CAP 203 - Computer Animation III

Chapter 8: Character Animation

Objectives:

This lesson covers material from chapter 8 of the text. Objectives important to this lesson:

  1. Biped overview
  2. Animating a biped
  3. Fitting a biped
  4. Skinning
  5. Inverse Kinematics
Concepts:

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, however, 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 156 and 157 is meant to be an overview of the steps you will go through when animating a character with a biped 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 to the mesh, and custom fit the biped to it. The text tells us that the biped must be sized to fit inside the character mesh. This is not absolutely true, since the biped itself will be set not to render. It is, however, a good idea to avoid being distracted by the biped when working in the scene, and it will make the mesh deformations more accurate. The what? You will animate the bones, and the mesh will move along with them.
  • The text tells us to test the biped 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 biped, or you might create a new one with a very different structure.
  • Once your biped is properly constructed and sized, you skin it. As the text explains, you use a Skin modifier to connect the biped to the mesh. 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 biped 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 biped must be attached to a character mesh 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 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 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 cyle 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 158 and 159. 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. 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.

Biped fitting

This exercise is on pages 160 and 161. 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.

  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 an open-toe shoe possibility.)
  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.

Skinning

This exercise is on pages 162 and 163. This exercise attaches a biped system inside a character mesh.

  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, follow her instruction to hide the character mesh, then play the animation she provided. This will also give you an opportunity to 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.
    Assume she 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.) Adding the bones at this stage is also called binding the bones to the mesh.
  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. 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 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 a finished arrangement.

    Select one of the bones whose influence needs modifiying. 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. We should have played the two help file videos about skinning in class by now. 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.