maya.超真实人物角色教程

Hyper Real | Creature Anatomy II:

Skinning & Musculature

Presented by Erick Miller & Rudy Grossman

Disclaimer: The object of this class is not to advocate any one method, but instead to bring ideas and concepts together as clear-minded solutions to problems based on the combination of pre-existing and publicly defined successful workflows developed using Autodesk Maya 3d software’s architecture and tools. The techniques/workflows and concepts in this class may possibly contain similar methods used in many large digital production facilities; due to the breadth of this topic, these similarities are either purely coincidental in nature or are simply based on commonly available knowledge. There is in no way any proprietary information contained in or even hinted at in this course or documentation. The materials contained here-in are in no way whatsoever advocated by nor created in conjunction with Digital Domain, Sony Pictures Imageworks, Weta Digital, Industrial Light & Magic, or any other digital production facility, for that matter. Any MEL scripts, plug-ins, or project files included are supplied for learning purposes only, and will be used at your own risk. Autodesk is not responsible for the support of any script, plug-in, example file or any executable code supplied for the purposes of this course. Sample files, scripts, or plug-ins supplied with this presentation are only guaranteed to function properly when used as described with the version of Maya that this class was authored for (version 7.0+). All files and materials included with this course, including this document, are the sole property of the respective author, and are absolutely not to be re-distributed or used for any commercial, or non-commercial reasons, or any reasons at all, without prior written contractual permission given by all authors and is subject to licensing terms set forth by Autodesk, Inc.

Hyper Real | Creature Anatomy II

Skinning & Musculature

Introduction

This Master Class will discuss and demonstrate techniques used to create and manipulate the anatomy of a realistic cg creature. This is the second

class in a two part series, Hyper Real Anatomy I & II. The focus of this

second course will be to expand upon, elaborate, and continue to develop,

a thorough understanding of anatomy and how it relates to the features of

the deforming skin of a fantastic cg creature. Whether during the

modeling process, joint/skeletal placement or deformation process, a

strong foundation of anatomy combined with having great reference at

your fingertips is a truly priceless asset to have. This second course will

continue to reinforce this concept by outlining many of the various aspects in which knowledge and understanding of anatomy comes into play when

deforming a hyper realistic cg character or creature, and will explain each

concept and technique in the documentation, as well as through examples with supplied sample files as shown during the class.

Also, be sure to check out Paul Thuriot’s Hyper Real Body Rigging I and II classes and DVDs. The body rigging courses by Paul are excellent, and

many of the data and topics that are covered in this course pick up from

various topics in Paul’s excellent courses.

Topics covered in this course will be:

I. Muscle Rigging Solutions

o Home-brewed lofted muscles

o Using muscles as skin influence objects

II. Advanced Rigging & Skinning Solutions

o Wrist - radius/ulna setup that properly twists

o Clavicle/shoulder rigging & skinning

o Using HyperRealCorrectiveShape.mel & advanced skinning effects

III. Rigging Anatomical Transformation

o Blending techniques for blending the human to beast transition

o Advanced skinning techniques for transforming a human into a beast & using skeletal matrix data to avoid double transformations on the skin

IV. Skin/Flesh Simulation

o Overview of essential principals in cloth, skin, and flesh simulation.

o Simulating skin sliding over bones & muscle during monstrous transformation. o Adding volume preserving fatty flesh simulation onto an animated character.

I. Muscle Rigging Solutions

Creating simple anatomically based musculature for your cg creature does not have to be a complicated or difficult matter, in fact, the basic volumes

of anatomy (as described in the first course) can easily be replicated

within Maya using some nurbs curves and a loft node. This technique is

fairly popular, and works pretty well for creating basic forms of

musculature within a cg creature.

Home-brewed lofted muscles

Let’s start out by creating an object that could be used as a simple muscle influence object. Begin by creating a fresh new scene in Maya, and

performing the following steps.

