CAP 203 - Computer Animation III

Chapter 8: Character Animation


This lesson covers material from chapter 8 of the text, and from lessons posted in YouTube by Autodesk. Objectives important to this lesson:

  1. Biped overview
  2. Animating a biped
  3. CAT lessons
  4. Fitting
  5. Skinning

Something to think about as we begin this chapter: you can animate many kinds of objects and viewers will not complain, but if you animate a humanoid figure, viewers will have strong opinions about whether the animation looks natural, or believable. Animating something that looks familiar is more demanding because viewers will expect it to move in familiar ways. Think about this each time you animate any figure the audience will think they are familiar with.

The lesson begins with a discussion of a 3DS Max tool called Biped. The tool provides a functional skeleton with a built-in hierarchy and inverse kinematics for a two-legged figure. This means that there is programming in the figure itself about how connected bones must move together.

Some terminology may help:

  • When objects are linked in a hierarchy, any of those objects can be selected and animated as needed, without breaking the links in the hierarchy. Such animation will, naturally, cause animation to propagate to other members of the hierarchy.
  • Forward Kinematics - the process by which a child object inherits motion from its parent object
  • Inverse Kinematics - the process by which we examine the motion of a child object to determine the necessary motion of a parent object

Although the tool starts with human proportions by default, you can modify the bones (objects that represent bones of a figure) to represent other bipedal character types (ape, T. rex, alien, etc.).

The lesson on workflow on pages 176 and 177 is meant to be an overview of the steps you will go through when animating a character with a biped or CAT structure. This lesson does not give you any details about each step, but it gives a good summary of what takes place.

  • You will typically create a character from one or more meshes, add a biped or CAT parent to the mesh, and custom fit the skeleton to the mesh. The text tells us that the skeleton must be sized to fit inside the character mesh. This is not absolutely true, since the rig itself will be set not to render. It is, however, a good idea to avoid being distracted by the rig when working in the scene, and it will make the mesh deformations more accurate. The what? You will animate the rig, and the mesh will move along with it.
  • The text tells us to test the rig by moving it into planned positions. It does not tell us what to do if it will not move as desired. You could modify the structure of the rig, or you might create a new one with a very different structure.
  • Once your rig is properly constructed and sized, you skin it. (More like adding a skin to it.)
    As the text explains, you use a Skin modifier to connect the mesh to the rig. Now the good news: once this is done, you can use this character model in other scenes as well. And the bad news: that's not as easy as it sounds.
  • Animate the rig to animate the character in the scene.
  • Check your work and render.

So, how do you use a biped? Your curriculum has not presented material on bipeds up to now, so some background may be helpful.

As noted above, a character mesh must be attached to a biped with a skin modifier. Accept that the bones of the biped should fit completely inside the character mesh. It is better to size the bones to the mesh first, then to apply the skin modifier. This will require less modification of the envelopes that enclose the character mesh. More on that shortly.

To create a biped, you can select Create, Systems, Biped, then drag in one of two ways:

  • If you select Drag Height, you should place your mouse pointer where you want the feet of the biped in the Front viewport and drag up or down. This will give you better control over placement and height of the biped.
  • If you select Drag Position, you will create a default size biped when you begin a left click. You can place the biped anywhere you want in the viewport by moving the mouse pointer before you release the left mouse button.

On the modify panel of the biped object, you can name the biped, which will cause all the bones of the biped to inherit that name. By default, each biped you create is numbered and its head is a different color from the last one you made, but they will look a lot alike. Naming them will help tell them apart when you are trying to select a specific bone in your scene.

You can also choose from four shapes for the biped: skeleton, male, female, and classic. The classic shape was the default in previous versions of the program, and is included to allow the use of older files.

You will have noticed by now that a biped object's right arm and leg are green, and its left arm and leg are blue. This is to help you remember which side is which: there are four letters in left and in blue, five letters in right and in green. (These colors are not continued in the fingers and toes of the figure.) Like a person or an animal, a biped has a left and a right of its own, which will be used for reference.

