Autodesk 3ds Max Tutorials > Character Animation Tutorials > Biped Beyond the Basics > Interacting with Objects > 
Creating the Illusion of Weight
 
 
 

There are two techniques for giving the illusion of weight to a biped object in an animation. Both affect the biped center of mass.

The first technique uses Balance Factor, which moves the center of mass. Balance Factor is available on the Body Horizontal track. This technique creates the illusion of lifting a heavy object. It lets you keyframe the center of mass moving in and out of the body.

The second technique uses Figure mode to turn on Rubber Band; you then move the center of mass in front of or behind the body. This technique creates the illusion of the biped pushing or dragging a heavy object. You can’t keyframe the center of mass with this technique; instead, you set it for the entire animation.

Lifting Heavy Objects

Set up for this part of the lesson:

  1. Open balancefactor_start.max.

    Two bipeds have planted keys set on their feet, with the pivot points set at their toes.

  2. Select any part of the biped on the left, and then open the Motion panel.

Use Balance Factor:

  1. On the Track Selection rollout, click the Body Horizontal button.
  2. In the Key Info rollout, expand the Body expansion bar so you can see the Body parameters. Then set Balance Factor to 0.

    The Balance Factor is available because this file contains a keyframe on the Body Horizontal track at frame 0.

    TipMany parameter settings will not be available unless you have a key set on a particular track. If a parameter is unavailable, click Set Key in Key Info rollout and see if it becomes available.
  3. Move to frame 15, then turn on Auto Key mode.
  4. Select Bip01 Spine object (the first, or lowest, spine object).
  5. Use Arc Rotate to orbit the viewport so you can see the biped from the side.
  6. Rotate the spine about the Z-axis.

    Notice that the upper body rotates, while the legs stay firmly planted.

    Only the upper body rotates.

Animate the Balance Factor:

  1. Select any part of the other biped.
  2. In the Track Selection rollout, select Body Horizontal.
  3. In Key Info rollout, click Set Key.

    Now Balance Factor is available.

  4. Change Balance Factor to 2.
  5. Select Bip02 Spine. Rotate this spine.

    This time the hips move back as the torso rotates forward. If you rotate the torso enough, the feet move off the floor.

    Notice also that the center of mass is now in front of the body.

    Hips move back, torso rotates forward.

  6. Move to frame 0, and select Body Vertical in the Track Selection rollout. In the viewport, use the Transform gizmo to move the center of mass down, so the knees are bent.
  7. At frame 15, also move the center of mass down so that the knees stay bent.
  8. Use the time slider to view the motion. Notice that you no longer have the original standing poses at frame 0.
  9. Go to frame 0. Rotate Bip02 Spine so the standing pose is re-established. Adjust the COM so the knees are not bent.

  10. At frame 23, move the COM way down so the chest goes right through the leg (this will look wrong, but don't worry). Then rotate the Bip02 Spine so the knees almost touch the chest.

    TipThere's nothing that prevents biped body parts from intersecting. Check your animation for these intersections visually and correct by adjusting keyframes.
  11. At frame 30, move the center of mass so the biped starts to lift the imaginary object using its legs, rather than its back.

  12. At frame 38, in the Track Selection rollout, click the Body Horizontal button.
  13. On the Key Info rollout, click Set Key.
  14. Change the Balance Factor to 1.

    The center of mass moves back closer to the biped.

  15. Select and rotate the spine.

  16. At frame 45, rotate the spine more.
  17. Move the center of mass so the biped stands up straight. Now the center of mass is back inside the body.

  18. Move the time slider back and forth to view the animation.

    Watch how the center of mass moves outside the biped, then back again.

    The biped appears to be lifting something heavy because of the positioning of the knees and spine.

  19. For extra credit, finish the animation of the other biped. Save your work as my_balancefactor_final.max, or open balancefactor_final.max for comparison.

Pushing Heavy Objects

Set up for this part of the lesson:

  1. Open pushbox_start.max.
  2. Play the animation.

    The biped is pushing a box along the floor. Notice that the center of mass is behind the biped.

Adjust the center of mass with Rubber Band:

  1. Select any part of the biped.
  2. In the Motion panel, in the Biped rollout, turn on Figure mode.

    The biped moves so his hands are no longer touching the box. Notice that, for this figure, the spine objects are rotated so the biped has a rounded back.

  3. In Track Selection rollout, click Body Horizontal.
  4. Expand the expansion bar in the Biped Rollout. In the Modes group, turn on Rubber Band Mode. Then move the biped’s center of mass so it's back inside the body.
  5. Turn off Figure mode and play the animation.

    The upper torso moves back over the feet. The illusion of weight is diminished.

  6. Turn on Figure mode. Move the center of mass far behind the body, and then turn off Figure mode.

    The biped leans into the box, as though the box were quite heavy.

  7. Play the animation.

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