Animating a Biped

When it comes time to animate a biped, there are two modes that you can use: Footstep Mode and Freeform Mode. Both have advantages. Footstep Mode is useful for characters that need to walk, run, or jump. It ensures that the feet stay parallel to the ground at all times and can be used to walk over rough terrain. Freeform Mode doesn't constrain the biped and is used for all other actions. Most animation sequences use a combination of both modes.

Using Footstep Mode

In Footstep Mode, you animate the movement of the biped by placing footprints for the biped to follow. These footprints can be positioned anywhere within the scene, and the biped automatically creates the motion required to have the biped follow the footprints including all the realistic secondary motion such as swinging arms. You can also select to have the biped motion be walking, running, or jumping using the buttons in the Footstep Creation rollout.

Using the Walk Footstep and Double Support values located under the Walk button, you can set how quickly the footstep animation happens. The Walk Footstep value is the number of frames for which the foot remains within the footstep, and the Double Support value is the number of frames during which both feet are touching the ground.

For the Run and Jump options, the Run Footstep and 2 Feet Down values define the number of frames in which the foot is (or feet are) within the footstep, and the Airborne values define the number of frames in which both feet are in the air.

Footstep Mode is enabled by clicking the Footstep Mode button in the Biped rollout of the Motion panel. This button turns light yellow when enabled and opens several additional rollouts.

The easiest way to create footsteps is to click the Create Footsteps (at current frame) button in the Footstep Creation rollout and then click in the viewport where the footprints are to be located. By default, the left foot is placed first and the footprints alternate between right and left. Left footprints are marked in blue, and right footprints are marked in green. Until keys are created, you can select, move, and rotate footsteps.

Tip 

If you look closely at the cursor, you can see which footstep will be created with the next click. If you press the Q key and then reselect the Create Footsteps button, the opposite foot is placed with the first click.

After several footprints are added to the scene, clicking the Create Keys for Inactive Footsteps button in the Footstep Operations rollout creates the keys for the biped following the footsteps. After the keys are created, the biped is moved to the location of the first footprint, and dragging the Time Slider (or clicking the Play Animation button) moves the biped through the available footsteps.

After a sequence of footprints exists, you can add more footsteps using the Create Footsteps (Append) but-ton in the Footstep Creation rollout. This button lets you place more footsteps, continuing the sequence already laid out.

To automatically create multiple footsteps that are equally spaced, click the Create Multiple Footsteps button in the Footstep Creation rollout. This opens a dialog box, shown in Figure 42.9, that lets you specify which foot is first, the total number of footsteps, the stride width, and many other settings associated with the first and last steps.

Figure 42-9: The Create Multiple Footsteps dialog box lets you specify details such as Stride Length.

If a footstep is selected, the Footstep Operations rollout includes buttons to deactivate, delete, or copy the selected footstep. If you copy a footstep and then forget what is in the copy buffer, you can enable the Buffer Mode button in the Biped rollout to see the motion of the footstep that is in the buffer.

The Dynamics &Adaptation rollout includes a GravAccel value. This value is used to determine how quickly the body returns to ground during a run or jump cycle. To simulate a character jumping on the moon, reduce the GravAccel value. There are also several Footstep Adapt Locks that can be set.

Tutorial: Making a biped jump on a box

Although we could easily make our biped dance the two-step, this tutorial has the biped walk a few steps, transition to a run, and then jump on top of a stationary box.

To make biped jump on a box, follow these steps:

1. Select the Create Standard Primitives Box menu command, and drag in the Top viewport to create a simple box.

2. Select the Create Systems Biped menu command, and drag in the Top viewport to create a biped object. Be sure to leave enough room between the biped and the box so the biped can get a running start.

3. Open the Motion panel, and click the Footstep Mode button in the Biped rollout to enter Footstep Mode.

4. In the Footstep Creation rollout, select the Walk button and click the Create Footsteps button. Then click in the Top viewport to create four footsteps in front of the biped object starting with the right foot.

5. Choose the Run option in the Footstep Creation rollout, select the Create Footsteps (append) button, and add four more steps that are spread out slightly more than the first steps.

6. Choose the Jump option in the Footstep Creation rollout, and click the Create Multiple Footsteps button. In the Create Multiple Footsteps dialog box that opens, set the Number of Footsteps to 2, and click the OK button.

   The two footsteps where the biped lands should be in the center of the box object.

7. Select and move the final two footsteps upward in the Left viewport to be on top of the Box object.

8. Click the Create Keys for Inactive Footsteps button in the Footstep Operations rollout to create the keys for the available footsteps.

9. Click the Play Animation button to see the biped walk, run, and jump on the box. Figure 42.10 shows the biped as he hops onto a box object.

Figure 42-10: By positioning footsteps, you can control exactly where the biped moves.

Converting biped animation clips

The Convert button in the Biped rollout lets you convert all footstep animations to Freeform keys. If you don't plan to append any additional steps to a biped, then converting the keys makes it easier to work in Freeform Mode.

Clicking the Convert button opens a simple dialog box with the option to generate a keyframe for every frame.

Using Freeform Mode

Freeform Mode is enabled by default when none of the other buttons in the Biped rollout are enabled. Using Freeform Mode, you can select and set keys for any of the biped bones, blend several animations into one using Layers, import Motion Capture data, and define Dynamic properties.

Setting Freeform keys

The easiest way to set biped keys is to drag the Time Slider to the frame where you want the key to be, select and transform the bone, and then click the Set Key button in the Key Info rollout, shown in Figure 42.11. The Key Info rollout also includes buttons for deleting keys; setting a planted, sliding, or free key; and viewing trajectories.

Figure 42-11: The Key Info rollout includes specialized buttons for setting biped keys.

The Key Info rollout also includes several expandable sets of controls including TCB, IK, Head, Body, and Prop. The TCB controls let you configure the Ease To and Ease From curves of the selected key using a Tension, Continuity, and Bias curve.

The IK controls let you set IK Blend and Ankle Tension values and select a pivot about which to apply the IK solution. The Head section lets you select a Look At Target. The Body controls include a Balance Factor that is used to determine the amount of sway in the biped's motion. The Prop controls lets you select which bone the current prop is linked to for position and rotation.

Using the Keyframing Tools, Layers, and Motion Capture

The Keyframing Tools rollout, shown in Figure 42.12, includes several useful buttons for enabling and manipulating subanimations. You can also delete a range of keys or all keys, mirror keys, set multiple keys, and anchor the right hand, left hand, right foot, or left foot. By default, the animation key for each bone is rolled up to its parent, but you can have the key for each individual bone appear as a separate track using the check boxes in the Keyframing Tools rollout.

Figure 42-12: The Keyframing Tools rollout includes a variety of features.

The Layers rollout lets you layer sets of animations while maintaining existing animations. For example, if a character is animated walking about a scene, then adding a layer with the character's arms held straight up will have the character walk about the scene in the same manner with its arms held up.

After several layers are added onto a biped, you can use the Activate Only Me button to see the animation of a single layer or the Activate All button to view all layer animations.

The Motion Capture rollout lets you load, apply, and manipulate motion capture data. Motion capture data is saved using the .bip file extension.

Loading and saving biped animation clips

In addition to creating an animation, you can simply load an existing animation using the Load File button in the Biped rollout. This button opens a file dialog box where you can select the .bip file to load. A preview of the selected biped animation sequence is displayed in the Open dialog box, and you can select to restructure the biped to match the file.

Animation sequences can also be saved using the Save File button. All biped animations are saved using the .bip file extension, and you can select to save any Max objects and List Controllers along with the file.

Using Motion Flow Mode

The Motion Flow Mode button in the Biped rollout opens a rollout of options that let you create a graphical flow of animation sequences that flow together to create a large animation sequence.

With the Motion Flow Mode button enabled, click the Show Graph button in the Motion Flow rollout. This opens the Motion Flow Graph dialog box, shown in Figure 42.13.

Figure 42-13: The Motion Flow Graph interface displays each motion clip as a separate node.

Using the Create Clips button, you can add a node to the graph window. Right-clicking the node lets you open a .bip file. The name of the clip file appears in the node. The Create Transition button can be used to define transitions between nodes, which are represented by arrowed lines. After a transition is defined between two nodes, clicking the Create All Transitions button causes transitions to be created for each defined transition that ensures smooth motion between the various motion clips.

After the nodes of the motion are established, you can use these same nodes to create a motion script that creates the keys needed to view the animation in the viewports. To create a script, click the Define Script button in the Motion Flow rollout and then select the nodes in the order that you want them to appear. Each selected node is added to the rollout list, and dragging the Time Slider displays the biped's motion in the viewports.

Scripts can be saved and loaded using the Load and Save buttons in the Motion Flow rollout. The Motion Flow Graph interface also includes buttons for optimizing, randomizing, and checking all motion clips and their transitions.

Previewing a biped animation

After a biped has been animated using footsteps of Freeform keys, you can preview the animation in the viewport using the Biped Playback button found in the Biped rollout. This animates the biped as a stick figure moving through the various keys and is useful for understanding the timing of the biped's motion.

Moving a biped with its footsteps

When footsteps are created, the biped and its footsteps can be moved independent of one another. The Move All Mode button lets you select and move a biped along with all its footsteps all together to a new location.