We can also observe that the pelvis bone is a shade of yellow because it is in the center or middle of the figure (six letters). The spine and neck are in the middle, too, so why aren't they yellow? The pelvis structure is special: it is the root bone, the first bone, of the biped. It also represents the Center of Mass (COM) of the biped.

save file buttonThe biped model may be used with non-human characters that are still four limbed. You can modify most aspects of the structure to accommodate bipedal and quadrupedal animals, aliens, and fantasy characters. Once you have made such modifications, you can use the Save File button on the Motion panel to save a new .fig (figure) file. The save button is on the Biped rollout of the Motion panel. It has an image of a 3.5 inch disk on it. This button saves a figure file when figure mode is enabled, and saves a biped animation file (.bip) when figure mode is disabled. A figure file contains only the biped figure currently selected, not the entire 3DS Max scene.

You will want to be aware that the Modify panel for a biped is mostly empty. You make modifications to a biped on the Create panel or the Motion panel. The reasoning may be that a biped is not just a structure, but a structure that is intended to be animated. Look at the Motion panel with a biped selected to see the choices presented in the next image.

biped links list Many body parts in a biped can be subdivided into a series of links. The spine, for example has four links by default, but it may have as few links as 1 or as many as 10. Other parts have specific features. In the Body Type section of the Create Biped rollout, shown on the right, you see that arms can be turned on and off. There are either two or none. By default a biped, as shown above, has only one finger on each hand, one toe on each foot, and no tail or pony tail.

You can modify all of these parameters before you create your biped. You can also turn on the three props. Prop 1 will attach to the right hand, prop 2 will attach to the left hand, and prop 3 will float on the left side of the body. (Useful for Mr. Cotton's parrot , or Jack the monkey?)

Note, also, the default value of the Ankle Attach field. A value of 1 would attach the ankle at the front of the foot, and a value of 0 would attach it at the back of the foot. The default value (0.2) is a more average human location.

In addition to the Save File button shown above, the Biped rollout has eight other buttons. Clicking any of the mode buttons changes what other rollouts are available on the panel. At this point you will want to know the following:

  • Figure mode - allows you to save a figure file, and allows you to edit the bones of your biped
  • Footstep mode - allows you to animate the biped by placing footsteps on the scene
  • Move all mode - allows you to move the biped and its footsteps, allowing you to keep the animation, but change where it takes place

This becomes confusing, so let's simplify it.

  • Select the biped object, and use the regular Move tool to place the biped at the start of a scene. Do not use this tool for animation of the biped as a whole.
  • Avoid the footstep mode, because it often leads to jerky, zombie-like animation.
  • If the character is to walk, run, crawl, or otherwise move in the scene, animate the bones separately, creating a walk cycle where possible to take advantage of repeated motion.
  • Your should change the position of the feet, arms, hips, and other body parts as needed during a walk cycle and use the Center of Mass tools discussed below to move the body as needed across the scene. Why? To animate every step gives you ultimate control, but takes a long time. Using a walk cycle can be done faster, as long as the terrain is constant. The text does not discuss the theory that people (and animals) walk in a series of repeated movements (cycles). Follow the link above for more information in a lesson by David Atkinson.

Biped 101

This exercise is on pages 180 and 181. This is less an exercise than a set of tips, but we can use the tips to make an exercise.

