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Game Character Creation Series: Kila Chapter 9 - Hair Rigging

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This post is part of a series called Game Character Creation Series: Kila.
Game Character Creation Series: Kila Chapter 8 - Rigging with Blend Shapes
Game Character Creation Series: Kila Chapter 10 - Level of Detail Variations
Final product image
What You'll Be Creating

At this stage in the series, after eight information packed tutorials, we have pretty much covered all the bases. You have worked through both high and low polygon modeling, UV mapping and also texturing. This was then followed closely with skeleton building, rigging and even two approaches to facial rigging, leaving you with a character ready to be animated.

From here we have only two more areas to investigate, and the first being a pretty obvious one. As your moving and posing Kila don't you get the feeling something is missing, perhaps we have overlooked a key area?

Well, look closer at her head and you will soon realize that her hair, no matter how much you move her, remains static. Yes her body moves, her face emotes, but the hair is still a solid, inanimate block of polygons.

In this, the ninth part of the Game Character Creation Series we will investigate a number of ways to rectify this and bring Kila's hair to life.

Your Options

These days, real time hair and cloth animation can be approached a wide variety of ways, with most being dictated by the target platform and the game engine you’re using.

At the lower end of the scale most hair or cloth remains static, or is simplified so it needs the minimum amount of hand animation. This is to help reduce the strain on the processors as it tries to include the extra calculations needed for the additional joints and deformation data.

Jump to the opposite end of the scale and most hair and cloth simulations can be done in code, being calculated in real time as the character interacts with the game world.

Take the recent Tomb Raider reboot, which utilized AMD's TressFX technology to create and simulate each individual strand of hair on Lara's head. Granted, it wasn't a perfect representation of hair, but it wasn't far off, yet it demonstrated how close we are to replicating hair in the virtual world.

In this tutorial we will investigate two techniques to help drive the movement of Kila's polygon strip based hair.

Firstly we will add joints which, with the help of a simple rig, will allow you to pose and animate the hair. Once we have those in place we can then look at taking things a step further and introduce dynamics into the rig, allowing Maya to do some of the work for you.

1. Add More Joints

Unless you’re choosing to use blend shapes or a real time dynamics system like TressFX, you are going to need something to help drive the hair geometry. In this instance you will be calling on your old friend the joint, to help you out again here.

Note: In this tutorial you will be using the joint based facial rig version of Kila created in Part 7, but feel free to follow along with the blend shape version if you prefer.

  • Begin by going to Modify > Evaluate Nodes > Ignore All. This will disable the IK, and other influences on the Kila rig.
  • Next select the Kila model and go to Skin > Go To Bind Pose, resetting her to her default pose.

Note: If this doesn't work, or you get an error, simply Right Click on the root joint of Kila's skeleton and go to Assume Preferred Angle as an alternative way to reset her.

What you need is a single joint chain which you can then duplicate and reposition around her head at key points. There is no need to add a joint chain for each and every strand of her hair, this wouldn't be economical, plus as Kila's hair is built from simple polygons strips you only want subtle movement.

Note: In hindsight, if you want more dynamic movement from your future character's hair, you could add in more geometry from the start to take this into account.

So let’s start building the first chain.

  • Go to Skeleton > Joint Tool.

  • Now create Five new joints following the hair down the left side of Kila's head.

  • Before you move on, rename these joints appropriately as shown below, to maintain a tidy scene.
  • With the first strip created you can now duplicate this to populate the left side of Kila's head, and also add a one at the rear, making sure you adjust them to fit the flow of the geometry.
  • Finally, rename the joints in each chain to ensure you, or someone else, can quickly find each in the scene.

2. Edit Orientations & Mirror

Now the joints for the left side of Kila's hair have been created you can think about generating them for the right. However, before you do it’s a good idea to adjust each chain's rotational pivot so, when animated, they naturally fan away from the head, rather than move in one direction and end up passing through it.

  • Select each new joint you created for the hair.

  • Now go to Display > Transform Display > Local Rotational Axis.

What you are seeing now are the orientations of each joint. By default they will simply be trying to mimic the world orientations, so the Z axis will be pointing forwards in most cases.

