In traditional keyframe animation, realistic-looking motion for trajectories can be difficult and tedious to achieve. The animator would need to set multiple keyframes along an arc in an attempt to create a realistic trajectory, and changing any properties of the trajectory (for example, height or distance) would require the animator to update several poses and keyframes along the arc.

With the use of physics-based motion, animators can roughly key important poses, and the motion and spacing between poses are replaced with physics-based trajectory. This can significantly speed up and simplify the animation workflow when working with trajectories in your scene.

Houdini’s Projectile Motion and Pendulum Motion SOPs are used to determine physics-based motion. These nodes take as input the character skeleton animation, and replaces sections of the animation with projectile/pendulum motion. These nodes change the character’s position, but not the character’s pose.

In a basic setup for keyframe animation, you start with a character animation rig, set keyframes on the rig, add the projectile/pendulum motion, and put the motion back onto the rig.

In this example, we have a simple keyframed animation of a character jumping.

1. Set up the node network:

   • Connect the character animation rig node (animated skeleton output) to the Projectile Motion SOP (1st input). Keyframes are set on the character animation rig for the jump motion.

   • Connect the Projectile Motion SOP (1st output) to another copy of the character animation rig node (animated skeleton input). This second character animation rig will have the animation with projectile motion.

2. On the Projectile Motion SOP, the Joint Group is set to the locomotion joint of the skeleton. The locomotion joint is the joint that defines the motion of the entire skeleton. It usually has a name like C_COG.

   See How-to for more information on using specific joints to determine the projectile/pendulum motion path.

3. When the Projectile Motion SOP is first plugged into the network, the Projectile Sections multiparm is empty, so no projectile motion is added to the animation. To add projectile motion:

   • Click beside the Projectile Sections multiparm. This will add a tab of parameters for each new section of projectile motion.

   • Turn on Create Projectile Motion.

   • Set Range to Replace with the range of frames in the original animation that you would like to replace with projectile motion.

     Tip

     Select the first character animation rig node to see the frames that are keyframed, and then use those frames in Range to Replace.

In this example, we have an animation of a soccer ball being juggled. Keyframes are set for the different ball positions in the ball_positions HDA.

Turn on the display flag for ball_positions to see the original keyframed animation (soccer_juggler is templated in the below animation).

1. On the Projectile Motion SOP, set Joint Group to the joint at the center of the ball.

2. To quickly add projectile motion between multiple keyframes, use the Bake option on the Projectile Motion SOP:

   Click the Bake button. In the Projectile Motion Baking window:

   • Bake To defaults to the current Projectile Motion SOP.

   • Set Bake From to the character animation rig with the keyed positions. In this example, it is ball_positions.

   • The Frame Start and Frame End values are filled in with the frame range of the animation by default.

   • Click Bake All.

In the Projectile Motion SOP, a section of projectile motion is created between each key in the Bake From node. Each projectile motion section has a tab of parameters in the Projectile Sections multiparm. To remove a section of projectile motion, click in the tab you want to remove.

The projectile motion output is wired back into the ball animation rig, ball_positions2. Turn on the display flag for ball_positions2 to see the updated animation with projectile motion.

In this animation, keys are set on the character animation rig to create a swinging animation:

The following physics-based motion is added to the animation based on the approximate path of the original animation:

• Frame 1-40: pendulum motion

• Frame 40-50: projectile motion

• Frame 50-100: pendulum motion

In this example, we add physics-based motion to a motion retargeting workflow. We start with a rough jump animation that is transferred onto a capybara geometry.

The rough jump animation is stored in a Stash SOP as a motion clip. The motion from the Full Body IK SOP is then transferred onto the capybara.

Add physics-based motion by connecting the Projectile Motion SOP between the Full Body IK SOP and the Bone Deform SOP. Optionally connect the 2nd output of the Projectile Motion SOP to the Merge SOP to see the path of the projectile motion in the final output.

On the Projectile Motion SOP:

• Click beside the Projectile Sections multiparm to add a new section of projectile motion.

• Turn on Create Projectile Motion.

• Set Range to Replace to the frame range in the original animation to replace with projectile motion.

• Adjust the Start Position as necessary. Turn on Shift Animation Before Projectile so that the animation before the section of projectile motion transitions smoothly into the projectile motion.

• Adjust the Target Position as necessary. Turn on Shift Animation After Projectile so that the projectile motion transitions smoothly into the remaining animation.

Multiple projectile and pendulum motion sections can either be put into one Projectile Motion or Pendulum Motion SOP, or the different sections can be kept in their own separate node.

In the node network below, the Projectile Motion and Pendulum Motion SOPs on the left each contain one section of projectile/pendulum motion. In pendulummotion3 on the right, there are 2 sections of pendulum motion, with the Range to Replace parameters set to those in pendulummotion1 and pendulummotion2.