Understanding Biped

Biped is a 3D Studio MAX system plug-in that you access from the Create panel. Once you create a biped, you animate it using the Biped controls on the 3DS MAX Motion panel. Biped provides the tools you need to design and animate the motion of characters.

The Biped

The biped structure has a number of properties designed to help the animator.

Biped Center of Mass

Moving or rotating the Biped center of mass (COM), an object shaped like a diamond in the pelvis area, positions or orients the entire biped. The center of mass uses three tracks to store motion keys: Body Vertical, Body Horizontal, and Body Rotation.

Body Vertical and Body Horizontal keys have additional parameters for gravity and balance, referred to as Biped Dynamics.

Body Vertical keys have Dynamics Blend and Ballistic Tension, respective parameters that control gravity and the amount of knee bend when a biped lands after an airborne period. These parameters only apply for lift-off and landing keys on either side of an airborne period. In an animation containing footsteps, an airborne period is an interval of time when neither of the biped feet is on a footstep. In a footstep animation using Biped Dynamics, you won't have to create Body Vertical keys for the apex of a jump or for the lowest part of the landing period; the Biped Dynamics will automatically calculate a trajectory for the center of mass. Gravity plays no role when a character is walking. In an animation containing footsteps, walking is defined as the biped having at least one foot on a footstep at all times.

Body Horizontal keys have a parameter called Balance Factor. Balance Factor, whose parameters are stored in the center of mass horizontal track, helps the animator by automatically shifting all of the biped limbs to naturally position the character's center of mass, over the feet when the spine is rotated. You can actually see the biped pelvis moving away from the center of mass object if the spine is rotated. This relieves the animator of having to set extra keys to simulate the character's balance. Balance Factor positions the biped for you.

Keyframing the Biped

There are two primary methods used in creating biped animation: Footsteps Method and Freeform Method. Each method has advantages, and you can convert from one method to the other, or you can use a combination of both techniques in a single animation.

Footstep Method

In the viewports, footsteps represent support periods in space for the biped feet. Moving or rotating footsteps in space is done in the viewports. In Track View, each footstep appears as a block that represents a support period in time for each of the biped's feet. Moving footsteps in time is done in Track View. The footstep position and orientation in the viewport controls where the biped will step.

There are three ways to create footsteps for the biped. The first way is to place footsteps individually, or one at a time. The second way is to invoke Biped's multiple footstep creation tools to create a walk, run, or jump animation. The third way is to extract footsteps from raw motion capture data.

A key advantage of the footstep method is the natural adaptation of the biped that occurs when the footsteps are edited in time and space. Editing footsteps in the viewports allows you to reposition all of the footsteps to move the entire animation. In Footstep mode, stride, length, width, and direction can be changed quickly for an entire animation and the biped automatically adapts. Using the Footsteps Show/Hide button on the Display rollout, all footsteps can become visible. Move the footsteps in the viewports to position them for proper ground collision with the terrain object. For example, if the biped toes are rotated for the Lift key at the last frame of a footstep (to create more toe curl as the character walks) the leg automatically repositions itself to maintain foot contact with the ground (footstep).

These adaptations speed up the process of creating and editing animation for the biped. If necessary, the animator can prevent biped adaptation by using the Adapt Locks parameters on the Animation Properties rollout.

Foot States

Within a footstep animation, there can be four foot states: move, touch, plant, and lift. These correspond with the state of the biped feet in relationship to the footsteps. Use the foot state displays in the General rollout to determine the state of the biped feet when you are editing the biped foot or leg keys. Foot states are used on the Set Multiple Keys dialog to select multiple keys in Track View.

Freeform Method

A freeform animation contains no footsteps; instead it relies on the transforms of the biped body objects and center of mass. Use Freeform for motions like swimming or falling where footsteps are not necessary. If you are a familiar with creating all of your keyframes manually to animate a character, you may want to use the freeform method exclusively.

To start a freeform animation, turn on the 3DS MAX Animate button and start positioning the biped. Or you can leave the Animate button off and use the red set key buttons to create keyframes.

You can also create freeform animation by importing motion capture data and choosing freeform rather than footstep.

Tip: Take advantage of both methods by combining footsteps and freeform animation. You can create a freeform period for any airborne period between footsteps. A freeform period replaces the ballistic motion calculated using the GravAccel value with a user defined spline motion.

If you are using footstep extraction with motion capture data, you often need a freeform interval to accommodate falling or tumbling motion in the data. The Fit to Existing option on the Motion Capture Conversion Parameters dialog allows for a combination of both methods. Extracting footsteps from motion capture files eliminates sliding feet, a common problem with motion capture data.

Note that while you can add a freeform period to a footstep animation, you cannot add a footstep period into a freeform animation. If you want to add a footstep animation to an existing freeform, you can use the motion flow editor to create a script that sequences the footstep with the freeform.

Inverse Kinematics

Footstep and Freeform animations use the same IK constraints and extensions. This means that in a footstep animation, you can now edit keys to change footstep duration. By definition, a footstep is the start and end of a sequence of IK constraints in World Space with an IK Blend value greater than 0. Deleting and inserting keys or changing IK space or IK blending alters footstep duration.

In cases involving edits that alter the length of ballistic intervals (when a biped is airborne), the software ensures that there is a vertical key occurring at the lift-off and touchdown frames. This calculates the correct ballistic motion, so vertical keys are automatically inserted if not present.

There are three types of IK keys you can create: planted, sliding and free keys.

IK constraints are implemented with a pivot-based system. This allows you to pivot a hand or foot around a selected pivot. For example, in a walking motion you can select a pivot on the heel of a foot and rotate the foot around it,. You can then shift the pivot to the ball of the foot.

Key Interpolation

The way that Biped Dynamics affect the biped center of mass is discussed earlier in this topic. Spline Dynamics is an alternative method of keyframe interpolation. If you are starting a new animation, use the Animation Properties rollout to select the method you want to use. Spline Dynamics will be most familiar to new users. Selecting this method and creating new footsteps generates Body Vertical keys using Spline Dynamics for interpolation; Balance Factor and Gravity are turned off. You can convert from one method to the other by selecting one or all of the Vertical Center of mass keys in Track View and setting the Dynamics Blend parameter to 1 for Biped Dynamics, or 0 for Spline Dynamics.


Motion Capture Import, Motion Flow Mode, and Footstep creation tools are designed to increase your output.

See also