Parameter Motion Effects - Houdini online help

Overview

Motion Effects let you easily add secondary motion to keyframed parameters (usually translate, rotate, or scale parameters), such as jitter or overshoot.

You can access the effects on the Motion Effects submenu on the context menu of parameters in the parameter editor, or of the timeline (for scoped parameters). You can also use the Jiggle tool or Lag tool on the Modify tab of the shelf to set up motion effects. Overridden parameters have an orange background in the parameter editor.

Behind the scenes, Motion Effects build networks of CHOP (channel operator) nodes that override the values of a parameter. Because the effects are procedural, you can edit, refine, and augment them by editing the CHOP network. You can apply multiple motion effects to the same parameter, and Houdini will simply merge them into the network. You can view the network and edit the CHOP nodes to change the effect.

When you use Motion Effects, Houdini creates a CHOP network node called motionfx at the Scene level (/obj/motionfx) to contain the CHOP nodes it creates. You can double-click the motionfx node to go inside and inspect the nodes, or delete the motionfx network to remove all motion effects.

Available effects

Advanced users can edit and add to the list of available effects.

Cycle	Offsets and repeats keyframed values. For example, if you keyframe one cycle of a ball moving forward and bouncing, then apply Cycle to the translate parameter, the ball will continue to move forward and bounce after its keyframed animation runs out. See also Repeat below.
De-spike	Removes “spikes” (sudden large changes in the parameter value). This may be useful for cleaning up noisy animation data imported from somewhere else. (This applies a Filter CHOP node set to “De-spike” mode.)
Lag	Slows and overshoots animation, creating an appearance of heaviness.
Limit	Clamps, loops, and/or quantizes parameter values. You can use this to keep an animated parameter from going outside certain limits, or snap continuous motion onto a grid.
Noise	Adds animated noise to the parameter values. When added to translate parameters, this makes an object jittery.
Repeat	Repeats keyframed values. For example, if you keyframe one cycle of a ball moving forward and bouncing, then apply Repeat to the translate parameter, the ball will move forward and bounce, then reset back to the beginning and repeat after its keyframed animation runs out. See also Cycle above.
Shift	Time-shifts keyframed values, moving the start time and end time of a piece of animation. See also Stretch below.
Smooth	Smooths out keyframed values, to remove jitter. (This applies a Filter CHOP node set to “Gaussian” mode.)
Spring	Adds overshoot and vibration to animation, as if the object was attached to its animated position by a spring.
Stretch	Time-compresses or expands keyframed values, making a piece of animation play slower or faster. See also Shift above.
Wave	Overrides the value of a parameter with the result of a simple wave function. For example, you can apply the Wave effect to the Translate Y parameter of an object to make it bob up and down.

How to

To...	Do this
Add a Motion Effect to a parameter	Right-click a parameter and open the Motion Effects submenu. Choose an effect.
Add a Motion Effect to the scoped parameters	Right-click the timeline and open the Motion Effects submenu. Choose an effect. A floating parameter editor for the CHOP node will appear. You can use this to alter the effect.
Edit an effect’s parameters	Right-click a parameter and open the Motion Effects submenu. Open the Edit effect parameters submenu. Choose the effect. Houdini opens a pop-up parameter editor you can use to edit the parameters of the effect.
Disable an effect	Right-click a parameter and open the Motion Effects submenu. Turn off the Enable effect checkbox item.
Edit the CHOP network	Adding effects to a parameter builds a network of CHOP (channel operator) nodes. You can jump to this network to edit node parameters, rearrange or delete nodes, and so on. Right-click a parameter and open the Motion Effects submenu. Choose Jump to effect network. Tip Press Space + G to center on the selected node.

Clips and poses

The Channel CHOP can hold channel animation which you can then manipulate using other CHOP nodes and export back onto the original parameters. The Motion Effects sub-menu gives you quick access to create Channel CHOPs for parameters.

To...	Do this
Start a new Channel CHOP for a parameter	Right-click a parameter and open the Motion Effects submenu and choose Create clip. See help for the options in the Create Clip dialog box below.
Create a new Channel CHOP just with the current values of a parameter	Right-click a parameter and open the Motion Effects submenu and choose Create pose.

Tip

When you create a clip, Houdini will be clever and still allow you to modify handles on the original node but forward the parameter value changes onto the Channel CHOP instead.

For example, if you create a clip from the Transform parameter on a Geometry container object, if you move the transform handles for the Geometry node, the values are changed on the Channel CHOP you created, not on the Geometry node.

To prevent this behavior (for example, to prevent artists from accidentally changing values on the Channel CHOP by manipulating handles on an object), turn on Lock clip in the dialog box that appears when you choose Create clip.