  1. In step 1, create a biped in a new scene. Note the author's tip: once the biped is in the scene, you need to create a keyframe in frame 0. It is not created automatically, or in the regular way.
    It may help to think of a biped as an add-on system for 3DS Max. (It originally was.) This may make it easier to remember that a biped has its own set of animation controls in a rollout, as shown on page 159.
    Follow the instructions in this step to set all the required keys for the first frame on the timeline.
  2. Turn on the regular Auto Key feature. Move the time slider to frame 10, and change the positions of several bones in the biped as the text suggests The author hints that something will go wrong if you try to pose the biped in frame 0. Take her advice, and consider frame 0 a staging area for bipeds that will be used in our scene. We can choose not to render frames before 10 if need be.
    Body Horizontal
  3. Step 3 introduces the three Center Of Mass (COM) buttons that are mentioned earlier as being on the Track Selection rollout. The three buttons described in the text stand for horizontal movement (left-right arrow), vertical movement (up-down arrow), and rotation (circular arrow) of the center of mass. If you move the center of mass, the rest of the biped moves as well. Note the use of the buttons: to animate the entire biped at once.
    Is the biped being shot out of a cannon? No, you will want to use this for general motion in the scene. You can animate the bones as well to indicate specific motion, like a walk or a run. As noted above, the center of mass of the biped is its pelvis bone. So, the weird thing to accept is that you will animate the biped separately from relocating the biped in the scene.
  4. Step 4 continues the discussion of moving the entire biped as a unit.
  5. Step 5 is confusing to me, since the words pose and posture mean two different but related things in 3DS Max.
    Pose refers to the arrangement of the entire biped. Posture refers to the arrangement of a particular bone. Let's try to remember that: we pose the whole rose, we posture each petal.

    The author is not using these words this way. The instructions in this step relate to postures, copying and pasting them from one frame to another.
    Use the instructions in this exercise to move a biped as you desire, save the animation, and show it to me. Use good taste, and imagination: you need to create something for your show reel.

The text is as light on its material about CAT rigs as it is about bipeds. CAT stands for Character Animation Toolkit, which became available in our animation lab when we switched to 3DS Max 2012. (It was available in 3DS Max in version 2011.) CAT provides a variety of pre-rigged bone sets for humanoid and animal models, as well as a new version of the Biped object that includes the other features of CAT that we will discuss.

Autodesk Lesson: CAT Animation Layers

We will begin a discussion of CAT with a video lesson that Autodesk has provided on YouTube. You should play the video, refer to the notes here, and carry out the lesson before you experiment with CAT.

  1. The narrator mentions other videos we are supposed to have seen (0:23). People always seem to forget that their other videos are not immediately accessible to someone who found the one that is playing in a web search. Oh, well.

    Create a CAT objectHe does not explain that he selects the Create panel, Helpers tab, and selects CAT Objects from the drop down list. (Indicated by the red ellipses in the image on the right.) He does mention that he clicks CAT Parent. He shows that he can quickly scroll through the list of CAT Rigs to choose his preferred rig for this lesson: Clown.(0:41)

    Start a new scene, set a project folder, and follow his example.

  2. The narrator quickly drags in the Perspective viewport to create a clown rig. He demonstrates that we can center the rig in the scene by selecting the root node (the triangular base of the rig), activating the Move Transform Type-In dialog box (right click the Select and Move button), and entering 0 in each of the Absolute: World boxes.(0:46) Remember that if a field has a spinner control, you can set the value in that field to the minimum allowed value by right clicking the spinner. In this case, right clicking the spinners will set the values to 0.

    Again, follow his example to learn/practice the centering technique.

  3. With the clown rig still selected, the narrator switches to the 3DS Max Motion panel (0:55), and calls our attention to the Layer Manager rollout of the Motion panel.

    He discusses two modes available by toggling the large red or green button that is the first icon on the first toolbar in the Layer Manager rollout. To change from one mode to the other, you normally just click the button. Don't click it yet: it won't work until you change one more thing.

    Background information:
    • When this button is red, you are in Setup mode for the rig. In Setup mode, the rig can be scaled, adjusted, and otherwise modified to fit the mesh you intend it to animate. This mode is for adjusting and sculpting the rig, not for animating it.
    • When the button is green, you are in Animation mode. This mode is not accessible until you add an animation layer to the scene with the first button in the second toolbar under Layer Manager, which adds one of four kinds of animation layers to the scene: Absolute, Adjustment Local, Adjustment World, and CAT Motion.

      The narrator adds an Absolute layer to demonstrate the process. At 3:23, the narrator adds a color to the layer, using the first icon on the third toolbar on the Layer Manager rollout. The value of this action is not immediately apparent. The narrator will add different colors to each of his layers to tell him visually which layers are affecting the rig as he works.

