high resolution shirt and pants, now appear in the scene
view.

3 Play back the scene.


Dolly and tumble the scene while scrubbing the Time Slider so that you can
closely observe any differences in the behavior of each nCloth object.
When you play back the simulations, you notice the following:

■ The low and medium resolution shirts are behaving in the same way.

■ The medium resolution pants are taking more time than the low resolution
pants to settle in the first frames of the simulation.

■ Between frame 1040 and 1045, the medium resolution pants ride further
up on the character legs than the low resolution pants.

■ At frame 1047, the cuffs of the medium resolution pants are further apart
than the cuffs of the low resolution pants.


■ The medium resolution pants appear to have more bounce during specific
areas of the simulation.

If you are not satisfied with the behavior of your medium resolution nCloth
shirt and pants, use the same attribute optimizing workflow described in
Lessons 3 and 4 to adjust the Collisions, Dynamic Properties, and Quality
Settings attributes.
You can remove some of the bounce from the medium resolution nCloth
pants by editing the Scaling Relation attribute. When Scaling Relation is set
to Link, the nCloth’s dynamic attributes, such as Bend Resistance and Stretch
Resistance are calculated relative to the mesh’s scale. When Scaling Relation
is set to Object, the dynamic attribute calculations are automatically scaled
based on the mesh’s resolution. Low resolution meshes tend to stretch more
when Scaling Relation is set to Object rather than Link. High resolution meshes
tend to stretch less when Scaling Relation is set to Object rather than if Scaling
Relation is set Link. For more information, see Scaling Relation on page 328.

To edit Scaling Relation

1 In the scene view, select the medium resolution nCloth pants.

2 In the Attribute Editor, click the nCloth_Pants_MedResShape tab.

3 In the Dynamic Properties section, set Scaling Relation to Object Space.


4 Cache the simulation (nCache > Create New Cache), and play it back.

Compare all three versions of nCloth

To compare all three versions of nCloth

1 Ensure that:
■ The Shirt_MedRes, Pants_MedRes, Shirt_LowRes, and Pants_LowRes objects
are hidden.

■ All objects in the Character_HighRes group are shown in the scene.

2 In the Display Layer editor, turn on Visible for the Reference_Character
object in the scene view.

3 Scrub through the Time Slider to compare how the nCloth meshes behave
observe any subtle differences in the nCloth.

character’s caches to their respective objects. See Reconnecting objects to


When you compare the three versions of nCloth, you have to look closely to
notice the subtle differences in behavior. Considering the amount of time
saved optimizing a low resolution mesh, this workflow provides a viable
alternative to simulating highly detailed character clothing. By wrapping your
highly detailed mesh onto a lower resolution nCloth instead of simulating it,
you can still model details into mesh without worrying about how it simulates.
By saving the low resolution nCloth attributes as a custom preset, you add
value to the time invested in optimizing the nCloth simulation. After applying
the preset to the medium resolution nCloth, only minimal adjustment is
required to obtain a simulation with good results.


Mayanclothadvancedtechniques2010

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