Create clip dialog box

Clip name	The name for the Channel CHOP to create. To add a new channel to an existing Channel CHOP, enter its name here and turn off Create new CHOP node.
Location	The CHOP network in which to create the Channel CHOP (or in which it already exists if Create new CHOP node is off). The default is the `/obj/motionfx` CHOP network created by other motion effects commands.
Channels	The channels to add to the Channel CHOP. The default value corresponds to the parameter you right-clicked, but you are free to edit and expand the list of channels.
Channel range	The range of frames from which to copy animation on the channels.
Create new CHOP node	If this option is off and the Channel node named in Clip name exists, Houdini will add the channels to the existing node. If the option is on, Houdini will always create a new Channel node in the CHOP network.
Apply immediately	Turns on the export flag on the Channel node, so that the parameters are immediately overridden by the CHOP network. If this option is off, you have to manually turn on the export flag of a node in the CHOP network to override the parameters.
Remove original animation	After copying the parameter’s channel animation into the Channel CHOP, delete the animation on the parameter.
Lock clip	Lock the parameters on the Channel CHOP that contain the copied values. This prevents Houdini from forwarding changes to handles on the original node to the corresponding parameters on the Channel CHOP.

Example

Use the shelf to create a sphere, and animate it. Try to add some sharp turns and direction reversals.

For example, move the sphere, set a keyframe with K , go to frame 24, move the sphere to a new location, set a keyframe, go to frame 48, move the sphere, set a keyframe, go to frame 72, move the sphere, set a keyframe.
Select the sphere object. In the parameter editor, right click the Translate parameter.

Right click the label. If you right click one of the individual X, Y, or Z text boxes, the context menu will only apply to that component, not all three components.
Open the Motion Effects submenu and choose Spring.
In the floating parameter editor for the Spring CHOP node, decrease the Spring constant. This will exaggerate the “springiness” of the effect.
Rewind and play the animation. The sphere will appear to be attached to its original animated position by a spring. It will overshoot and vibrate when it changes direction.
Close the floating parameter editor window. Right click the Translate label again and choose Motion Effects > Cycle.
Rewind and play the animation again. The animation will cycle past the last keyframe you set, offsetting from the last keyframed position. This effect is very useful for setting up walk and bounce cycles.

Next steps

CHOP (channel operator) nodes let you grab keyframed parameter values, procedurally manipulate them using the nodes in the network, and then override the original parameter with the changed value. The Motion Effects menu items make it easy to set up a quick, powerful CHOP network automatically, but you may also want to explore the full power of CHOP networks. CHOP nodes exist for many functions from math to time, audio, and frequency manipulations, letting advanced users achieve very complex effects on top of simple keyframed animations.

See the list of CHOP nodes available.

Editing the effects menu

The $HFS/houdini/MotionEffectsMenu.xml file defines the contents of the Motion Effects submenu (on the context menu for parameters). It is an XML file, where the XML defines the structure of the menu, and embedded Python scripts define the action of each menu item.

See how to customize the main menus for general details on how Houdini’s menu configuration files work. Instead of <mainMenu>, the Motion Effects submenu configuration file uses <menuDocument> as the root element.

The basic structure is:

<?xml version="1.0" encoding="UTF-8" ?>
<menuDocument>
    <menu>
        <subMenu id="motion_effects_menu">
            <label>Motion Effects</label>

            <actionItem id="action_id">
                <label>Menu item label</label>
            </actionItem>

            <scriptItem id="item_id">
                <label>Menu item label</label>
                <scriptCode>
                    <![CDATA[

                    Python script to run when the menu
                    item is chosen.

                    ]]>
                </scriptCode>
            </scriptItem>

            <separatorItem />

            ...
        </subMenu>
    </menu>
</menuDocument>

actionItem nodes reference built-in houdini functionality.

The script contained in the scriptCode element of a scriptItem node automatically has the hou module imported. It also has a predefined dictionary named kwargs, containing the following keys:

`parms (list)`	A list of `hou.Parm` objects representing the parameter(s) the user right-clicked.
`toolname (str)`	The menu item’s hotkey id (the element’s id prefixed with `h.pane.parms.`).
`altclick, ctrlclick, shiftclick, cmdclick (boolean)`	True if the user held the given key while selecting the menu item. You can use this to provide alternate “power-user” behavior based on modifier keys. Note that `cmdclick` is only possible on Mac OS X.

The value of the id attribute corresponds to a Houdini hotkey action starting with h.pane.parms. For actionItem nodes this is what defines the action to run when the menu item is selected. For scriptItem nodes this lets you assign a hotkey to the menu item.

For example, an item with ID revert_to_prev_val refers to the h.pane.parms.revert_to_prev_val action in the hotkeys manager.