IK Key Info Rollout

Select the Biped > Motion Panel > IK Key Info rollout

This rollout is the heart of the IK system. Controls here allow you to set IK constraints and pivots for the biped hands and feet. There are three preset set key buttons that apply the most common IK constraints. A biped limb can be put into the coordinate space of the world or an object in the scene as well as body space.

Inverse Kinematics

Footstep and freeform animations use similar IK constraints and extensions. All edits become unified as keyframe based in character studio 3. This means that in a footstep animation you can edit keys to change footstep duration. Deleting and inserting keys or changing IK space or IK blending will alter footstep duration also. By definition a footstep in both a Freeform and Footstep animation is the start and end of a sequence of IK constraints in World Space with an IK Blend value greater than 0. If this rule is adhered to then you can convert easily between footstep and freeform animation using Convert on the General rollout.

Sliding footsteps are understood as footsteps with moving IK constraints. A sliding footstep is created by using the Sliding Key set key button, which automatically sets IK Blend to 1 and turns off Join to Previous IK Key.

In cases involving edits that alter the length of ballistic intervals (biped is airborne), the software insures that there is a vertical key occurring at the liftoff and touchdown frames in order to calculate the correct ballistic motion, so vertical keys are inserted if needed.

Footstep constraints are implemented with a pivot-based IK system. This allows you to pivot a hand or foot around a selected pivot. In a walking motion you can select a pivot on the ball of a foot and rotate the foot around it for example.

Since both footsteps and freeform use the same IK system:

Converting from freeform to footstep will always preserve the IK constraints. In fact, if there is no Biped dynamics at work during ballistic motion (that is, the Dynamics Blend for vertical keys is zero), the conversion to footsteps will not change the motion at all. Therefore, it is often desirable in the workflow to convert to footsteps to simply move clusters of IK pivots around using footstep edits. You can then convert back to freeform without losing the coherence of your work.

Unlike previous releases of the software, you are free to edit the IK pivot constraints of the footstep. This means that a footstep is best thought of as a convenient "gizmo" for moving IK pivots. The IK pivots will move as if they are "linked" in the coordinate space of the footstep, but they no longer need to be rigidly locked onto the footstep's plane. In cases where you want to eliminate foot sliding, the "Join to Prev IK Key" (Join to previous IK key) in the IK system itself provides for precise control.

When motion capture files are imported, the IK pivots can now be computed from the footsteps. Using the default sliding footstep parameters, the extracted footsteps will not alter the coherence of the raw data coming in. In cases where the motion capture data is already of high quality, the footstep extraction will retain its precise motion. This allows you to edit the motion with footstep operations, even if the data is not key reduced.


To put the biped legs into world space

In a freeform animation (no footsteps), putting the character's feet into the coordinate space of the world during footsteps will simplify the process of animating the biped. Setting a planted key anchors the foot to world coordinate space and prevents it from sliding.

If you are manually creating a walk or run then you should alternate the IK constraints for the biped feet. If a foot is on the ground use Set Planted Key or Set Sliding Key. If a foot is in the air between footsteps then use Set Free Key.

If you plan on converting a freeform animation to a footstep animation later, the keys representing footsteps should have IK blend equal to 1. IK constraints are used by the Convert command in the general rollout to extract footsteps.

  1. Select a leg on the biped.

  2. On the IK Key Info rollout click Set Planted Key.

    This turns on Object, sets IK Blend to 1.0, and turns on Join to Previous IK Key.

  3. Test the leg by moving the center of mass up and down, the foot should stay planted on the ground as the biped body moves up and down.

To set a pivot point for a hand or foot rotation

  1. Select a hand or foot.

  2. Find or set a keyframe that is in world or object space.

    A key that has an IK Blend of 1 with object selected is world space, if an object is also selected and displayed in the Object Space Object field then the hand or foot is in object space. You can click Set Planted Key to place a limb into world space.

  3. Turn on Select Pivot and select one of the displayed pivots in the viewports.

  4. Turn off Select Pivot and rotate the limb about the pivot.

  5. Set a key.

  6. Click Select Pivot to turn it off.

Using IK Blend to vary limb trajectory

Default trajectory for the hand between two keys. In the Kinematics group, Body is selected and IK Blend=0 for both keys.

Trajectory for the hand when both keys are set with IK Blend=1. In the Kinematics group, Body is selected and IK Blend=1 for both keys.

With IK Blend=0 for the first key and 1 for the second key, the trajectory starts as a forward kinematic motion (curve) and ends with inverse kinematics.

To link the biped hand to an object using Object Space Object

This attachment is set on a per key basis and can be blended. Blending would work well if the character was catching a ball, as the blend value increases between two keys the hand moves to match the velocity of the ball.

  1. Select the biped's hand and move it to the correct position, relative to the ball.

  2. Click Set Key, or turn on the Animate button before positioning the hand.

    A key must exist to store Object Space parameters.

  3. Turn on Select Object Space Object (button with the arrow).

    Select the ball in a viewport.

    In the viewports, the cursor changes to a plus sign.

  4. On the IK Key Info rollout turn on Object in the Kinematics group; set IK Blend to 1.

    Anywhere the ball moves, the hand will follow at this keyframe. The duration of the attachment is created when another key is set with similar parameters at a different frame or time.

    Turning on the anchor, on the Keyframing rollout, for the selected biped limb will lock the hand to the object until further keys can be set to establish the period of attachment. Anchors are an interactive aid for keeping attachments in place while you are moving either the object, the body of the biped, or the time-slider, which can change both elements simultaneously. You should turn anchors off once your keys have been set that perform the desired attachments. The anchor buttons are on the Keyframing rollout, on the Motion command panel.