      If you have not done so yet, add an Absolute layer, toggle to Animation mode, and turn on Auto Key as the narrator does at 3:44.

  4. The narrator selects the rig's left hand. Note that the hand has a thumb and three fingers, like many classic cartoon characters. (Quickly: if the rig is facing you, is its left hand on your right or left? Cue Jeopardy music...)

    He moves and rotates the hand a bit at frame 30, and shows us that, with the hand selected, we have a rollout for Digit Manager. The Digit Manager has some preset poses (e.g. Peace, HangLoose). The narrator selects the Peace pose, then the HangLoose pose, and demonstrates changing the value of the Weight variable (4:20) to make the hand follow the pose less closely. He mentions that we can save more poses here as well.

    Carry out his steps, but choose your own value for the pose Weight.

  5. The narrator sweeps the timeline slider to demonstrate that the animation he added to the scene has been captured so far.

    Do this as well to check your mirroring of the exercise. (No animation captured? Did you turn on the Auto Key feature?)

  6. The narrator creates a new absolute animation layer. Make sure you put the time slider back to frame 0, then follow the narrator: first he selects the word "(Available)" which should always appear below the last layer in the Layer Manager stack. Then he activates the drop down for the layers by holding the mouse down over its selector button, and once again he selects the first choice, Absolute. He assigns a green color to this layer to differentiate it from his first/red layer.

    The narrator demonstrates that the rig does nothing when he moves the time slider. This is because the new animation layer is selected and it has no animation in it yet. He also explains that the animation layer stack works from top to bottom, which is the reverse of the way the modifier stack is evaluated: animation layers are evaluated top to bottom, modifiers are evaluated bottom to top. So, why isn't it evaluating both layers? Well, it is, and the bottom layer is now empty. For now, be glad that we can work in one layer at a time, creating simpler movements that will become more complex when they are combined.

    The narrator proceeds to add animation to the right hand of the rig in the new animation layer (5:34). Note that he selects that layer, turns on the Auto Key mode, and then animates the hand, as before. Note that the right hand animation is placed at the same time on the timeline (in the green layer) as the left hand animation (in the red layer). That will be useful in a moment.

  7. The narrator continues the lesson, calling our attention to the Local Weight and Global Weight fields for the currently selected layer (6:02). Weight refers to effect or influence, not to the simulated heaviness of the rig.

    Local Weight
    refers to the relative effect or influence of the layer over a portion of the rig. He demonstrates that he can select various portions of the rig to change the influence of the selected layer only on the part of the rig currently selected.

    Global Weight describes how much influence a given animation layer has on the entire rig, compared to other layers. By default, each layer has 100% Global Weight, which means that each layer completely overrides the layer that preceded it. That's a problem if we want to see both animations. The narrator selects the green layer, selects the left hand group, and reduces the layer's Local Weight to 0%. This tells the green layer to have no effect on the left hand group, allowing the left hand animation stored the the red layer to show through it. (7:20)

    Change the Local Weight of the green layer to 0% for the left arm, to match the change made by the narrator.

  8. The narrator shows us a new tool that makes use of the colors he assigned to the layers. The second icon on the Layer Manager's first toolbar is used to set the coloring mode for the rig. Click that button, and select the second option (7:42), which changes the color scheme used on the rig from default to a scheme that displays the color of the layer that is currently affecting each portion of the rig.

    The narrator creates a new layer (his third) as an absolute layer, which he colors codes as yellow. Note his mini-lesson about creating the new layer:
    • if you want to create the new layer at the bottom of the stack, select Available first, then use the layer creation tool
    • if you want to create the new layer above some layer already in the stack, select that layer first, then use the layer creation tool
    The narrator continues, reminding us that the new absolute layer, being third in the stack, is automatically set to 100% Global and Local Weights, obscuring the animation in the previous two layers. This is why the rig appears all yellow at video time 8:52.

