Character Skinning Demo

This demo focuses on Maya's ability to use overlapping deformers to skin up the shoulders and arm of a character.

Part I: Setup

Use the following file: skinChar.mb

Part II: Demo

Load skinChar.mb into Maya.
Go to a perspective view.
Go to shaded mode (5 on the keyboard) and twirl the camera around to show the body we're going to skin up. (fig 1)
Select different parts of the body and mention how this body is made up of 28 different patches.
Turn off your joint pick mask by clicking off the joint icon in the pickmask area.
Click and drag over the entire geometry to select it.
Hit 1 on the keyboard to put it in low resolution mode.

fig. 1

Switch to a front view.
Go to wireframe mode by hitting the 4 key on your keyboard.
When approaching a complicated area to skin like a shoulder, we found that using lattices really helped make the skinning much easier.

Hit F8 on the keyboard to go to component mode.
Click and drag with the left mouse button to select the cv's designated to the right (fig 2).

Note: You can tell where to select on the arm by making sure to stop your selection after the two slanted rows of cvs on the arm. (fig 3).
Note: if you're having trouble selecting the required cv's using a box-pick, you can go Edit-->Quick Select Set-->Torso Set.

fig. 3

fig. 2

Notice how in figure 3 the cv's that are pretty close to being straight vertically aren't selected, but the ones to the right of them which are slanted are selected. That is your cutoff point.

Make sure you are in animation mode.
Go Deformations-->Lattice.

This will create a default lattice for you around the points you had selected.

Go to a perspective view. Spin the camera around to show how the lattice was created around the selected cvs.
Make sure the channel box is visible.
With the lattice selected, and the channel box up, change the divisions.

S Divisions = 14
T Divisions = 7
U Divisions = 2

Note: A really cool way of doing this is to select the proper channel in the channel box, then with the middle mouse button held down, drag in the 3D view to increase or decrease the number of divisions.

Now we're going to perform the bind skin where we bind the lattice and the rest of the cvs which aren't in the lattice to the joint.
Go back to the front view. This makes things much easier for right now.
Make sure you're in wireframe mode.
Click with the right mouse button over the right arm. This will bring up a context sensitive marking menu. Sweep the mouse to the left and choose Control Vertex. This will hilight all the cv's in that arm without making you switch between object and component mode. (fig 4)
Click and drag with the left mouse button over the cv's which aren't inside the lattice box. This will select them. (fig 5)

fig. 4

fig. 5

Now on over the left arm, click with the right mouse button and again choose Control Vertex. (fig 6)
With shift held down, click and drag with the left mouse button over the cvs on the left arm which are not inside the lattice. (fig 7).

fig. 6

fig. 7

Now hold down shift and select the lattice with the left mouse button.
Turn back on the joint pick mask and select the bottom joint in the skeleton (spineHip). (fig 8)
Go Skinning-->Bind Skin to bind the cv's in the arm and the lattice to the skeleton.

fig. 8

Go to shaded mode, and to a perspective view.
Select shoulderL and rotate it up and down. Show how the skinning works by default. Point out how when the arm lifts up, the chest stretches way too far. Also, when the arm goes down, the chest compresses very funny. If you rotate the arm forward and back you can also see many problems with the skinning.
Go Skinning-->Go to Bind Pose. This will return the skeleton back to it's default state.
The first step to making the skinning work best in this case would be to make sure the correct lattice points are attached to the correct joints.
To do this, go Deformations-->Edit Membership Tool.
Go back to a front view, and go to wireframe mode.
Every time you click on a joint with the left mouse button, you'll see the associated lattice points hilight. To add to a set, hold down shift and click and drag with the left mouse button. To remove from a set, hold down ctrl and click and drag with the left mouse button. When you're done, go to the select tool.
Check the images below to see which lattice points go with which joints.

spineHip	spineLow	spineMid
spineTop	scapulaL	shoulderL
scapulaR		shoulderR

Now go to shaded mode (5 on the keyboard).
Select shoulderL and rotate it up and down to show how much better the skinninng is now that we've reordered some of the points. (fig 9)
We can improve the skinning even more, however, simply by changing the weights of some of the points.
In the images below, you'll see a selection of lattice points, and an associated weight with the lattice point. The workflow for modifying the weight is as follows:

Select the lattice point.
If the set editor isn't up, go Windows-->General Editors-->Set Editor to bring it up.
Inside the set editor, go List-->Update Now to view all the clusters that the lattice point is in.
Open the appropriate cluster group (mentioned next to the image) by clicking on the arrow next to the name.
In the bottom of the set editor there is a little field for setting weights. Type the associated weight in there and hit enter. This will set the weight for the lattice points.