    Anchors are no longer critical. By using Set Planted Key on two consecutive key frames the limb will snap to the first keys location.

To activate an anchor, do the following steps in either order

To set keys that define animatable IK attachments

  1. At the frame for the start of the attachment, move your hand or foot to the desired location in relation to the Object (or World Space, if no object is specified)

  2. On the Keyframing rollout, set an anchor for that limb so that the hand or foot remains attached in the same place.

    This step is optional since you can also snap to the same pivot point by checking the Join to Prev Key option later on in step 5.

  3. On the IK Key Info rollout, click Set Key with IK Blend=1.0 and Object Space chosen or, as a shortcut, click the more convenient Set Planted Key button.

  4. Click on Select Pivot Point and then select one of the end effector's red spheres displayed in the viewpoint

  5. Move to the frame of detachment and reposition the hand or foot (if desired).

  6. Click Set Key with IK Blend=1.0 and Object Space chosen or, as a shortcut, if you wish to snap to the same pivot point as the previous key, click Set Planted Key button again. This will turn on the Join to Prev Key checkbox. If you wish to allow the pivot point to move, click Set Sliding Key. This will turn off the Join to Prev Key checkbox.

  7. If you used anchors, turn off the anchors and move between the attachment and detachment keyframes, noting whether the hand or foot is attached for the desired interval.

  8. If the hand or foot detaches during the interval you should delete the default keys for the attached limb in the interval between the start and end attachment keys.

    Note: The attached hand or foot may drift from its attach point as the object tries to maintain a smooth, continuous trajectory. To insure linear motion between keys, you should also set Continuity to 0 for the keys defining the attach interval. You can also adjust the tension to 50 to eliminate any undesired overshooting.

    You can continue to animate each limb's IK attached object conventionally -- moving or otherwise transforming it in different keyframes. The arm or leg moves to follow it using the kinematic solution specified by the IK Blend setting.

    Note: The procedure above can be embellished by setting more than two consecutive keys with IK Blend=1.0 in order to move the hand or foot over the object in some specific way during the attachment. Also, subtle changes in the dynamics of the attachment are possible by manipulating the IK Blend values.

    An eased detachment can be created, for example, by setting the key that follows the detach key to be in Object space with IK Blend=0.0. This will cause the blended IK values to remain in Object space while the IK Blend values ramp gradually down from 1.0 to 0.0.

    Saving a .bip file from a motion containing IK Blend keys will store the IK Blend information in the file. When the .bip file is loaded onto a new biped, the IK Blend settings and the body/object state are mapped onto whatever Object Space Object is already defined for the existing biped. If no Object Space Object is assigned, then the IK Blend setting will be stored in World space.

    Note: You can change the position of the elbow or knee without affecting the attach point of the hand or foot. While the anchors are turned on, select the forearm or a calf and rotate it around its X axis. The hand and foot stays put while the limb is rotated.

    Warning: You must save the animation as a 3DS MAX file in order to save the animation keys for the Object Space Object.


Footsteps will adapt automatically to changes made in IK values of leg keys. In Freeform and Footstep animation a footstep interval is the start and end of a sequence of IK constraints in World Space with IK Blend > 0.

Next Key-Previous Key: Find the next or previous keyframe for the selected biped part.

The field displays the key number.

Set Key: Use either Set Key or turn on the 3DS MAX Animate button to create keys when you are moving biped objects.

Set Planted Key: Set a biped key with IK Blend set to a value of 1, Join to Previous Key turned on and Object selected.

In a Footstep or Freefrom animation all footsteps that do not slide should have Join to Previous Key turned on.

Set Sliding Key: Set a biped key with IK Blend set to value of 1, Join to Previous Key turned off and Object selected. This creates a sliding footstep. Sliding footsteps display in the viewports with a line running through the middle of the footstep. Sliding footsteps are understood as footsteps with moving IK constraints.

In a Footstep or Freeform animation if the foot slides rather than being planted then use Set Sliding Key.

Set Free Key: Set a biped key with IK Blend set to a value of 0, Join to Previous Key turned off and Body selected.

In a Footstep or Freeform animation a biped leg in a move state should have a "free" key.

Time: Enter a value to specify when in time the key occurs.

Use this to fine tune keyframe timing on a character by moving a key backwards and forwards in time.

Ankle Tension: Adjusts the precedence of the ankle joint over the knee joint. When set to 0, the knee takes precedence. When set to 1, the ankle takes precedence.

This effect is only visible between keyframes.

Select Pivots: Turn on to select pivots on the biped hands and feet to rotate around. After a pivot is selected in the viewports turn off Select Pivots and rotate the hand or foot.

Pivots are only available if the biped hand or foot is in world or object coordinate space.

Join to Previous Key: Turn on to put the biped foot in the coordinate space of the previous key. Turn off to put the biped foot into a new reference positon.

Turn off and move the biped foot to create a sliding footstep for example.

IK Blend: Determines how forward kinematics and inverse kinematics are blended to interpolate an intermediate position. Using an arm to move a hand is an example of forward kinematics. Using the hand to move the arm is an example of inverse kinematics.

Activates when a biped arm or leg (hand and foot) key is current.

Body: The biped limb is in biped coordinate space.

Object: Object Space, the biped limb is either in World coordinate space or the coordinate space of the selected object. Coordinate space can be blended between keys.

Select Object Space Object (button with an arrow in it): Link a biped hand or foot to an object in your scene.