1. Create a new scene – File->New Scene

2. Create a nurbs circle – Create->NURBS Primitives->Circle

3. Duplicate this circle, by hitting Ctrl+d, and translate it two units up in

the Y axis.

4. Duplicate the circle again, by hitting Ctrl+d one more time, and

translate it two more units up in Y again (so that the 3rd one is 4

units up in Y) as seen in the image below:

5. Next, shift select each of the circles in order, so that all three are

selected at once, selecting the first, second, and third in that order.

6. Next, under the modeling sub-set of menus, perform – Surfaces->Loft

7. You should now see a surface that has been created by performing the

loft command, this surface can be used as a basic muscle surface

8. Next, play with the cross sections. Try scaling the top and bottom

cross sections down to around .1, and then move them around.

9. Experiment and image where you could add muscle using this

technique into your character’s skin as a part of the crucial underlying anatomical structure. In the next section, as well as later in this course you will see a couple practical examples of using a surface such as this to help define the underlying structure. Open the file

simple_muscle.mb to see the Maya file used to create the above

example.

Using muscles as skin influence objects

The simple lofted muscle we created in the previous section can then be

used to deform skin very easily by attaching the geometry into a

skinCluster using the influence object option. This is a fairly common

technique, but, is definitely worth mentioning as it is very powerful and

definitely can help to achieve a realistic result when done with some

careful attention to anatomy.

The next example will be a more simple example of using a muscle as an influence object, but the same technique can easily be expanded upon to create more sophisticated skin deformation effects. Later in this course we will also show you how to perform flesh simulation on lofted muscle

surfaces, but for now, let’s take a look at the skin cluster influence object

example.

1. Start out by opening the file home_brew_muscles.mb. This example

was created quite simply by translating, scaling and parenting the

nurbs circles used to create the lofted muscle surface into position in a

rough approximation to a bicep muscle for the beast.

2. The beast was then skinned to the hierarchy of joints, and the weights

were painted in order to get decent looking deformations

3. Next, the lofted muscle surface (as created in the previous example)

was added as an influence object:

4. Select the smooth skinned mesh, and the influence muscle

5. Under the animation subset of menus, choose – Skin->Edit Smooth

Skin-> Add Influence (options box) – as seen in diagram below:

6. Be sure the Use Geometry option is checked on, and hit the Apply

button.

7. Now the lofted muscle surface influences the deformation of the skin

8. You can paint the weights of this muscle just as you could paint the

weights of the transform influence joints of the skin cluster.

II. Advanced Rigging & Skinning Solutions

Creating realistic deforming skin that is motivated by anatomy and

underlying structure does not always have to include the creation of actual muscles below the skin – in fact, often times it is simpler to take some

shortcuts and use joints to fake the appearance of underlying anatomy.

Of course only actual musculature results in certain realistic skin effects

that using muscle-like objects beneath the skin can give you, but, often

times - even on hero characters - this detail is not always needed. In

these cases, more straight forward – yet still advanced skinning

techniques can be used to create a very realistic and anatomically based

result.

Wrist - radius/ulna setup that properly twists (Wrist twist exercise) One of the hardest aspects of creating the deforming skin on a character

is giving it the ability to properly deform through the full range of realistic

motion that a real human-like character has.

For a moment, take a break from reading this document, and try rotating

your wrist around – just put your arm out in front of your body, and rotate

your palm facing up and twist your hand as far as it will comfortably go,

now, twist in the opposite direction. As you can see, the range of motion

of your wrist is almost a full 180 degrees.

Of course in cg animation, the whole purpose of creating something as a

cg creature is usually to make it do things that are not possible in reality,

so, this makes things like an animation friendly usable range of motion on the radius ulna fairly difficult to achieve without any collapsing or tearing to occur during deformations.

Here we will show you a quite simple way of building a wrist that properly

twists, without the need to build a full radius/ulna rig, you will get the visual result of properly twisting radius ulna skin deformations.