fig. 9

	Lattice Points Selected: ffd1Lattice.pt[7][4][0:1] Cluster Set: scapulaLSet1 (scapulaLCluster1) New Weight: .2
	Lattice Points Selected: ffd1Lattice.pt[8][4][0:1] Cluster Set: scapulaLSet1 (scapulaLCluster1) New Weight: .5
	Lattice Points Selected: ffd1Lattice.pt[9][4][0:1] Cluster Set: scapulaLSet1 (scapulaLCluster1) New Weight: .7
	Lattice Points Selected: ffd1Lattice.pt[10][4][0:1] Cluster Set: shoulderLSet1 (shoulderLCluster1) New Weight: .2
	Lattice Points Selected: ffd1Lattice.pt[11][3][0:1] Cluster Set: shoulderLSet1 (shoulderLCluster1) New Weight: .6
	Lattice Points Selected: ffd1Lattice.pt[10][5][0:1] Cluster Set: shoulderLSet1 (shoulderLCluster1) New Weight: .5
	Lattice Points Selected: ffd1Lattice.pt[11][6][0:1] Cluster Set: shoulderLSet1 (shoulderLCluster1) New Weight: .5

Now move the shoulder around and show how good the skinning is. Remember to point out that this surface is made up of multiple patches, and the skinning we've done is holding together really well.

Let's also add some weighting to the torso, so that when the character twists around, the body doesn't look broken.
Select the two joints in the middle of the back (spineLow and spineMid). It's easiest to do this by switching to wireframe mode, selecting both joints, then switching back to shaded mode.
Rotate the joints in y to show how the body twists (fig 10 & fig 11).

fig. 10

fig. 11

Now it's time to add the weighting to the body's torso. Go to component mode, and follow the below images to give the proper weighting to the associated lattice points.

Lattice Points Selected: ffd1Lattice.pt[0:13][1][0:1]

Cluster Set: spineLowSet1 (joint2Cluster1)

New Weight: .5

Lattice Points Selected: ffd1Lattice.pt[0:13][3][0:1]

Cluster Set: spineMidSet1 (joint3Cluster1)

New Weight: .5

Note: In this case you'll be selecting lattice points which belong to the arms. If you don't open those sets in the set editor, it won't matter when you change the weights because the set editor doesn't change weights for sets which aren't opened.

Go back to object mode, select a joint, and to Skinning-->Go To Bind Pose. This will return the body back to it's original position.
Select the two joints again (spineLow and spineMid) and rotate them around in all different axis.. the skinning should look good.
Notice that when you bend the body forward, the belly doesn't stick out at all. We're going to add a cluster to control that sticking out.

Go to component mode and select some of the lattice points on the front of the body. We're going to use these to make the belly stick out when the character leans over.
Create a cluster by going Deformations-->Cluster.
Open up the Attribute Editor for the cluster by either going Window-->Attribute Editor, or by hitting Ctrl-a on the keyboard.
Turn on Relative mode for the cluster. Turning on relative mode allows you to parent the cluster to another bone without getting double-transforms.
Go back to object mode and shift-select the bottom joint (spineHip).
Hit p to parent the cluster to spineHip.

fig. 12

Next, we're going to set up some of the weighting for the lattice points affected by the cluster.
Pull the cluster out a little bit in Z. This will stretch out the lattice points so you can see how they deform (fig 13).
Go to component mode, and select the bottom row of lattice points (fig 14) which are in that cluster and bring up the set editor.
Go List-->Update Now, and open up the cluster1set. Change the values to .5. You should see the lattice points you had selected snap back a bit (fig 15).

fig. 13

fig. 14

fig. 15

Go back to object mode.
Before we go any further, we're going to need to reorder the deformations on the lattice. To demonstrate why this is necessary, with the cluster still pulling the belly out, select the bottom joint (spineHip) and rotate it in y. See how the belly flattens? This is because the cluster's transformations are happening to the lattice after it's already being deformed by the joint. We want the cluster's deformations to happen before the joint.
To do this, keep the joint rotated in Y so it the deformation looks incorrect. Select the lattice and hold down the Right mouse button. A context-sensitive menu will pop up.
Choose Inputs-->Complete List.... This will bring up a window with all the deformations occuring on ffd1Lattice.
With the middle mouse button, click and drag Cluster(cluster1) from the top of the list down to the bottom. This will reorder the deformations. When you release the mouse button, you should see the lattice deforming correctly.
Select the skeleton and go Skinning-->Go to Bind Pose to return the skeleton back to it's bind position.
Now we're going to set up a set driven key relationship between the cluster's translation, rotation, and scale and the spineLow joint's rotation. Select the cluster.
Go Keys-->Set Driven Key-->Set-->Option Box. This will open up the Set Driven Key dialog box with cluster1Handle loaded into the Driven section.
Select spineLow in the 3D view (it may be helpful to go to wireframe mode to do this), and click Load Driver in the Set Driven Key dialog. This will load spineLow into the Driver section.

Select the rotateX attribute for spineLow.
Select the translateY, translateZ, rotateX and scaleY attributes for cluster1Handle. (fig 16).

This sets up a relationship where the rotateX of spineLow is going to be controling the translation, rotation and scale of cluster1Handle.

