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.

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

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.

1. 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.

2. 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.

3. Open the Motion Effects submenu and choose Spring.

4. In the floating parameter editor for the Spring CHOP node, decrease the Spring constant. This will exaggerate the “springiness” of the effect.

5. 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.

6. Close the floating parameter editor window. Right click the Translate label again and choose Motion Effects ▸ Cycle.

7. 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.

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.

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.