Animation clips are combined and edited in the motion mixer pane, where animation and motion effects for multiple characters are displayed along a timeline. The motion mixer pane is populated with tracks and clips if you have a Motion Mixer SOP selected in the network editor. If you have another SOP selected, the motion mixer pane will be empty.

In this example, we input the Electra test geometry into the motion mixer. The input to the Motion Mixer SOP must be an animation scene in a character folder structure:

1. On the Electra SOP, set Output to APEX Scene. This outputs Electra as a character in an animation scene.

2. Select the Motion Mixer SOP.

3. Open the motion mixer pane by clicking the New Tab icon at the top of a pane and selecting New Pane Tab Type ▸ Animation ▸ Motion Mixer.

4. When you first open the motion mixer pane, it displays one track. The top left of the pane displays a hierarchy of elements (tracks and clips) that are available in the motion mixer. If you select the motion mixer (top level) element in the hierarchy, the parameter editor in the motion mixer pane displays the top-level parameters in the Motion Mixer SOP:

   

   The parameter editor in the motion mixer pane is a filtered subset of the parameters in the Motion Mixer SOP. For example, if you click the track0 element in the hierarchy, the motion mixer parameter editor displays the same parameters that appear in Houdini’s regular parameter editor under the Mixer ▸ Track 0 section.

See the motion mixer pane for descriptions of the toolbar buttons.

Tracks, clips, and weights ¶

A track is a way to group multiple animation clips for a character, and is represented as a row on the timeline. A track can contain multiple animation clips.

Each track has an associated weight that is represented as a row below the track. The weight determines the extent to which the clips in the track contribute to the overall animation.

The headers in the motion mixer pane provide settings for configuring the tracks and weights. There is also a click menu on the track header, which includes some of the functionality provided by the track header icons.

To have access to animation clips in the motion mixer, there must be animation in the input scene to the Motion Mixer SOP. The animation clips must be added to the /animation folder in the input scene’s character folder structure.

Add animation from separate APEX Scene Animate SOPs ¶

If you have existing animation on multiple APEX Scene Animate SOPs, you can copy all the animation into one scene using an APEX Scene Copy Animation SOP or Packed Folder Copy SOP, then input the scene into the Motion Mixer SOP.

APEX Scene Copy Animation SOP ¶

In this example, we create animation on multiple APEX Scene Animate SOPs, copy the animation into one scene, and pipe the scene into the motion mixer:

1. On the Electra SOP, set Output to APEX Scene. This outputs Electra as a character in an animation scene. In the rig tree view, we can see the following character folder structure:

   -- electra.char/
   ...
   

2. When animation is performed in the animate state (on an APEX Scene Animate SOP), an /animation folder is created in the SOP’s character folder structure and the animation is stored in a clip folder named default.clip. After adding animation, the two APEX Scene Animate SOPs have the following character folder structure:

   -- animation/
       -- default.clip/
   -- electra.char/
   ...
   

3. The APEX Scene Copy Animation SOP copies the animation from the second input’s scene to the first input’s scene. In the To parameter, set the new name for the second input’s animation, for example, /animation/punch.clip. We now have the following character folder structure on the APEX Scene Copy Animation SOP:

   -- animation/
       -- default.clip/
       -- punch.clip/
   -- electra.char/
   ...
   

4. In the motion mixer pane, the default (jump) and punch animations appear in the list of available clips:

   

Packed Folder Copy SOP ¶

In this example, we create animation on multiple APEX Scene Animate SOPs, then bring the animation into the character folder structure and rename them using a Packed Folder Copy SOP:

1. On the Electra SOP, set Output to APEX Scene.

2. Add the jump animation clip to the character folder structure. On the Packed Folder Copy SOP:

   • Drag and drop the APEX Scene Animate SOP, jump_animation, onto the Node parameter textbox. This automatically populates Node with the path to the jump animation, ../jump_animation.

   • Set the From parameter to the jump animation clip in the jump_animation node, /animation/default.clip. Use the rig tree view to see the character folder structure on the jump_animation node.

   • Set the To parameter to the path in the character folder structure to copy to, for example, /animation/jump.clip.