Select cluster1Handle and make sure it's translateZ value is back to 0.
Make sure spineLow's rotateX value is also set to 0.
Hit Key in the Set Driven Key dialog.
Rotate spineLow forward in X until it's value is around 40.
Select clusterHandle1 and move it forward in Z until about 3.5, and down in Y until it's around -.37. Also rotate it in X until it's around -12.5, and scale it in Y until around 1.58. (fig 17).
Hit Key in the Set Driven Key Window.
Now rotate spineLow back and forth in X. Watch how the belly sticks out whenever the body roates forward.
Go Skinning-->Go To Bind Pose to return the body to it's neutral pose before continuing.

fig. 16

fig. 17

Now let's start to add some more control for the arm. Let's take the example of the elbow.
Select elbowL and rotate it in y. Point out the stretching on the outside of the elbow area. (fig. 18)

fig. 18

Go Skinning-->Go To Bind Pose to reset the arm back to it's bind pose.
Now add a flexor to the elbow by going Skinning-->Create Flexor..
Choose flexor type of lattice. Make sure it's set to At Selected Joints.
Hit Create.
A flexor has now been created around the joint. Rotate the elbow and show how the skinning around the elbow is much smoother.
Also point out that although the elbow doesn't stretch as poorly, it still doesn't look much like an elbow. When a person bends their elbow, it looks a bit boney. We're going to fix that by using a cluster.
Go to Bind Pose to return the joint to it's normal position.
Go to component mode and select 2 cv's that are on the outside of the elbow. (fig 19 & fig 20) (armLGeo.cv[2:3][11]) Note: you can tell which cv's to select by looking at how they're colored. Look at the row of cvs which has colors in it for both the top part of the arm and for the bottom part of the arm. This is where you want to grab cv's from.

fig. 19

fig. 20

Create a cluster for these two points by going Deformations-->Cluster-->Option Box.
Turn on Relative mode.
Hit Create.
Go back to object mode.
Shift-select shoulderL and hit p to parent the cluster under the character's shoulder bone.
Now we have to reorder the deformers, or else we're going to get strange results with our elbow not deforming properly.
Select the geometry (armLGeo) and hold down the right mouse button to get the context-sensitive menu up.
Choose Inputs-->Complete List and drag Cluster(cluster2) from the top down below Ffd1(ffd1) with the middle mouse button.
Now we're going to set up a Set Driven Key relationship whereby the elbow's rotation drives the cluster's translation.
To make it a little easier to see what's going on in the scene, you can hide the two lattices by going Display-->Hide-->Hide Deformers-->Lattices, or by selecting both lattices and hitting ctrl-h.

Select cluster2Handle and go Keys-->Set Driven Key-->Set-->Option box. Cluster2Handle should be showing in Driven.
Select elbowL and hit Load Driver. This will load elbowL into the Driver section of the Set Driven Key Dialog.
Choose elbowL's rotateY attribute t odrive cluster2Handle's translateX and translateZ attributes.
Hit Key in the Set Driven Key dialog.
Now select elbowL and rotate it in Y to about -95.
Select cluster2Handle and start to move it in x and z. You'll notice that because the geometry is in low rez, it's hard to tell what it's going to look like (fig 21).
Select armLGeo and hit 2 on the keyboard. This will increase the resolution so you can see what the elbow will really look like (fig 22).
Move the cluster around until you get the elbow to look exactly how you want (fig 23).
Hit Key in the Set Driven Key dialog.
Rotate the joint back and forth to see how the elbow works.

fig. 21

fig. 22

fig. 23

Now we're going to add a sculpt object to the bicep in order to simulate muscles.

Select all the geometry in the left arm (armLGeo, upArmBackLGeo, upArmTopLGeo, upArmFrontLGeo, armPitLGeo) (fig 24).
Go Deformations-->Sculpt to create a sculpt object.
Select both the sculpt object (sculpt1Sphere) and the locator (sculpt1StretchOrigin) and group them together by hitting Ctrl-g.
Go Modify-->Center Pivot.
Shift-Select shoulderL.
Hit p on the keyboard to parent the grouped sculpt object and sculpt origin (group3) to the shoulder.
Move the sculpt object and sculpt origin to a location like the one in fig 25. This is where the muscle will start from.
Now we're going to set up another Set Driven Key relationship between the muscle and the rotation of the elbow.
With group3 selected, go Keys-->Set Driven Key-->Set-->Option Box.
Select elbowL and hit Load Driver to load elbowL into the driver section.
Choose rotateY for elbowL.
Choose translateX, translateY, translateZ, scaleX, scaleY, and scaleZ for group3.
Hit Key to save a key.
Rotate elbowL to an angle around -95 in Y.
Select group3. Note: an easy way to do this is to click on group3 in the Set Driven Key dialog. It will automatically select it for you.
Translate and scale the sculpt object until it looks like a rediculously large muscle being bulged (fig 26).
Hit Key.

fig. 24

fig. 25

fig. 26

Rotate the elbow back and forth to show how the bulging works.
By using the above techniques of layering many different types of deformers, it's possible to skin up extremely complicated characters in Maya with very complex behaviors.