    CAT figure with three layersHe selects the new (yellow) animation layer, moves the timeline slider to frame 30 again, and animates the rig's left leg with the Auto Key mode turned on. The narrator seems to make an error next, by not turning off the Auto Key mode. He makes good use of it, however. At frame 30, he sets the Local Weight value for the yellow layer to 0% for the right leg, the right arm, the left arm, the chest, and the head of the rig. Note that he does this for each major portion of the rig separately. In my version on the right, I have left the Local Weight of the yellow layer at 100% for both legs, and I have posed the hands differently..

    He notices (10:12) that Auto Key was on when he set the weights. The interesting effect is that the Weight changes are now animated: they start at 100% in frame 0, and change to 0% by frame 30. Note the gradual change from yellow to red and green in the affected portions of the rig.

    Complete the animation as the narrator has for this lesson. Save your scene so far. It should occur to you to wonder why he did not animate all the actions in one layer. He certainly could have, but chose to do it this way to introduce layers as a working concept.

You may want to know that absolute layers work as you have seen in this lesson, overriding layers that are higher in the stack. Adjustment layers work differently, allowing you to adjust the pose of the model. For example, you might have animated the model's arm to wave at the audience in an absolute layer. Later, you realize that the arm should have been positioned higher. You can adjust the overall position of the arm with an adjustment layer added to that absolute layer.

Adjustment Layers

Adjustment layers come in two types: Local and World. Let's look at a short lesson on the difference between them. In this animation, the narrator creates a new CAT rig instance, and adds a motion layer to it, pointing out that a motion layer has a built in walk cycle, which is handy if you like it, and at least a basic motion to modify if you don't like it.

In this case, the narrator demonstrates the walk cycle, then adds a Local Adjustment Layer to add a new animation feature, turning the figure's head to its right, holding for a moment, and turning it back to the front. The adjustment layer adds the head animation without disturbing the overall motion of the body already contained in the motion layer. The narrator explains that the local adjustment has modified the rig's animation like an additive blend. The advantage of this method should be apparent: the basic walk cycle of the character can continue, while specific animations that occur only in specific parts of the walk can be added wherever needed.
Do this in a scene of your own to get the idea, then come back to these notes.

The narrator continues by removing the existing adjustment layer and adding a new World Adjustment Layer. He places the timeslider at frame zero, and shows that he can adjust the initial pose of the character, which is continued from that frame. He could have done this in a different frame. He could, for example, make the figure crouch only in the second thirty frames. To leave the crouch pose, the animator would also need to copy the keys for standing straight from the beginning of the timeline to the desired position on it.
Try this as well, to change the nature of the walking figure's pose for a few frames.

It is not clear when to use a Local adjustment, and when to use a World adjustment. Other lessons make it clear that you can get similar results with either one.

Other useful CAT lessons:


This exercise is on pages 182 and 183. The exercise fits a biped system inside a character mesh. The key in these steps is to fix the center first, then work your way out to the extremities. This technique is valid regardless of your choice of biped or CAT rig.

  1. Open the start file for this exercise. Note the description the author gives us in step 1 of the ideal way to pose the character mesh as you prepare it to receive a biped. Sort of like a gym class pose?
  2. Recall the commands (not given in the text) to make the character mesh see-through and frozen. What would be the point of doing this to the character mesh at this time?
  3. Step 3 describes creating a biped, and reminds us that we modify the basic characteristics of a biped on the Create panel, unlike any other object you have worked with. The tip for this step is that you can set the biped's toe count to 1 (per foot) and its toe link count to 1 as well if the character is wearing shoes that do not display the toes. (I am surprised she ignored the possibility of a sandal.)
  4. Turn on Figure Mode as instructed. Begin sculpting the biped by moving the biped pelvis to match the character mesh pelvis. Expect that this will put all the other bones in the wrong place, and don't worry about those bones yet.
  5. Next, follow the instructions to fit the biped legs inside the character mesh legs. It may be much easier to scale matching bones at the same time: right and left thigh, right and left shin, etc. Once they are sized correctly, set the posture for the bones on one side, then try her advice to copy the posture to the other side.
  6. Scale the spine, but watch the biped shoulders. When the spine is scaled correctly, the shoulders should fit inside the character mesh shoulders. Rotate and scale the arms as needed, working from upper arm, to elbow, to lower arm. Leave the hands for step 8.
  7. Scale and rotate the neck and head. Then move down to fix the feet and toes. These steps could have been reversed with no adverse effects.
  8. The author tells us that she saves the fingers for last, because they take longer to get right. She works on the wrists and the palms next, then begins the fingers. Something to be aware of: this character mesh has a thumb and four fingers on each hand, unlike many cartoon characters. Follow the instructions in the text to complete your custom biped for this mesh.