3. Click beside Count to add another set of parameters for the punch animation. Populate the parameters.

4. In the motion mixer pane, the jump and punch animations appear in the list of available clips.

Add multiple animation using one APEX Scene Animate SOP ¶

In this example, we add two empty animation clips to the character folder structure, then populate the clips with animation using an APEX Scene Animate SOP:

1. On the Electra SOP, set Output to APEX Scene.

2. On the first APEX Scene Add Animation SOP:

   • Set Character to electra.

   • Specify a name for the Animation Clip, for example, anim1.

   We now have the following character folder structure:

   -- animation/
       -- anim1.clip/
   -- electra.char/
   ...
   

3. Do the same for the second APEX Scene Add Animation SOP, but set Animation Clip to anim2:

   -- animation/
       -- anim1.clip/
       -- anim2.clip/
   -- electra.char/
   ...
   

4. In the APEX Scene Animate SOP, add animation to the animation clips:

   • Enter the animate state - select the APEX Scene Animate SOP, turn on its display flag, and click Animate on the left toolbar.

   • If there are multiple animation clips (*.clip) in the character folder structure, a drop-down menu will appear on the top toolbar of the animation layers pane with a list of the available clips. Select the clip you want to add animation to.

5. In the motion mixer pane, the anim1 and anim2 animations appear in the list of clips.

In this example, we have a scene that contains Electra and a jump animation. In the rig tree view, we can see that the character folder structure contains the following character and animation data:

/
-- animation/
    -- jump.clip/
-- electra.char/
...

1. Add Electra’s jump animation to the track:

   • Drag and drop Electra’s jump animation onto the track:

     or

   • Move the playhead to the frame where you want to add the clip, select the jump clip, and click on the top toolbar.

   The animation is applied to Electra:

2. Drag the clip to move it along the timeline. To cycle (repeat) the animation, drag out the ends of the clip:

   The Cut Start and Cut End parameters define the part of the original clip that is played. This is shown by the clip’s light blue region in the timeline. The dark blue regions are the frames where the animation is cycled. The cycled frame range is specified in the Start and End parameters for the clip.

3. To cycle only a portion of the animation, resize the clip’s light blue region:

   

   If you don’t want to cycle the animation but instead want the character to hold the first or last pose, set the clip’s Cycle Before/Cycle After parameter to Hold.

   Note

   The Cycle Before and Cycle After parameters specify the clip behavior within the clip boundaries. To set the behavior outside the clip boundaries, use the Extend Before and Extend After parameters.

4. To smooth out the transition between the cycled regions:

   • Increase the value of the Blend Frames parameter.

   • Click the button beside the Cycle Joint parameter and select a joint on Electra to match between the two cycled regions. In our example, we pick Electra’s foot joint, ball_r.

   Tip

   If your cycled animation is veering off at an angle, set Rotations to Off in the Cycle section of the clip parameters.

5. To reposition your animation:

   • Enter the Motion Mixer viewer state by clicking on the toolbar to the left of the viewport.

   • On the top toolbar of the Motion Mixer state, turn on Handles. When you select a track or clip, a transform handle appears in the viewport. When you manipulate the handle, the Transform parameters for the track/clip are also updated. There is a separate handle for tracks and clips.

In this example, we add a second animation clip to Electra’s track:

We want the punch clip to begin where the jump animation ends, so to fix the transition between the clips, we specify a joint to match between the clip transition:

1. In the motion mixer pane, select the second clip, punch.

2. Select Edit ▸ Match Joint or click on the top toolbar.

3. In the Match Joint window, click .

4. Select the joint to match, in our example, we select one of Electra’s foot joints, ball_l.

5. Click Accept.

6. Back in the Match Joint window, click Match All. This updates the values in the Transform section of the punch clip’s parameters.

7. Display the character in different ways, for example, the character’s controls, skeleton, or outline:

   • Enter the Motion Mixer viewer state by clicking on the toolbar to the left of the viewport.

   • On the top toolbar of the Motion Mixer state, choose from the Selection drop-down menu.