First, lets start by taking a look at the file arm_wrist_skinned.mb – examine how the joint structure was created, essentially we are distributing our wrist setup so that the skin can be weighted between 4 different joints, instead of simply 1 joint, this will allow us to have the skin twist all the way up the forearm more appropriately as the wrist twists.

Creation of this wrist twist skinned setup was fairly simple, here are the steps:

1. First, begin by creating a basic wrist, elbow, shoulder joint structure for

your character (by using, under the Animation subset of menus,

Skeleton->Joint Tool and using the defaults for the tool)

2. Next, duplicate the wrist joint, delete any duplicate children (i.e. any

duplicate hand that may be there now) so that just the new wrist joint is left – do this three times. Each time you duplicate the wrist and delete the new duplicate children so that only the wrist is left, move the new wrist upwards towards the elbow, as seen in the diagram above, so that each new duplicated joint moves closer to the elbow each time.

These new joints will function as extra skinning joints to distribute the wrist twist along.

3. Now, we want to extract the X rotation (this is the twist axis) from the

wrist, so that we can re-distribute it into the new joints that go along the forearm. Since Euler angles (this is the representation of rotation

channels in Maya) can have more than one solution during a rotation, we cannot reliably just directly connect the x channel of the rotation

from the wrist – instead, we will use a simple technique to extract this value – a weighted orient constraint.

4. Duplicate the wrist joint two more times, and delete any left over

children of the new duplicates so that only the wrist joint is left - but this time, do not move the new duplicates at all. Keep them directly

overlapping on top of the current wrist joint.

5. Now that you have duplicated the wrist joint twice, you will have two

new joints under the elbow, for clarity I will refer to the node names

included in the sample file: HRR_handJARt_2 and

HRR_handJARt_3.

6. Next, you will create a weighted orient constraint, but only on the x-

axis. The constraint will be used to extract the x axis rotation from the overall rotations of the wrist – but, instead of a single channel direct orient constraint, we will create a single channel constraint with two weighted inputs in the constraint in order avoid getting an incorrect

value when gimbal lock or Euler flipping occurs. This is the reason we have duplicated the wrist two times instead of just once.

7. Select HRR_handJARt_1 (the original wrist joint), then shift select the

new duplicate wrist joints, HRR_handJARt_3 and HRR_handJARt_2.

8. Next, go to the orient constraint options window, under the animation

menu sub-set, choose Constrain->Orient (options box).

9. Make sure that maintain offset is un-checked, and then check the “X”

checkbox. You should notice that when you check the “X” checkbox, the “Constrain Axes – All” is automatically turned off.

10. Next, with the 3 wrist joints all still selected from step 7, hit the “Apply”

button. You have now created a new orient constraint that is only

applied to the x rotation channel of a duplicate copy of the original wrist joint, and is equally weighted between the original wrist joint, and an additional duplicate wrist joint.

11. Now, HRR_handJARt_2 has the newly created constraint applied.

As you rotate the original wrist joint, you should notice that the

HRR_handJARt_2 joint only gets half of the original joint’s twist

orientation values. This is because it is weighted between the current wrist that moves around, as well as a wrist joint that is kept as a place holder of the original wrist location. This is what we want at this stage, and the reason is that by applying this weighted constraint, we help avoid flipping that could normally occur by simply doing an un-

weighted constraint.

12. Next, in order to get this new twist distributed along the skin of the

wrist, we will want to skin the joints that we created in step 2 of this

section. Each of these wrist joints should have smoothly distributed weights along the length of the wrist, as seen below.

13. Now, we will simply drive the x rotation of these evenly distributed

joints that are laid out along the wrist - as seen below.

14. Each distributed joint should get a smaller twist value as it gets closer

to the elbow, this way the twist will smoothly fall off and will not cause in-correctable collapsing to occur when the wrist is twisted. The

easiest way to distribute the twist quickly is to simply use an

expression.