This exercise is on pages 184 and 185. This exercise attaches a character mesh to a biped system.

  1. Open the starting file for this exercise. Note that you will work from the mesh to the biped this time. The starting point assumes you already have a biped that has been customized to fit your mesh.
    The author seems to be of two minds about this and other exercises. She asks you to create a simple animation with the biped, then reveals that she has already done so. She may have meant her instruction to be a general principle, or she may have changed her mind.

    In any case, you don't need to follow her instruction to hide the character mesh, because it is already hidden in the scene. Play the animation she provided. It is only ten frames long. Examine her fitted biped, comparing it to your own work in the exercise above.
    Complete her instruction in this step, to create a Selection Set for the biped.
  2. Figure ModeHer next instructions are puzzling. She wants you to enter Figure Mode, which will enable editing the biped model, then she asks you to unhide only the body part of the character mesh, if it has separate parts for the hair and/or head. If? Didn't she supply us with the model? She seems torn between giving general advice for this kind of procedure, and giving specific advice for this exercise.
    Unhide by NameSo how do you unhide only one portion of the mesh?

    Right-click in one of the viewports, and choose Unhide by Name from the upper right quad menu. This will give you a list of hidden items in the scene. Choose the Body, and click Unhide.

    The author is giving more general advice when she tells you to remove any smoothing modifiers that would interfere with seeing the actual mesh, and to collapse any other modifiers in the modifier stack before proceeding.

  3. With the character mesh selected, add the Skin modifier as instructed.
    Continue following the instructions by looking in the Parameters rollout for Bones, and by clicking the Add button. (Why do they always write that kind of instruction backwards?) Choose all the biped parts as instructed. (How do you choose all the items in a list? Think about it. It will happen again. Look at the Select menu.) Adding the bones at this stage is also called binding the bones to the mesh. It is important to add all of them.
  4. Select the biped again to switch back to it, and turn off Figure Mode as instructed. Hide the biped for practice. (Remember, you don't want it to appear in a final render.)

    Get ready to have your heart broken, then run the animation again. The character mesh probably does not deform correctly yet. Watch the animation dispassionately, and note what parts of the mesh do not flex properly during the animation.
  5. In the next several steps, the author demonstrates further necessary customizations for this biped/mesh pair. Begin by selecting the character mesh again, and select the Modify panel.

    This step introduces Envelopes. The envelopes around a bone provide a graphic expression of the influence that bone has on the portions of a character mesh surrounding that bone. They also provide a toolset to adjust the bone's influence. In the illustration below, a head bone (of a different model) has been selected. You see an envelope displayed that is affecting not only the head of the mesh, but parts of the neck, the shoulders, and the katana that the character is wearing. This is not an acceptable arrangement.

    Select one of the bones (from the list on the Modify panel) whose influence needs modifying. In this example, the author has chosen the left calf bone, which is likely an appropriate choice on most models. (Yes, Virginia, a biped has a calf bone, not a tibia and a fibula. Our robot is simpler than a human.)

    Look on the Parameters rollout, and select Edit Envelopes. You want to modify how the bone affects the vertices of the mesh, so find the Select group and choose Vertices.

    Follow the instructions to select the vertices on the mesh that should be affected only by this bone. Remember that a vertex can be affected by more than one bone, but the sum of their effects must equal 1 (100%). In this case, you want to identify only the vertices that are affected 100% by the thigh bone. Continue with the exercise and show me your work.