Animating Fall Stunts with Falling Bodies and Softimage|3D

SOFTIMAGE^®|3D version 3.7 or later on NT/Intel is required for this tutorial.

  Download free demo version of Falling Bodies plug-in with tutorial scene (1.0 MB)
View completed movie (tutorial.avi) (1.5MB)

The fall stunt is a cliche of action movies and video games, and a tough bit of character animation. Fall stunts are hard to animate with keyframes, because the collisions with obstacles are hard to get right. Motion capture is hard on the actors. With Falling Bodies, fall stunts can be animated quickly and accurately. This effect is achieved in four steps.

1. Built a character with IK chains that has "anatomy" (which joint connects to what) matching a standard anatomy supported by Falling Bodies.
2. Set joint limits on the IK joints that limit the range of motion appropriately.
3. Animate the character using keyframes or motion capture up to the beginning of the fall
4. Use Falling Bodies to animate the fall.

Building the character

For this tutorial, a pre-built scene, "Tutorial runfall-start" is provided. This scene is included in the download of the demo version.A character is provided; it's basically the standard Softimage|3D motion capture model.

You can change anything in the background, but when using this demo version of Falling Bodies, you can't change the character, other than moving him and bending him at his IK joints. For details on character-building, see the Falling Bodies manual.

Setting the joint limits

Joint limits keep body parts connected to the same joint, like an upper arm and forearm, from going through each other. Everything else is handled by collision detection, which is complete and fully automatic.

Joint limits are set by selecting an IK joint and then using Motion->Constraint->Rotation Limits. The damping width should always be zero, and the damping strength should usually be 1.0. The joint limits should be set according to what real bodies can do.

Falling Bodies needs the last 2 degrees of rotation in each joint to slow down swinging limbs.So allow at least 4 degrees between MIN and MAX, and when animating, the last frame before Falling Bodies starts animating a fall can't have any joint within 2 degrees of its limits.

This character already has joint limits set.

Animating the character up to the fall

The character is running along a railed catwalk, away from something behind him. He doesn't see the spiral staircase ahead. He runs off the end of the catwalk onto the spiral staircase and falls down the stairs, gets caught in the railing.

• Start by creating a simple run cycle. Save explicit translation keyframes for the top element of the hierarchy, and rotation keyframes for all IK joints. Remember to save all rotations for joints that have three rotations, like hips and, in this model, clavicles. Save only the Z rotation for joints that have only one rotation. Don't save any other keyframes for the character, or your fcurves and those generated by Falling Bodies will both be pulling on the character at the same time, in different directions. Falling Bodies will produce error mesages if you get this wrong.
• Duplicate the run cycle enough times that the character runs off the end of the catwalk.

Using Falling Bodies to animate the fall

• Move the play control slider to the first frame at which both feet are clear of the ground just before reaching the stairs. The fall will start from here.
• Set the play control to allow another 100 frames or so after that frame.
• Select some part of the character and use Motion->Effects->Falling Bodies.
  The fall will be animated automatically, with automatic collisions and dynamics. Falling Bodies will automatically match the newly generated motion to the keyframed motion, matching the position and velocity of the character and all joints.
  

More fun things to do

Once you've tried the basics above, you can try some other things. A few suggestions:

• Try making the little guy hit the stairs at different speeds. The difference between the frame at which you start Falling Bodies and the previous frame determine the velocities, so you can keyframe different speeds. If the little guy is going fast, he'll go over the railing. If he's going slow, he'll slide down the stairs. Somewhere in between, he'll end up doubled over, sliding down the railing.
• Give the little guy different initial rotation rates. You can get a backflip that way.
• Try removing a step from the staircase or a section from the railing.
• Try the little guy in some scene of your own. Falls and slides will work.
• You can change the elasticity and friction of materials with Dynamics->Physical Properties. The elasticity and friction of both colliding objects are used in the collision.

Troubleshooting

There's a Falling Bodies manual if you need it.

The message "You can't do that with the demo version" usually means you changed the character's geometry.

Messages like "axis Z of joint clavicle-D within 2 degrees of joint limit" mean that the indicated joint is too close to the joint limit. Falling Bodies needs the last 2 degrees of joint travel to slow down moving limbs. It's easy to get this if you use IK to position joints and IK was up against a joint limit. You may have to back off a joint angle a bit for the frame at which Falling Bodies gets control.

Sometimes Falling Bodies has to slow down for a difficult collision, but it usually speeds up again within a minute or two. These are the situations where most other dynamics systems break or give up.

Contact support@animats.com if you get stuck.

Animating Fall Stunts with Falling Bodies and Softimage|3D was created by John Nagle of Animats.

This tutorial is an example of the material that will be available to Softimage^® Full Support customers as of January 1998 in the new Technique section of softimage.com. Technique will feature Tutorials, Demo Scenes, Plug-Ins, Shaders, and Standalones from animators, programmers, educators, and technical support engineers at Softimage.

If you are currently a SOFTIMAGE|3D user, you can find out about the status of your Softimage Full Support contract from your Softimage Reseller. If you are unable to contact your reseller, Anick @ i-anickg@microsoft.com will be glad to help you with any questions.

To receive further information about Softimage products, please use the Softimage Info Request Form.

To receive further information about Animats products, visit www.animats.com, or mail to info@animats.com

Questions or comments about the Softimage tutorial series can be sent to Robert McFadden, SupportContent Webmaster @ robmcf@microsoft.com.

©Copyright 1997 Animats.  All rights reserved.
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