Note: Don't worry if your orientations differ from the example shown.

If you rotate the joints around the X axis now you will see that they will all rotate forwards and pass through Kila's head, which isn't an ideal setup for this rig.

To rectify this, first use the Skeleton > Orient Joint tool to quickly realign each axis, just as you did in Part 5 of this series. Ensure that initially, each joint is oriented in the same way.

  • Next click the Select By Component button on the top menu.
  • Now move over to the right of that button and Right Click the Select Miscellaneous Components button. This is the button with the question mark icon.
  • While clicked make sure Local Rotation Axes is enabled.

With this selected you can now go in and physically select, and then rotate the rotational axis for each joint, making sure the X axis is pointing perpendicular to the curve of the head.

Now when you rotate the joints around the X axis they should all move away from the head, almost as if Kila is falling.

With the rotational axes fixed it’s now a simple case of creating the joints for the right side of Kila's hair.

  • First Parent each new joint chain to the Head joint.
  • Now go to Skeleton > Mirror Joint and open the options.
  • You should be familiar with this tool by now, so set Mirror Across to YZ and Mirror Function to Behavior.
  • Finally, under Replacement Names for Duplicated Joints, set Search For to L_ and Replace With to R_.
  • Click Apply and the right side of the head will now be generated for you.
  • As a final step edit these joint chains to match the flow of the hair. You may need to delete some joints as her hair on the right is tucked behind her ear.

3. Binding

With the joints added you now need to attach these to the hair geometry, and adjust how the hair will deform by editing the weighting information.

Kila already has a skinCluster attached, which is the node that stores all the weighting information, so all you need to do is tell the new joints to jump into that same skinCluster.

  • Select the new hair joints.

  • Add the Kila model to the selection.

  • Now go to Skin > Edit Smooth Skin > Add Influence using the default options.

The new joints will now influence the Kila model, but the problem is, particularly if you are using the joint based facial rig, all the weighting information you edited around the face has been destroyed.

  • If you move the hair joints now, parts of her face and body will also move with them.

You could start again and rework all the weights in her upper body, and then move on to work on her hair. Fortunately there is a simple solution which will restore the weighting on Kila, and also give you back that precious information on her head and face.

  • First Save the current scene, giving it a new temporary name.

  • Now go to File > Create Reference.

  • Point the browser to the file you began this project with, which should be Kila_Hair00_Start.ma, and reference it into the scene.

This will temporarily bring the original file into the same scene and most importantly it will also give you a version of Kila with the weighting information intact.

  • Now select the referenced Kila model and go to Skin > Go To Bind Pose, making sure she is also in default pose.

Note: Again, if this doesn't work, or you get an error, simply Right Click on the root joint of Kila's skeleton and go to Assume Preferred Angle, to reset her.

Note: Remember to also temporarily disable your nodes too – Modify > Evaluate Nodes > Ignore All.

  • With the referenced Kila model still selected, now add the main Kila model to the selection. In effect you have selected the Source model with the correct information and then the Destination model where it will be transferred to.

  • Now go to Skin > Edit Smooth Skin > Copy Skin Weights.

What you have done is simply copied the weighting information from one version of Kila to another, restoring the weights in her face in the process.

Quickly test this by rotating the hair joints, and also animating her facial joints, (if you have them). If everything is working you can then move on to delete the referenced file.

  • To do this go to File > Reference Editor.
  • Right Click on the item called Kila_Hair00_StartRN Kila_Hair00_Start.ma and select Reference > Remove Reference.

Your now back to stage one, except your new hair joints are also contained in the main skinCluster and influencing the model, plus you don't have to waste time reworking the weights on her face.

4. Painting Hair Weights

If you're now rotating the hair joints and nothing is happening, don't worry. When you copied the weighting information across previously, it was from a version of Kila without the hair joints, so in effect, there was no information to be copied.

The hair is still connected, but currently not being told to influence any vertices. In a way this is good as it means you can safely work on the hair without the need to touch anything else.

All you need to do now is define the weighting information for the hair joints, a process which we covered earlier in Part 5 and also Part 7, but we will briefly recap here.