   • To change the character’s color:

     • Select the clip.

     • Click the Color parameter’s color chip and select a color.

     • Turn on the Display parameter.

     If a clip is selected, it is displayed in yellow.

You can decide to use the motion from part of a character. In this example, we only want the upper body motion of the character from the previous example:

1. Select the track.

2. Select the joints you want to use the motion of - click the button beside the Joints parameter and select the upper body joints.

3. In the Blend Parent parameter, specify the parent joint you want to put the motion on top of. In our example, we set Blend Parent to the chest joint, so the swinging upper body of the jump animation is placed on the lower body of the character at rest:

Blend between different tracks ¶

You can take the motion from one track and blend it into the motion on another track. This is called track blending.

In this example, we start with a jump animation, then replace the animation on Electra’s arms with 2 clips - rest and punch:

1. Drag and drop the jump clip onto the track.

2. Drag out the right edge of the jump clip to cycle the animation.

3. Set the cycling parameters, Blend Frames and Cycle Joint, to smooth out the transition between the cycled regions.

4. Add another track - select Add ▸ Add Track or click on the top toolbar. The new track is added above the first track.

5. Drag and drop the rest and punch clips onto the second track.

6. Adjust the punch clip to play and cycle only part of the animation.

7. Select the second track.

8. Select the joints to use the motion of - click the button beside the Joints parameter and select the upper body joints.

9. In the Blend Parent parameter, specify the parent joint to put the motion on top of, in our example, the chest joint.

   The first jump uses the arms from the rest clip, and the second jump uses the arms from the punch clip:

10. Adjust the extent to which the tracks and clips contribute to the overall animation:

    • To have Electra make small bunny hops, reduce the weight of the jump track.

    • To have Electra hold up the punch arms only after the end of the second jump, set keys on the weight of the arms track by holding Alt, and clicking the curve in the timeline or the textbox/slider in the weight header:

In this example, we have a scene that contains Electra, a chair prop, and animation clips for Electra and the chair. In the rig tree view, we have the following character folder structure:

/
-- animation/
    -- chair.clip/
    -- jump.clip/
    -- punch.clip/
    -- standup.clip/
-- chair.char/
-- electra.char/
...

1. Add Electra’s standup animation to the track:

2. Apply the track to Electra:

   • Click the icon in the track header.

   • In the Motion Mixer Track Filtering window, select Electra.

   • Click Accept. In the parameter editor, the track’s Characters parameter is set to electra.

3. Create a track for the chair:

   • Select Add ▸ Add Track or click on the top toolbar.

   • Drag and drop the chair animation onto the new track.

   

   Note

   The Scene folder at the bottom left of the motion mixer pane contains the clips for all the characters. If multiple characters have a clip with the same name, that single clip can drive multiple characters.

4. If you play the animation, only the chair animation plays. We need to apply the chair animation only to the chair:

   • Click the icon for the chair track.

   • In the Motion Mixer Track Filtering window, select the chair.

   • Click Accept. The clip now applies only to the chair.

Effects are additional changes that can be applied to a track. When an effect is added to a track, it is placed on its own row below the track weight:

Effects can either be local or global. Local effects are applied within a track. Global effects are applied after the track is blended with the previous tracks (tracks on lower rows).

You can control the extent to which an effect contributes to the final animation by adjusting the slider in the effects header or the Blend value in the parameter editor. To set keyframes on the Blend value, hold Alt, and click the curve in the timeline or the textbox/slider in the effects header:

To choose the data that is output by the motion mixer:

1. In the motion mixer pane, select the motion mixer (top-level) item in the hierarchy.

2. Set the output in the Mode parameter. You can use the rig tree view to see the geometry that is output in each mode.

   The Animation Clip mode outputs the full animation scene, and the updated animation is saved in the file specified in the Animation Clip parameter.

You can also grab the motion mixer output using a Motion Mixer Fetch SOP or export the data using the motion mixer’s baking functionality. In this way, you can get the motion mixer data into another SOP and continue editing the animation independent of the motion mixer.

See the motion mixer pane for ways to navigate in the timeline and descriptions of the toolbar buttons.