15. To create the expression, load up the expression editor by going to

“Window->Animation Editors->Expression Editor”.

16. Type the following expression into the expression editor, and then hit

the “Create” button:

twist_1.rotateX = HRR_handJARt_2.rotateX * 2 * .9;

twist_2.rotateX = HRR_handJARt_2.rotateX * 2 * .5;

twist_3.rotateX = HRR_handJARt_2.rotateX * 2 * .2;

17. A quick glance at this expression shows how incredibly simple it is.

Each twist joint is assigned a value that is equal to the constrained

wrist joint’s x channel, multiplied by two since the value is halved

because of the fact that the constraint is weighted between the current value as well as the original value. This causes it to be half of the

actual value – so, we multiply it by two in order to get a value that

represents the full twist amount. A cheat, for sure, but it works ?

18. Next, each of the doubled twist values are multiplied by a float value

between 0 and 1. These multiplier values will give you the ability to control how much of the twist from the wrist is applied across the extra weighted joints. The values used here (.2, .5, .9) may or may not work best for different examples – experiment with different values, for

example, .3, .6 and .9 may work well also. You could also create even more joints to distribute the twist along, and use a finer set of multiplier values – for example if you had 6 wrist twist joints, you could try using

.167, .333, .5, .667, .833, and 1. Just remember that you want a

higher multiplier value as the joints are closer to the wrist, and a lower

value as the joints are closer to the elbow.

19. Have fun & experiment distributing wrist twist using the simple orient

constraint technique!

Clavicle/Scapula/Shoulder rigging & skinning

The shoulder area of a realistic character is a difficult area to master, due to the obviously in-correct application of a single shoulder joint via direct

smooth skinning. Since the shoulder in a realistic creature is made up of many complex connections of bones, tendons, sinew, muscle and floating joints, this area of a creature can be deceptively difficult to skin with the

ability to perform a realistic range of motion. The clavicle, scapula, and

shoulder (deltoid) all connect together to form a unique intersection with

the pectorals, the biceps and triceps, the trapezius, latissimus dorsi, as

well as the ribcage and serratus areas. There really isn’t a more crucial

or critical area in the body when it comes to complexity of anatomical

deformations – so we will focus on this part of the body next.

We can start by opening and examining a completed setup file that has a

skinned shoulder rig: arm_shoulder_skinned.mb

1. Let’s begin by examining the general joint layout used. Since it is

difficult to describe exactly where each joint should be placed, examine

the following image to get a better idea of joint placement prior to

starting this exercise.

2. The first thing to recognize for this tutorial is the series of joints used to

deform the overall mass of the shoulder – this is the long series of

joints that extends into the upper arm of the character. Start by

creating this joint chain. Begin drawing joints (using the joint tool)

starting somewhere close to just below the base of the neck,

somewhat parallel with the opening of the shoulder into the body.

Draw somewhere between 10 to 15 joints, extending the joints into the center of the upper arm, and ending the joint chain just before the

elbow as seen in the sample file and in the image above labeled

“Shoulder Joints (spline ik)”.

3. As you may be able to guess, this new joint chain will be used to skin a

major portion of meat on the shoulder, and a spline ik will be applied in order to connect this joint structure with the main shoulder joint that gets animated. This will be done by skinning the spline ik curve, and then extracting the twist using the weighted orient constraint technique as shown in the wrist twist exercise.

4. So, now lets add some spline ik to this new set of shoulder joints by

choosing “Skeleton->Ik Spline Handle Tool” options box. Reset all the settings to default in the tool settings window, and then un-check “auto simplify curve” – next, create an ik spline by selecting the first and last joint in the shoulder chain.

