apex clothing artist guidelines

APEX Clothing

APEX Clothing Artist Guidelines

February 2011

APEX Clothing Artist Guidelines (APEX 1.0 beta)

© NVIDIA Corporation. All rights reserved. 2

Clothing Artist Guidelines ......................................................................................................................... 3

Overview ................................................................................................................................................... 3

General ...................................................................................................................................................... 3

Folds ...................................................................................................................................................... 3

Uniform 100% Scale .............................................................................................................................. 3

Scene Scale ............................................................................................................................................ 4

Frame Rate ............................................................................................................................................ 4

Underlying Geometry ............................................................................................................................ 4

Mesh Criteria............................................................................................................................................. 5

Uniform Quad Topology........................................................................................................................ 5

Balance .................................................................................................................................................. 6

Mesh density ......................................................................................................................................... 6

Mesh Ornamentation ............................................................................................................................ 8

Multi Layer Cloth Mesh ......................................................................................................................... 8

Simulated Vertices vs. Non-Simulated Vertices .................................................................................. 12

Vertex count ........................................................................................................................................ 13

Collision Criteria ...................................................................................................................................... 14

Backstop .............................................................................................................................................. 14

Bone Collision Shapes ......................................................................................................................... 18

Animations .............................................................................................................................................. 19

Animation Sampling ............................................................................................................................ 20

Common Attributes used for Clothing .................................................................................................... 21

Paint Channel Attributes ......................................................................................................................... 21

Tips, Tricks, and Notes ............................................................................................................................ 23

Tutorials .................................................................................................................................................. 26



Clothing Artist Guidelines

Overview

The APEX Clothing module for DCC tool allows for artists to create a variety of cloth and

clothing types with minimal effort and a fast turnaround. This document is intended as a quick

start guide to get an artist up to speed in a timely manner. After going through the following

tutorials and having a properly prepared model, an artist should be able to make clothing effects

in a production environment in a relatively fast and effective.

General

Simulated clothing and the graphical clothing are 2 different components that together can create

a fluid and realistic cloth effect. To make cloth simulate physically correct the following items

should be considered.

Folds

Traditionally, folds are modeled into a characters clothing. With cloth this becomes unnecessary,

as the simulation will create natural folds as the character animates

Uniform 100% Scale



It should go without saying that anything in 3d should be at 100% uniform scale in the x, y, and z

directions. Scale and rotation don’t always play nice with each other and can sometimes cause

non-uniform scaling or skewing. Always reset your x-forms and set your pivot point where you

want it before skinning a character.

Resetting bone scale after setting up a character hierarchy is also extremely important. Every

bone in the chain should be at exactly 100% uniform scale. If it is not, once the animators get a

hold of it, there will most likely be skewing artifacts and it will take a significant time out of

production to go back and fix it.

If scale is not managed properly before Cloth is applied, the negative and unpredictable artifacts

of skewing may also take place.

There is also no guarantee that the game engine that is used for your particular production will

handle scale very well either. And if it does, it may handle it differently than other

engines/software.

The best defense against skewing and the best way to protect the art pipeline is to avoid scaling!

Scene Scale

It is crucial to work at the scene scale that is appropriate for your production’s end product. For

the tutorials related to this doc, the PhysX DCC tool is set to a centimeters scale in anticipation

of exporting the assets to Unreal 3 Engine. A character in Unreal 3 averages about 100 unreal

units tall. Upon exporting from content packages such as 3dsMax/Maya, a 1:1 conversion takes

place. So, that means that our Crash Test Dummies are about 90-100 cm tall. In the real world

this is half scale, but in Unreal, we can say that they are about 180cm tall or about 6ft.

Multiple scene scales are acceptable; make sure the scene scale is correct and consistent for

your specific project!

Frame Rate

Cloth is meant to be authored at 50Hz. To an artist this means, the DCC tool should be set to

play at 50fps. It is common for animators to work at 30fps however. Just keep in mind that to get

the best feedback and to export correctly set it to 50fps.