  • First it’s important to pose the hair joints, so as you paint you will see the geometry move showing how it’s being influenced.
  • Now select the Kila model and go to Skin > Edit Smooth Skin > Paint Skin Weights Tool and open the options.

If you recall, this is the stage where the model will go black, showing in white or grey the influence of the currently selected joints.

  • Right Click on the first hair root joint and go to Select Influence in the marking menu.
  • Back in the Paint Skin Weights window, now select Replace as your Paint Operation.
  • Next to Profile select a softer brush.
  • Set Value to 1 and Opacity to around 1 – This time around you want some more definite influences painting onto the hair.

Note: Opacity will paint a percentage of Value and gradually build up the weighting for you until it reaches 1, which means the vertices are effected fully by the specific joint.

  • With those set you now need to paint over the vertices you want to be influenced by the L_Hair_01_Root joint.

Note: If you paint in too much influence, simply set Value to 0 and gently remove the joints effect.

  • Continue this for each joint in the current hierarchy, adding in more influence until the full length of the hair strip is effected by the joint chain.
  • As a final step, before you move on to the next joint chain, set Paint Operation to Smooth, with a small Opacity of around 0.25.
  • Clicking Flood will wash over the whole model, smoothing the weights for you, which can help to make the hair appear less rigid and more organic as it moves.

Tip: When using the Smooth operation it’s always a good idea to follow it up with the Skin > Edit Smooth Skin > Prune Small Weights option, just to tidy up any stray values.

  • You can now repeat these steps for each of the left hand side joint chains for the hair, and also the one at the rear.

Now that you have her left side working well, you can focus on the right, but luckily you can take the work you have already done and copy this across to her right side.

  • To do this simply select the Kila model and go to Skin > Edit Smooth Skin > Mirror Skin Weights and open the options.

  • Set Mirror Across to YZ, and also make sure Normalize is checked to clamp the weighting between the values of 0 and 1.

  • Now click Apply and you are done.

This will mirror the weighting information for her entire body, so make sure everything is working as it should be before you proceed. Also check the weighting on any areas which are not symmetrical, like her fringe, as these may need some final adjustments.

5. A Simple Rig

The joints are now driving the hair allowing you to pose and even animate it, and from here you could call her complete and move on, letting the animator go in and select each joint in turn.

This is an “OK” approach, but not very intuitive, so instead lets now look at applying a basic rig to add a little more control into the hair while also making it easier to animate.

How you approach your rig is entirely up to you. You could choose to simply connect attributes to a main controller, unifying them so they are in one place, or you could use Set Driven Keys, as you did with the facial rig in Part 7, to lock in key hair poses.

What you will do in this section is use a series of IK handles. This will simplify the rig meaning that the animator is manipulating a few key controls, rather than juggling five times as many joint attributes.

  • To begin go to Skeleton > IK Handle Tool and open the options.

  • Make sure Current Solver is set to Single-Chain Solver. For the hair you don't require the extra rotation plane, so this solver will work well here.

  • Now click the first, root joint of the hair chain followed by the tip.

  • It’s quite a simple setup, but if you now move the IK handle around you will see that the joint chain will follow.

Adding the IK Handle will allow you to quickly pose the entire joint chain just by manipulating a single control.

However, with any rig you ideally want visual controllers for the animator to work with, rather than just the basic IK handles, so let’s add one next.

  • Create a new curve based icon, or duplicate one of the existing ones in the scene.
  • Move this new icon to the tip joint of the hair joint chain, the same place as the IK Handle, and go to Modify > Freeze Transforms to zero out any attributes on it.
  • All you need to do now is Parent the IK Handle to the new controller, and then hide the IK Handle.
  • With one hair joint chain rigged you can now repeat this process, adding a simple IK Handle and control icon to the others.

These icons are a good and simple approach to animating Kila's hair but at present they don't move with her head, plus with a little bit of extra work you could make them even better.

  • Select all the new icons and Group them twice.

  • Call the new groups HairGroup and HairControllers, with HairControllers being at the top of the hierarchy.