5. Now, a new nurbs curve has been created, and the shoulder joint

chain is being controlled by this curve through the use of the spline ik

solver. Next, select the HRR_clavJARt_3 as well as the

HRR_armJARt_1 joint, then shift select the newly created nurbs curve,

and skin the curve (Skin->Bind Skin->Smooth Bind) so that the

shoulder and clavicle joints control the spline ik nurbs curve.

6. The only thing missing now for the shoulder joints is parenting the joint

chain into the hierarchy of the spine, as well as putting the twist from

the shoulder into the spline ik shoulder rig.

7. Use the weighted orient constraint technique to extract the twist value

(exactly as shown in the wrist twist exercise). Since the technique for

extracting twist was already covered in detail, please refer to the wrist

twist exercise section in this document for detailed information on how

to extract the twist value.

8. Once you have the twist joint created, you will want to drive the twist

attribute of the spline ik handle, instead of the x rotation of each

individual joint. The spline ik automatically distributes twist along the

length of it’s chain, and this works quite well, especially for the use we

are making of it here in the shoulder to arm region. We are going to

only use 75% of the full twist of the shoulder in order to drive the spline

ik twist. This gives us a more natural looking deformation and creates

less collapsing and a bit extra distribution of twist along the shoulder

and arm regions.

9. Plug the following into a new expression in the expression editor in

order to set up the driving of the twist:

ikHandle2.twist = (HRR_armJARt_2.rotateX * -2) *.75 ;

Note: The node HRR_armJARt_2is the joint with the weighted constraint applied in order to extract the twist value (once again, check out the wrist twist tutorial). Also note that we are multiplying here by negative two instead of positive two simply because the x twist orientation direction is flipped on the joint chain being used for the shoulders in this particular scene.

10. Now that these shoulder joints are added, keep in mind that careful

weight painting will have to take place in order to smoothly spread the

weights out across the skinning on the shoulder so that even and clean

deformations take place across the arm and the shoulder. Check out

the sample scene for a rough example.

11. Next, we will want to add a clavicle and scapula joint in order to

complete our shoulder skinning rig. These setups are a bit more

simple, but none-the-less important aspects of the shoulder that can

not be ignored.

Above, the scapula is on the far left side, and the clavicle is on the far right. Notice the shoulder joint chain we just finished making is running directly down the middle.

12. Using the sample arm_shoulder_skinned.mb file as a guide, create a

clavicle joint and a scapula joint (as seen in the diagram above). For

the clavicle joint you can just create it using the joint tool, but – for the

scapula, you should duplicate the driver joint, HRR_clavJARt_3 so

that it will have the exact same rotation orientation and order as it’s

driver.

13. Now, parent the clavicle joint under the closest spine joint, in our case

this is joint HRR_spineJD_1

14. Next, parent the scapula joint below the joint be driven by the

animation controls for the clavicle/scapula motion – in our case this

will be the joint named HRR_clavJARt_3

15. Now, to rig the motion of the scapula, simply create a “y” channel

weighted constraint and drive the y rotation of the scapula – but, the

trick here is to again use an expression, but this time multiply the

scapula’s y rotation by negative one, so that as the arm moves up, the

scapula’s y rotations counter animates it downward. This can be

create via a simple expression:

HRR_clavJARt_5.rotateY = HRR_clavJARt_7.rotateY * -2 ;

Note that the joint HRR_clavJARt_7is a weighted constraint, as covered in the wrist twist example, thus the value is -2 instead of -1. Take careful note that we are using a negative value here for a different reason then why -2 was used for the shoulder spline ik rigging. The reason we are using negative two in this example is so that the scapula counter

animates down as the arm moves up. In the previous example we

used negative two simply because the orientation of our joints vs. our spline ik twist was going in the opposite direction so we had to flip the value.

16. Next, rigging the clavicle is fairly simple. Create an Ik handle, using

the “Skeleton->Ik Handle Tool (options box)” and be sure to choose “ikRPsolver” so that a proper pole vector can be created to stabilize any flipping by constraining the twist.