Underlying Geometry

It is important to remove any geometry on the underlying character model that the moving cloth

will not reveal during simulation. If it will never be seen, it shouldn’t be there. Artists generally

follow this guideline for many reasons in production, speed, authoring time, render time, etc.

Clothing is no different and should be treated the same way. It also has the added benefit of not

needing to have as precise of collision volumes since you are just implying the underlying

character with the ragdoll. Only leave the geometry that can be seen when the simulating

clothing reveals it. Don’t build what you never see.



Mesh Criteria

Uniform Quad Topology

There are many reasons in the 3d pipeline to maintain uniform quad topology where possible. It

is easier to manipulate and iterate on models due to many of the advanced ring and loops tools

built into modern DCC applications. Quads also UV easier and deform better and more

predictably. Ultimately clothing is solved as triangles and that is important to note for behavioral

reasons that we’ll get into later. The reasoning behind the importance of quad geometry is

because it is, again, easily editable and results in even and consistent triangles at solver time.

It’s important to note that the density of the simulated quads doesn’t need to be the same as the

rest of the mesh. It is acceptable to use triangle to blend to a different density of quads, so long as

they remain as uniform as possible. The more consistent the uniformity, the more predictable the

cloth will be. Keep in mind that by the time simulation rolls around, along with rendering, all

quads are broken down into triangles.



Balance

It is important to strike a good balance between simulated and animated portions of a mesh. If

cloth is tight fitting, it is usually best to just use the animated mesh. If the mesh is a little more

loose fitting, therefore giving it the ability to move more freely, the allowing this portion of the

mesh to simulate is desired. There is a balance that can be struck between these two scenarios as

well. If there is available compute power, then sometimes painting a small max distance on areas

such sleeves and have very good results.

Use cloth in areas of the character that will be bold and obvious to the user. Skirts, trench coats,

and dresses are good examples of utilizing cloth on the lower half of the body. Areas such as the

shoulders aren’t as noticeable since clothing generally fits this area fairly tight.

Mesh density

Mesh density can influence the aesthetic of cloth through bendiness and stretchiness. A lighter

density mesh can be made to behave stiffer than a high density mesh can. Therefore knowing

what material you want as far as the modeling stage is beneficial to the overall production.



While in the above picture the coarse mesh on the left behaves like leather, the mesh on the right

will produce silk like behavior. As fine resolution meshes take more simulation time than coarse

ones, the mesh should be uniformly subdivided only in regions where fine detail is desired.



Mesh Ornamentation

Any ornamentation on the mesh should be simplified. Items such as buttons and zippers may

look nice on a model, but they don’t work to well with cloth. It is difficult to get ornaments to

behave properly on cloth, and in most instances its best to just remove them.

Cloth can handle basic pockets with ease, but when the pocket become a bit more complex; cloth

can have issues, including unwanted penetrations and artifacts. It’s recommended to use simple

pockets as much as possible.

Multi Layer Cloth Mesh

Simulating the graphical mesh one to one as a physical mesh can yield visual artifacts and

undesired behavior. The following image shows a problematic case where the artist designed a

two layered mesh to create the effect of a cloth with a certain thickness. If those meshes were

simulated directly, the two layers would intersect. This can only be avoided by turning on self-

collision handling, which is computationally very expensive.

A better way to handle those cases is not to simulate the additional interior geometry (shown in

white), but passively move it together with the underlying base mesh using mesh-mesh skinning.

For this to work properly, the artist needs to create a connected single-layered base mesh that

captures all of the physical behavior. In the above example, the base mesh is the outer dress

mesh all the way to the bottom (excluding the ring shaped, down facing border and the interior

surface). The artist can simply paint the interior dress topology as ‘Latched’ using the ‘Latch to

Nearest’ channel. These vertices now will ‘Latch’ to the nearest simulated vertices on the

simulated side of the cloth (outer dress), therefore simulating both layers in concert with one

another while maintaining its thickness.



Simple pockets and features can also be achieved by using Latch to Nearest as seen in the right

side of the following image. The feature meshes are then put on top of the base mesh. This

means that there needs to be a non-visible part of the base mesh below the pocket mesh. If this is

not the case, the simulation would even out the sharp edges around the pocket. Now the artist can

simply paint the feature meshes as non-physical using the Latch to Nearest channel and turn on

simulation for the base mesh only.



For this to work properly, the artist needs to create a connected single layered base mesh that

captures all of the physical behavior.

It is also important to note that the non-visible, diagonal triangle edges on the quad topology

need to have the same direction on both the simulated side of the geometry and the latched side

of the geometry, or else visible artifacts will take place.

If edges are not turned the same direction on the interior and exterior, then the interior cloth can

penetrate through the exterior due to crossing edges as seen in the following image.



When working with thick or 2-sided cloth, it’s convenient that the vertices on the inside of the

cloth line up with the vertices on the outside. When simulating the cloth, you’ll be turning the

Latch to Nearest feature ‘on’ on the inner or outer vertices, depending on the mesh. These turned

‘Latched’ vertices will be projected to the nearest triangle then on its simulation. In practice,

lining up the vertices, your mesh will be better organized and edits can be done in a more

efficient and clean manner. This also allows for the edges to line up better and for the inner mesh

to simulate more accurately along with the outer, therefore enabling results that do not penetrate

one another.



The red vertices are part of the inner surface and the yellow vertices are part of the outer surface,

the blue line shows how the inner and outer vertices have been lined up.

Ultimately when a vertex is marked as latch to nearest, it is projected on to the nearest triangle.

So it is not necessary to line up the vertices to for Latch to Nearest to work. However, planning

the interior geometry to line up with the exterior, both the vertices and the edges, allows for a

more fluid workflow.

Simulated Vertices vs. Non-Simulated Vertices

Only the subsection of a model that is designated as cloth counts as simulated vertices. A vertex

is designated as simulated if it has a max distance not equal to zero. In addition to these, any

vertex that directly shares an edge with a non-zero max distance vertex is also simulated (see

picture below). If Latch to Nearest is applied to a given vertex, it overrides any max distance

applied to it, therefore turning its simulation off.

Any vertices that are marked as latch to nearest are not counted as simulating cloth

vertices during the simulation. The number of cloth vertices that are simulating can be a

useful statistic when trying to analyze performance.

Any vertices on all adjacent triangles to a vertex marked Latch to Nearest is also

removed from the simulation mesh.



It is important to note that any border vertices are also counted as simulating vertices,

despite the fact that visually they remain in their skinned positions.

Vertex count

Before diving into production, it’s important to know your target platforms and budgets. Ask

yourself some questions about the production before you get too deep into design and execution.

For example:

What are my target mediums, PC games, Playstation 3, Xbox 360, etc..?

How many characters do we need on screen at any one time? How many will have

simulated clothing?

How stiff or flexible should the cloth be?

What other effects will be in the game and how should the cloth be balanced against

them?

Typical Vertex counts for clothing asset are:

o 250 for consoles

o 1020 vertices for 200 based GPUs

o 3000 for Fermi based GPUs



Collision Criteria

The cloth on a clothed character is often in close proximity to the character’s skin. To handle this

situation properly, the simulator would have to perform collision detection and handling between

two dynamically deforming high resolution triangle meshes in real time. This task is too

expensive computationally even on today’s fastest computers. Therefore, APEX provides three

simplified techniques for collision handling, backstop, capsules, and convex. Backstop is the

most efficient, with capsules slightly more expensive and convex shapes being the most

expensive. A convex collision shape consumes up to 6 times more computing than a capsule

shape. As described above, high resolution cloth meshes produce more interesting cloth behavior

than low resolution ones. Therefore, the collision process is accelerated by simplifying the

underlying representation of the character. This can be achieved with two different methods,

which can be used discreetly or in concert with one another.

Note

Only the parts of the character that will interact with the cloth need be addressed.

Backstop

Use case: Backstop is generally used in areas with small max distance and minimal twisting

motions. Areas such as the arms or upper back are ideal for backstop.

A technique provided by APEX is Backstop, i.e. collision of the simulated mesh against a

simplified version of the animated mesh. Note that the cloth mesh is not tested for collision

against a separate mesh for the skin but against the animated configuration of itself! It is crucial

to understand out this works so that an artist can reliably use backstop in the correct situations.

The following schematic shows how backstop is computed:



This technique is valid and useful when the cloth mesh is a good approximation of the body

underneath it and the simulated vertices are expected to stay close to the animated mesh. It

produces better results on bones that are not rotated drastically during animation. Places such as

the upper back, arms, or even pants are ideal uses of backstop if they fit the character tight

enough and have a short max distance applied to them. Hips and the lower torso are notoriously

bad cases and usually cause artifacts. The lower torso generates a lot of twisting motions during

game play, this can yield unwanted artifacts.

In this example the upper cape is a good case to use Backstop because it prevents the cape from

going in front of the character, acting as if there is a wall which prevents the movement of the

cape from going in front of the character.



Backstop is much faster than collision volumes because all cloth vertices have to be tested

against all collision volumes while in the Backstop case, each vertex has to be tested against one

single sphere centered around its animated position only. This sphere is only valid as a collision

shape as long as the simulated vertex stays close to it. If this is the case, Backstop represents the

body of the character much more accurately than convex shapes. The top of the cape/torso is

ideal for using Backstop because the cloth is not expected to move substantially away from the

animated positions. When a small max distance is chosen and Backstop is turned on in that

region, the simulator will generated small features such as wrinkles as expected in a real world

scenario.

Larger max distance also allows the vertex to move tangentially; therefore the approximation of

the underlying geometry gets worse the further away a vertex is from its normal. When the max



distance is large, Backstop might still work, but legs are a very good example where collision

volumes are the better choice. The strong bending of the hips and knee joints will strongly

change the normal of the vertices, leading to unpleasant results with Backstop.

The left image shows a good combination of collision volumes (all capsules) for the legs and

feet, and Backstop for the torso.

The right image however shows that when Backstop is used offset behind the knees it can fold so

much that neighboring vertices have opposing normals and thus get opposing backstop forces

(notice the blue Backstop rendering at the knee). The artifacts are also clearly visible for the cape

above the left foot. The Backstop collision is far off because the simulated vertex moved a bigger

distance in the tangential plane. Backstop collision approximation becomes worse and leads to

wrong collisions.

Below is the same scenario but with normals turned on for visualization feedback.



Bone Collision Shapes

Use case: Bone Collision shapes should be used in areas where the cloth has a lot of movement,

such as dress, trench coats, skirts, or other loose fitting cloth, especially on the lower torso.

Capsule collision shapes are the preferred method since they are 6 times faster than convex

collision shapes.

Another technique is to create collision shapes for certain bones. In contrast to Backstop

described earlier in this document, collision shapes can handle situations where the distance

between the characters skin and the clothing varies greatly during the simulation. In such cases,

the relative velocity of cloth with respect to skin gets high as well. Also, cloth and skin are not in

constant contact but collide only at certain points in time. In many cases, accurate representation

of the underlying geometry is not essential and can be assumed by use of simplified collision

shapes. Whenever possible, capsule and sphere collision shapes should be used if it approximates

the body close enough. Convex, or custom collision shapes are much more computationally

expensive, but represent the silhouette of a character better if the situation calls for it.



Animations

When properly set up the simulation is providing all the correct clothing behavior. Less time

needs to be spent on the actual animating of Cloth parts, since they are going to be simulated

anyways. But a few things should be kept in mind.

1. If not enough resources are available for all clothing that the game requests, some further

away clothing actors will be turned off. In this case they will fall back to pure animation.

Thus animation should still satisfy some quality standards.

2. If the animation is causing penetrations by itself already, it will become more difficult to

find the correct settings to have a non-penetrating clothing simulation at all times. The

better the quality of the animation the easier to configure the simulation.

3. Animation should avoid doing any extremely fast accelerations. Any hitching or popping

in the animation will cause problems. Anything that causes big accelerations even only

on some of the bones is a potential source of clothing being entangled with itself or

causing jittering.

4. Self intersection of geometry should be avoided if possible, not just for the simulated

mesh itself, also within the whole character. Sitting animations for example can force two



collision volumes to intersect, putting any piece of clothing into a sandwich case. This

can lead to high oscillations and jittering. This can be very difficult, since areas of a

typical bipedal character such as the hips, shoulders, knees and elbows typically have

some degree of self intersection due to the resolutions of geometry used, bone influence

counts, and bone limits themselves. The more this can be reduced the more reliable the

cloth simulation be.

Animation Sampling

In order to get smooth simulations it is important that animations are sampled evenly w.r.t. time

as in this time sequence:

0.0 - 0.1 - 0.2 - 0.3 - 0.4 - 0.5 - ...

An uneven sampling like this produces artifacts:

0.0 - 0.1 - [0.1] - 0.3 - 0.4 - 0.5 - ...

The reason is that the velocities in the first case are constant:

0.1 - 0.1 - 0.1 - 0.1 - 0.1 - ...

While in the latter case, they change instantaneously:

0.1 - 0.1 - [0.0] - [0.2] - 0.1 - ...

Sampling typically happens at run time in the game engine rather than in the content creation

tool. Still, the artist has to make sure that there are no abrupt velocity changes in the trajectories

before sampling. Abrupt velocity changes do not refer to a restriction of typical game moment,

but rather hitches and pops in the animation.



Common Attributes used for Clothing

Density Higher values cause the clothing to be heavier (basically a multiplier of gravity for this clothing

model)

Friction Values between 0.0 and 1.0 control how the clothing slides when rubbing against rigid bodies,

super-slipper no friction vs rough and sticky, respectively.

Bendiness The (inverse) amount of force applied to counter bending. A value of 0.0 is full force (less little

bending), while a value of 1.0 is no force (allows the clothing to bend as much as needed).

Ortho Bending Enable a more complicated and expensive bending algorithm that is more stable for large angles.

Dampening

The amount of force to constrain the movement of vertices. This does not damp absolute velocity,

but rather situations like oscillations at the edges of the clothing. Should not be set to 0; we

recommend values above 0.2.

COM

Dampening

Dampens the simulated mesh by its center of mass as opposed to the vertices. This gives the cloth

a stiff, putty like behavior. It does not work well with clothing over a value around .7 and higher.

Typically, it is not used for clothing, but rather other cloth effects. This also comes with added

expense.

Stretchiness

The amount the vertices are allowed to separate beyond the skinned distances. A value of 0.0

indicates stiff, a value of 1.0 indicates fully stretchy. For maximum stiffness, also enable Anti-

Strecth (see below).

Anti-Stretch

Check this option to use additional code to enforce stricter limitations on stretching. A value of

1.1 allows vertices to stretch 10% of the original distances, while a value of 2.0 allows them to

stretch twice the original distance. A value of 1.02 to 1.05 for many common non-stretchy

clothing materials is recommended.

Thickness The distance that the clothing will stay away from rigid bodies in the scene. If you are seeing rigid

bodies penetrate clothing, increase this value.

Self Collision

Cause the clothing to collide against itself, attempting to stay apart by the amount specified by the

Self Collision Thickness parameter.

Use the Self Collision and Self Collision Thickness (below) to cause clothing to collide against

itself in addition to rigid bodies. However self collision is quite expensive, use only if really

needed.

Self Collision

Thickness

Thickness to be used by of the self collision. Self Collision is represented by a sphere at each

simulated vertex.

Simplification Simplifies the clothing mesh up to the specified edge length. This can be used to help make stiffer

cloth. This is achieved since there are less simulated vertices after simplifying the physical mesh.

Solver Iterations NxCloth solver iterations. Higher values can reduce stretching, but at a cost to performance.

Typical iterations are set at 5.

Paint Channel Attributes

Max

Distance

Represents the distance that each vertex may move, represented as a sphere around the skinned location.

A value of 0 prevents the vertex from moving away from the skinned location, disabling simulation for

that vertex.

Backstop Represents the distance that vertex may move backwards along the skinned normal. A value of 0



prevents the vertex from moving backwards at all from the skinned location, basically chopping the

‘sphere’ of Max Distance into a hemisphere; a value of 3 would allow the vertex to move backwards

3units from the skinned location; a value of -1 forces the vertex to stay at least 1 unit forward from the

skinned location.

Any value greater than or equal to the Max Distance value for that vertex will cause the Backstop to

have no effect for the vertex.

Latch to

Nearest

Controls whether the vertex is simulated or ‘latched’. There are only two valid values for this channel:

A value of 0 (the default) indicates that the vertex should be simulated if a max distance great

than 0 is applied to that vertex.

A value of 1 indicates that the vertex should latch onto the nearest simulated vertex and move

along with it.

For more detailed information on all the APEX Clothing attributes, please see the corresponding

documentation in the Max or Maya documentation.



Tips, Tricks, and Notes

Painting Max Distance

Paint variations in Max Distance to influence folding



Com Dampening enabled at high values, such as .9 -1.0 may be have unexpectedly for

clothing.

Com Dampening is only appropriate for clothing at damping values up to around 0.75 or

so depending on the asset. Typical use case is not using it on cloth.

Friction at high values ( > 0.6 ) can cause some unexpected results when Stretchiness

and/or Bendiness have higher values.

Stretchiness and Bendiness are related attributes. Higher Stretchiness values (approaching

1.0) cause bendiness to be less noticeable. This is due to how the internal springs work on

each attribute, as seen in the image below.

Turning on Ortho Bending overrides Bendiness and removes the spring relationship with

stretchiness. Ortho Bending measures the angle for each edge and computes a force that



is proportional to the angle error. It is more accurate; however it is slightly more

expensive.

Stretchiness values approaching 1 with high density topology are more likely to fall

around a collision shape. This is due to the vertex collision against the collision shape and

the vertices themselves being allowed to stretch. It is recommended that higher density

meshes use lower stretchiness values to keep the mesh tighter.

Backstop (also referred to as a-collision) is the cheapest method of controlling cloth

collision. However it is better suited for certain situations over others. This includes semi

tight sleeves, upper torso, and upper back regions.

If an artist needs to go back and adjust the skinning of a character, remove the clothing

modifier and ragdoll before making adjustments. A clothing template (.ctw) and a ragdoll

file (.rag) can be saved out and easily re-added for convenience.

If artist is using a Direct X shader to view the material in the viewport, it must be

disabled in order to see the color feedback for cloth painting.



Tutorials

The following tutorials will guide a new user through developing a clothing asset for use in

production. It is recommended to follow them in order. The tutorials will begin with simple

pieces of cloth that could be used as tapestries or other simple dressing, followed by low

resolution simulations that may be appropriate for console development, and finish up with

medium and high resolution meshes that can be used for PC gaming.

Tutorial 1 – Waving Flag

o Use case: Flags, tapestries, curtains, etc.

o 3dsMax

o Asset available in medium resolution.

Tutorial 2 – Cape Low Resolution

o Use case: Super heroes and/or console games.

o 3dsMax

o Asset available in low and medium resolutions

Tutorial 3 – Trench Coat Medium Resolution

o Use case: Primary or Secondary characters for PC games

o 3dsMax

o Maya

o Asset available in low, medium, and hi resolutions

Tutorial 4 – Pants Hi Resolution

o Use case: Primary or Secondary characters for PC games.

o 3dsMax

o Asset available in low, medium, and hi resolutions.



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