  • Now move the HairControllers group so it’s under the Kila_CS:Controls group.

In order for these new groups to move correctly the pivot point for each has to match that of the head joint.

  • Select each new group and press Insert.

  • Holding V, snap each pivot to the Head joint's position.

  • Now select the Head joint, and then add the HairControllers group to the selection.

  • Finally go to Constrain > Parent to lock the groups translations and rotations to the head joint.

  • You will find now that as you move Kila, all the hair controls will happily follow.

At this stage you are probably wondering about that second group, HairGroup aren't you? Well this extra node is going to give you even more control.

Note: As we have covered adding and connecting attributes extensively before we will just briefly cover the process here. If you are unsure then please refer back to Part 6.

  • Create a new curve based icon. For this example you could simply use an H symbol, as it will help to represent the hair.

Now add a series of new, float based attributes onto this icon using the Modify > Add Attributes tool. The attributes are listed below.

  • Control Visibility
  • Swing FB
  • Swing LR
  • Twist
  • Tilt FB
  • Tilt LR
  • Bounce

All you are going to do now is use the Connection Editor to directly connect these attributes with another on the HairGroup group.

Connect the attributes as follows –

  • Control Visibility > HairGroup.visibility
  • Swing FB > HairGroup.translateZ
  • Swing LR > HairGroup.translateX
  • Twist > HairGroup.rotateY
  • Tilt FB > HairGroup.rotateX
  • Tilt LR > HairGroup.rotateZ
  • Bounce > HairGroup.translateY

With these extra controls added you can now animate her hair globally without having to physically go in and select the HairGroup node, while also having the option to pose each controller individually if you desire.

Note: As always it’s a good idea with any rig to go in and Lock and Hide any attributes which aren't to be used.

That’s the first approach complete and what you have is a quick way to add movement to Kila's hair as she moves around the game world.

Again, this is a basic rig, but should be sufficient to get some movement into her once static hair.

6. Applying nDynamics

We all know Maya is a powerful application capable of many things, including simulations using its complex nDynamics systems. So at this stage, after you have spent days animating Kila, wouldn't it be nice to let Maya do some of the work?

In this instance her hair is a perfect area to bring Maya on board. As Kila moves around her hair should follow and react accordingly, movement which could easily be achieved using nDynamics.

This approach means you don't need to worry about animating her hair and can concentrate on polishing the animation elsewhere. Another plus side to this technique is it is still essentially joint based, so the animation, once simulated, can be happily baked and exported to use in your game engine.

So where do you begin? First you will need a curve to base the dynamics on.

  • First go back to a previous version of Kila where the joints for the hair were created and weighted, but not rigged. If you don't have one, then simply load the file Kila_Hair01_Joints.ma.
  • Isolate one hair joint chain for now. You will focus on applying dynamics to one chain, and then the same steps can be repeated for the others.
  • Now go to Create > EP Curve Tool.

What you need is a curve which is made up of the same amount of control points as you have joints so it can match its flexibility.

  • Holding down V now click on each joint in turn, starting at the root and ending at the tip. Holding V will enable Point Snapping, so each point will lock to the center of each joint.
  • Press Return to complete the curve.

Your new curve will now be created and follow the joint chain perfectly, although it’s still just a static curve, and isn't moving in any way. The next stage is to tell Maya that this curve needs to be dynamic.

  • Select the new curve.

  • Go to nHair > Make Selected Curve Dynamic.

Your scene will now be populated by a hand full of new dynamics based nodes, like a hairSystem and Nucleus as well as a few new groups. Don't let these worry you for now as you will only be using a hand full of controls, so most of this can be ignored.

Even with these systems in place you probably won't see much happening in the viewport, not until you press play in the Time Slider. Even then you may catch a slight wobble on the curve, but then it’s over all too quickly.

  • Adjust your Time Slider so the end time is something huge, like 100,000 frames. This will give Maya more time to evaluate the simulation before it returns to frame 0.
  • This time select the follicle1 node, which should be beneath the new hairSystemFollicles group.
  • Now, rather than pressing play in the Time Slider, go to nSolver > Interactive Playback. What this will do is play back the animation, while allowing you to interact with the scene.

Tip: You can also press the green arrow button found in the nHair shelf.

  • Now you can move the follicle1 node around and watch as the curve, (illustrated in red) wobbles and moves as you would expect.

Note: If your animation is still not playing back as it should, then you may also need to adjust your playback preferences, ensuring they are set to Play Every Frame.

Your curve is now dynamic, and as you have seen, is reacting to the movement of the follicle1 node.

7. Driving the Joints

The curve is now setup and reacting to the dynamics systems inside Maya, meaning if the follicle1 node is animated, Maya will take care of the curves movements for you. What you ideally need now is for the curve to drive the joints, and to do this you need to create an IK Spline Handle.

  • Go to Skeleton > IK Spline Handle Tool and open the options.

You already have a curve in the scene which you can attach the new spline IK, so you don't need a new one automatically created.

  • Deselect Auto Create Curve.

  • Now select the first joint, and then add the last joint in the chain to the selection.

  • Finally, holding Control, select the dynamic curve. This will be the curve found under the hairSystem1OutputCurves group called curve2.

With the IK Spline Handle created the curve will now also drive the joint chain.

  • You can see this in action if you select the follicle1 node, activate Interactive Playback and move it around in the viewport.

What you will also see is that you have another problem, and that is the dynamics are currently a separate entity to the rest of the Kila rig.

What this means is as you move the follicle1 node the joint chain moves, but also comes away from the head joint. You will probably also find that if you move Kila, the dynamic joints remain static, stretching the geometry and breaking the rig.

  • Select the Head joint.

  • Next add the follicle1 joint to the selection.

  • Now go to Constrain > Parent.

This will make the follicle1 node move with Kila's head, so if she animates the dynamics will now move with her.

8. Adjusting the Dynamics System

You now have one joint chain driven by dynamics, but before you repeat the process and apply dynamics to the remaining hair joints, let’s look at adjusting some basic settings to improve how this initial strand moves.

The first and most obvious issue is that when the hair moves it appears to be locked at the tip. Ideally you want the tip to flow freely.

  • Select the follicle1 node and open the Attribute Editor.

  • If you look under the follicleShape1 node you will see all the attributes associated with this follicle. As I said earlier, they may look intimidating but we only need to be concerned with a few key attributes.

Under Follicle Attributes you will notice a Point Lock drop down menu, currently set to Both Ends.

  • Change Point Lock to Base.

  • If you now move Kila around during Interactive Playback you will see that this has unlocked the tip of the curve, keeping just the base point locked.

With the tip unlocked you can turn your attention to the curves dynamic properties, which will dictate how it will move in virtual space. These are things like mass, drag and stiffness, all of which will contribute to the hairs movement.

Tip: It’s a good idea to apply some basic animation to Kila at this stage, so you can see the dynamics in action without having to physically move her each time. Just selecting and moving her root node should be fine, and the keys can easily be deleted when you are finished.

  • Select the hairSystem1 node and open the Attribute Editor, making sure you’re looking at the hairSystemShape1 node.

There are an awful lot of attributes to play with here, but remember that this is a simple dynamics setup so you can ignore most of these settings. In fact, the only tab you need to look at initially is the Dynamic Properties tab.

Note: I won't go into too much depth on what each attribute does, instead I will give a quick overview of key areas. With an nDynamics setup like this I find it’s better to experiment than be told what to do, as this will give you a much better idea of how each attribute effects the curve.

Initially your hair may have far too much movement, and even once Kila has stopped moving, it seems to continue like it has a life of its own.

  • To help reduce this, you can begin by adjusting the Start Curve Attract value to something small like 0.1.

This will essentially tell the hair to try and remain close to its start position, so a greater value will have more influence.

You can also control the effect this attribute has down the length of the hair through the Attraction Scale graph below it. Changing the curve will affect the falloff, so by default the effect is quite linear with the attraction being strong to begin with but then reducing to almost none at the tip.

In the example below you can see this has been adjusted so only the hair towards the very end of each curve will stray from the initial pose. This is a good setup for Kila as it means the hair closer to her scalp will have less movement.

These graphs occur quite frequently throughout the hair system attributes, and can be quite useful to adjust and experiment with to vary the overall effect of the dynamics.

  • If you now focus on the Forces tab you will see that this area allows you to adjust the properties of the hair as a whole, and in here you can adjust its Mass, Drag and other key attributes.

  • Mass dictates how heavy the actual dynamics curve is, so leave this at 1 initially.

  • Adjusting Drag will affect the amount of friction the hair has with the air around it. A setting of around 0.1 is a good starting point for Kila as it will slow down its movement slightly.

  • Damp controls the amount of momentum the hair will inherit from Kila's head. Leaving this at 0 will mean initially it inherits the full movement, but feel free to adjust this to 0.1 if you want her hair to be stiffer.

As you can see these are just a few key attributes, so the rest for now can be left alone. I do urge you to experiment with the other dynamic properties and forces, not only to get a feel for what they do, but also to achieve the movement you desire.

Tip: Adding some Turbulence into the system will give the hair a little variety in its movement, like there is a subtle breeze in the scene.

  • With your first joint chain now fully driven by dynamics it’s now your turn to work through and apply the same techniques to the others.

At this stage however I would leave editing any new hairSystem nodes attributes as next we will look at allowing each dynamics system to share the same hair system.

9. Scene Cleanup

You should now have dynamics setup on each hair joint chain, and in the process you have probably noticed your Outliner filling up with a number of new hairSystems and their associated groups and nodes.

This could be a good thing, as it means that each joint chain has its own unique system associated with it, meaning you could essentially use different attributes per joint chain. For Kila it makes sense for her hair strips to all use just one hair system, so it all moves and animates in the same way.

You can quickly and easily reassign hair systems in Maya.

  • Select all the hair system nodes and groups, except for the hairSystem1 node.

  • Now go to nHair > Assign Hair System > hairSystemShape1.

Once done you will see that most of the nodes which sat under the hair system groups have moved to be under the hairSystem1 nodes groups. There may however be some strays which linger behind.

  • Move all remaining follicle nodes so they are with the others under the hairSystem1Follicles group.

  • Now delete all the empty groups, and unused hairSystem nodes.

  • Finally rename the hairSystem1 node to KilaHairSystem, hairSystem1Follicles to KilaHairSystemFollicles and hairSystem1OutputCurves to KilaHairSystemOutputCurves, just to clean up the scene and make them more recognizable.

10. Adding a Basic Rig

Just as you did earlier when you added the basic rig, you also now need to create a simple controller for the dynamics. In this instance it’s probably even more important, as adding a few simple attributes to an icon will help to focus the animator on just the attributes they need to worry about, rather than allowing them to stare blankly at the full list in the attribute editor.

  • Just as you did previously, create a new curve based icon. The H will work nicely again here.

Now add a series of new, float based attributes onto this icon using the Modify > Add Attributes tool. The attributes are listed below.

  • Start Curve Attr
  • Attr Damp
  • Mass
  • Drag
  • Motion Drag
  • Turbulence

Note: Feel free to add any more attributes you may have found useful to this list.

Use the Connection Editor to directly connect these new attributes with the same ones on the KilaHairSystem node, connecting them as follows –

  • Start Curve Attr > Start Curve Attract
  • Attr Damp > Attraction Damp
  • Mass > Mass
  • Drag > Drag
  • Motion Drag > Motion Drag
  • Turbulence > Turbulence Strength

Again, this is a simple controller but adding it means the animator not only can animate these attributes, but isn't distracted by other areas of the system. In short, they can focus on animating.

Summary

We are now at the end of Part 9 of this series with just one more to go, but to be honest in this particular section there is a lot more I could have shared, like adding more control into the dynamics hair rig, or even baking out the animation when you are finished.

For now though I hope you have learned even the basics of adding movement into Kila's hair, be that through a simple IK based rig, or using nDynamics. From here you have the foundations to take things further, experiment and improve on these rigs to make them your own.

This leads us into the tenth and final part of the Game Character Creation Series, and although it’s the last, it’s probably one of the most important as we take a look at generating Level of Detail models.


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