17. Draw a single joint ik handle from the start joint of the clavicle (near the

center v of the chest) to the end joint that attaches at the crest point of the shoulder.

18. Now, create a locator, and parent it below the closest spine joint, in our

case it would be the HRR_spineJD_1 joint. Next, move the locator into a position somewhere in the center of the neck, directly above the clavicle (see diagram below).

19. Next, select the locator and shift select the newly created ik handle,

and create a pole vector constraint by choosing “Constrain->Pole

Vector”.

20. Finally, parent the clavicle’s ik handle under the main shoulder joint, in

this scene it is the HRR_armJARt_1 joint. Doing this will cause the clavicle to aim towards the shoulder and appear to be an additional bone that is attached and deform the skin in an anatomically expected manner.

21. At this point, you will now want to smooth skin these new joints, and

carefully paint the weights so that they are smoothly distributed across the geometry and there is no tearing or collapsing. This is a time

consuming part to get perfect, and the time should be spent at the

phase so that the skin looks as good as you can possibly make it by simply skinning it to the bones – this way when you need to make

additional changes later, you will have the best possible starting place to add more advanced deformation effects, such as corrective

blendshapes, sculpt deformers, or even skin simulation techniques.

You always wan to be sure that your base skinning is the best it could

possibly be, so being very attentive to the weight painting at this stage

is a crucial process.

By now you should have a finished Clavicle/Scapula/Shoulder deformation rig. Once again, check out the scene arm_shoulder_skinned.mb to see a completed example – a bit more time could have been taken on the weight painting portion in this scene, but over all it is a pretty good example.

Keep in mind that the technique of creating a spline ik joint chain and skinning to the joint structure below can be used on a large number of different areas in the body, and is not restricted to working well for only the shoulders. This technique would work very well for pectoral muscles as well as for the upper legs to hips attachment and many other areas in the body as well – it’s great for having anatomically based bending, but being able to apply the twisting across the skin of the model, and it’s all using simple techniques and tools already built into Maya’s skinning technology in order to do so. Experiment and have fun!

Using Hyper Real Corrective Shape for Advanced Skinning Effects The hyper real corrective shape script is a simple but powerful technique

for applying corrective blendshapes onto your deforming skin’s mesh, and being able to drive the shapes on using a single joint as the driver. This is

a really easy and fast way to start getting even more anatomically

motivated details to start appearing in the deforming skin, since as an

artist you have the ability to sculpt the poses into the shape that they

should look along the way.

To learn how to use the hyperRealCorrectiveShape script is simple – First, open up the example scene hyperreal_corrective_shape_example.mb

scene and play through the time line. Notice that as the elbow bends, the bicep bulges a bit and the forearm keeps it’s form fairly well. This is

because, in conjunction with some skinning techniques shown in the first

course, this file has a corrective shape that is being driven on as the elbow begins to bend.

It is important to note that this technique is not pose space deformations,

nor is it a substitute for such a system – instead, it is rather a simple

technique for applying actual blendshapes (parallel blend shapes) onto a

smooth skinned model – but, one of the coolest thing about this script is

that it requires no plugins, no custom nodes, and no complicated

calculations or algorithms - it simply uses Maya’s skin cluster, blend

shape, and a fairly straight forward expression with set driven keys to do

it’s magic.

To start creating your own hyper real corrective shape inside your scene, first source the hyperRealCorrectiveShape.mel MEL script by opening the script editor window, doing “File->Source Script” and choosing the hyperRealCorrectiveShape.mel supplied with this course. Next, follow these steps:

1. In the script editor window, or the MEL command line, run the following

command after you have sourced the hyperRealCorrectiveShape.mel script:

hyperRealCorrectiveShapeWin();

2. Running this command will open up a simple window that looks like

this:

3. Open a character rig, and pose a single joint into a position that you

would like to correct the deformations. Here is an example below of an fairly extreme twisted shoulder deformation that we can correct: