Tutorial 3: Animating a Biped with Footsteps

Lesson 4: Animating a Pratfall

In this tutorial you will animate the biped slipping on a virtual banana peel. It will teach you to change rigid footsteps to sliding footsteps and will also give you an introduction to creating a freeform period in a footstep animation using Track View.


  1. Load cs3_tut03_slipstart.max.

  2. Change the display to Wireframe.

  3. Select any part of the biped.

  4. Open the Motion panel.

    The Biped controls are now available.

Understanding the animation

  1. Play the animation by dragging the time slider.

    The biped walks about 5 steps

    You're going to make the biped slide on footstep 5. Imagine a banana peel on that footstep.

  2. At frame 75, select the blue Bip01 L Foot object.

    Frame 75 is where the heel hits the ground for footstep 5.

  3. On the Motion panel, open the IK KeyInfo rollout.

  4. Zoom into the viewport so you can see the foot easily.

    Heel touches down

    In the viewport, the pivot point is displayed in red, and the track bar displays keys. The key for the foot at frame 75 is a planted key. This means IK Blend is set to 1 and Join to Prev IK is on.

    IK Blend at 1 locks the pivot and the foot to the ground plane. Join to Prev IK Key considers the rotation of the foot at the previous key and attempts to match it.

  5. Continue to scroll through the animation, watching the pivot points move.

    The pivot point animates.

Creating a Sliding Footstep

  1. Turn on Key mode and go to frame 75.

  2. In the IK Key Info rollout, choose Set Sliding Key.

  3. Advance to the next key (at frame 78).

  4. Rotate the foot so the toes remain up in the air.

    Rotate the foot up.

  5. Move the foot forward, so it slides up on the footstep gizmo.

  6. In the IK Key Info rollout, choose Set Sliding Key.

    The pivot point jumps to the foot's new position. The footstep is now displayed with a line through it. This indicates it's a sliding footstep: the foot can slide over or off the footstep (and the ground plane).

  7. Select the foot again.

  8. Advance to the next keyframe (84).

  9. Set a sliding key.

  10. Choose Select Pivot and change the pivot from the toe to the heel.

  11. Turn off Select Pivot.

  12. Rotate the foot, then raise it up in the air, so the foot is starting to kick upward.

    The foot at frame 84

  13. Set a sliding key.

    Again the pivot point jumps from the footstep to the foot.

    If the foot is off the footstep and is in the air instead, use a free key. A sliding key, however, will also let you lift the foot off the footstep.

  14. Move the time slider to play the animation.

  15. The biped's left foot slides over and off footstep 5.

    Next you'll create a freeform period so you can freely animate the biped's movement in the air.

Creating a Freeform Period

Using a freeform period in a footstep animation allows you to suspend the automatic systems Biped uses during footstep animation.

If you set up a freeform period between footsteps 5 and 6, you'll be able to animate the biped slipping, falling and bouncing on the ground plane.

  1. Select the biped's center of mass.

  2. Turn on Footstep mode.

  3. Select a footstep in the viewport.

  4. Right-click the footstep and choose Track View Selected.

    Track View opens.

  5. Expand Bip01 Footsteps.

  6. Double-click footstep 6 in Track View.

    Footstep 6 highlights in white.

  7. In Track View, move footstep 6 so it is directly under footstep 7.

    Create a gap for a freeform period.

    The footsteps display the starting and ending frames in Track View. As you move the footstep in Track View, these frame numbers change.

    Tip: You can also change the duration of the footstep by using the Footstep Edge Selection arrows in the Footstep mode dialog. Select an edge to move that edge. Select the dot in the center to move the footstep in time, without changing its duration.

  8. Right-click the Footstep Track.

  9. In the Bip01 Footstep Mode dialog, turn on Edit Free Form (no physics).

    In Track View a hollow yellow box appears to the right of footstep 5.

    Click the hollow box.

  10. Click inside the hollow yellow box.

    The box becomes solid yellow. This indicates it is a freeform period.

  11. Scrub the time slider and watch the animation.

    The biped floats from footsteps 5 to footstep 6.

Changing the timing of the slip

The slip happens too slowly. You can change the timing of the sliding footstep so the freeform period can start sooner. Also, you need more time for the body to rotate and bounce on the ground.

  1. Click Time Configuration in the Time Controls.

  2. Change End Time from 123 to 150. Click OK.

    This extends the animation by adding 27 blank frames.

  3. Turn on Edit Footsteps in the Bip01 Footstep mode dialog.

  4. In Track View drag a selection rectangle around footsteps 6 and 7.

  5. Move footsteps 6 and 7 to the right so the yellow period is extended and the footsteps 6 and 7 don't start until frame 140.

    The last two footsteps extend beyond the end of the animation.

  6. Press ALT + R.

    The end of the animation now matches the end of the footsteps. The time slider now shows the animation has been extended to 158 frames

    Tip: Be sure the Plug-in Keyboard Shortcut Toggle is on, or the Biped keyboard shortcuts won't work.

  7. Click footstep 5 in Track View.

  8. In the Footstep mode dialog, in the Footstep Edge Selection group, click the right arrow.

    A white dot appears at the right side of the box for footstep 5.

  9. Adjust the right side so the footstep ends at frame 83.

  10. Play the animation. The biped slides faster over the footstep.

    Since you're still in Footstep mode, you can also move the footstep's position in the viewport.

  11. Select footstep 6 in the Perspective viewport and move it next to footstep 7.

  12. Move the time slider to observe the animation.

    Now you have enough time and space for the pratfall.

Keyframing the Fall

To keyframe the fall, you'll rotate the center of mass so the biped moves from vertical to horizontal. You'll also move the center of mass so the biped rises up and then falls to the ground plane. You'll turn off Dynamics Blend, so Biped physics are not used in the animation.

  1. Choose Edit Free Form (no physics) in the Bip01 Footstep Mode dialog,

  2. Close the Track View window.

  3. Turn Animate on.

    Since the center of mass doesn't use the planted, sliding, or free keys, you can just use the Animate button to set keys automatically.

  4. Turn off Footstep mode.

    Now you'll be able to see keys in the track bar.

  5. In Track Selection, select the Body Vertical Track.

  6. At frame 78, click Set Key on the Key Info rollout.

  7. On the Key Info rollout in the Body Dynamics group, change Dynamic Blend to 0.

    This turns off Biped Dynamics and applies Spline Dynamics instead. Biped Dynamics adds physics to the animation. Spline Dynamics doesn't. Dynamics Blend is only available on Body Vertical track keys.

  8. Move to frame 84.

    The left foot has a planted key at this frame.

  9. Move the center of mass forwards a little.

  10. Rotate the center of mass slightly so the biped begins its pratfall.

    The biped at frame 84

  11. Select the blue foot and set a sliding key.

  12. At frame 86 rotate the center of mass so the biped is horizontal.

  13. Select and move each foot so it's raised up and spread apart.

  14. Select and move each arm so it's positioned out from the body.

    The biped at frame 86

    The biped's feet will swing up for a frame or two after the rest of the biped begins to descend.

  15. At frame 88 move each arm and foot up some.

    Be sure Animate is on, otherwise set free keys for these movements.

  16. Rotate the feet a little so the toes point upwards.

    This adds some secondary motion to the animation.

  17. At frame 97 select and move the center of mass so the biped is close to but slightly above the ground.

    This creates the descent of the horizontal biped to the ground.

  18. Use the time slider to flip through the biped's slipping motion.

Adding a bounce

Next you'll add keyframes to make the biped strike the ground and bounce once. There is a Body Vertical key set at frame 115 you need to reset.

  1. Select the Body Vertical Track and move the time slider to frame 115.

  2. Make sure Footstep mode is off, then change Dynamic Blend to 0.

    Note: If Dynamic Blend is unavailable, click Set Key first. Dynamic Blend is only available where there are Body Vertical keys.

  3. Rotate the biped so it's horizontal.

  4. Move the biped to the ground.

  5. Keyframe the center of mass so the biped lifts off the ground at frame 102.

    Add secondary movement by keyframing the hands to trail the movement of the center of mass.

  6. A few frames after the center of mass rises, raise the hands.

  7. A few frames after the center of mass strikes the ground, make the hands strike the ground.

    Next you will keyframe the spine.

  8. Rotate the spine so the shoulders touch the ground at frames 95 and 110.

    Next you will keyframe the head.

  9. Rotate the head so the chin tucks in at frame 95 and the head rocks back at frame 102.

    Biped position at frame 95

  10. At frame 95, select the center of mass.

  11. In the Key Info rollout change the Tension and Continuity to 0.

    This sharpens the bounce.

    Tip: If the Tension and Continuity curves are not available, click previous key, then next key to return to this keyframe with the curves displayed.

    Tension and Continuity at zero

  12. Play the animation.

    The biped bounces more crisply.

    Biped position at frame 115

  13. Save your work to myslip.bip. Compare your work to slip1.bip.

    The timing is still off a bit, and the biped stands back up when the animation finished. Since character studio lets you change back and forth between footstep and freeform modes, you can convert between animation modes to correct this.

Converting from footstep to freeform

You can convert between the footstep and freeform modes without any loss of motion. The motions will remain identical, whether you convert from footstep to freeform or freeform to footstep.

  1. Continue in the current file, or load cs_tut03_convert.max.

  2. Turn on Animate.

  3. Select the biped's center of mass.

  4. In the General rollout, choose Convert.

    Do not turn on generate a keyframe per frame. Choose OK.

    The footsteps disappear from the viewport, but the animation is the same as before.

    Next use Copy Pose to eliminate the biped standing back up.

  5. At frame 115 select the entire biped.

  6. In the track bar select all the keys after frame 115.

  7. Delete all the selected keys.

Changing the timing

  1. Be sure Animate is still on.

  2. Select the entire biped.

  3. In the track bar, drag a box to select all the keys from frame 86 to the end of the animation.

  4. Move the time slider to frame 89.

  5. Slide all the keys down so the keys from frame 86 are now at frame 89.

    Tip: To move keyframes to a precise location, first select the keys. Then move the time slider to the desired location, then move the keys.

    The pratfall lasts a little longer now.

    If you like, you can add some more rotations to the head after frame 115. While the biped is laying on the ground make his head shake with disbelief. Also try keyframing some small movements in the feet, placing the feet flat on the ground and sliding them toward the pelvis, raising the knees.

  6. Save your work as myslip.max, or load slip2.bip to see the results.