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Channel node (CHOP) networks ¶
A CHOP network contains 
channel nodes (CHOPs) for manipulating time-based channel data such as animation curves or audio.
Houdini creates a default CHOP network at /ch. You can also create your own CHOP networks at any level of the scene (for example, inside a character to control kinematics).
Wiring CHOPs together controls the flow of channel data, from nodes that load or fetch the channel data through nodes that modify the channels.
You can view the data generated by a CHOP network in the Motion FX View tab.
CHOP flags ¶
Icon |
Key |
Description |
|---|---|---|
|
Q or B |
Bypass disables the node, making it pass its channels through to the output unchanged. This is useful for testing and visualizing the effect the node is having in the CHOP viewer. When a node is bypassed, the flag on the left of the node is lit yellow. |
|
E |
Export overrides the channel corresponding to the node’s channel name with the data from the node. For example, you can name the channel generated by a node Normally, clicking the export flag on a node turns off the export flag on all other nodes. You can ⇧ Shift-click export flags to export multiple nodes at once. When the export flag is on, the flag second from the left of the node is lit bright orange. |
|
T |
Output sets this node’s output as the output of the CHOP network. When the output flag is on, the flag second from the left of the node is lit dark orange. |
|
R |
Display marks the nodes whose channels appear in the CHOP viewer. You can set/clear this flag on multiple nodes to see their channels in the viewer at the same time. Normally, clicking the display flag on a node toggles the flag only for that node. You can ⌃ Ctrl-click display flags to set an exclusive display flag and turn off the flags on all other nodes. When the display flag is on, the flag on the right of the node is lit blue. |
Other CHOP flags ¶
You can set this flag by 
clicking a node and opening the Flags sub-menu.
Icon |
Key |
Description |
|---|---|---|
|
None |
Lock “freezes” and caches the node’s output channels. While the node is locked, the node uses the “frozen” channels instead of cooking. This is usually not necessary and can be confusing, (which is why the flag isn’t clickable on the node and doesn’t have a hotkey), but it has two potential benefits:
When a node is locked, a badge appears on the node. Don’t confuse this “lock” with locked digital assets. |
Nodes ¶
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Design audio filters and sound materials for the spatial audio system.
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Imports an animation clip from an agent primitive.
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Calculates the area under a channel’s graph, which is the same as calculating the integral of a channel, or integrating the channel.
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Adds, removes or updates attributes of the input chop.
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Receives audio input from the analog audio ports or the digital port.
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A 14-band equalizer which filters audio input channels in the same way that a conventional band equalizer uses a bank of sliders to filter fixed-frequency bands of sound.
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Manually tap the beat of a piece of music, and automatically generate a repeating ramp or pulse that continues to keep time with the music after the taps stop.
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Combines two or more chops in input 2, 3 and so on, by using a set of blending channels in input 1.
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Performs multi-dimensional, example-based interpolation of channels.
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Creates channels from the value of its parameters.
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Contains a VOP network that can manipulate channel data.
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Runs a VEX snippet to modify channel data.
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Layers (blends) the channels of one CHOP on the channels of another CHOP.
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Create up to forty new channels.
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Combines two or more CHOP inputs using a list of weights specified as parameters.
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Returns an Object Local Transform.
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Returns an Object Parent Transform.
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Returns an Object World Transform.
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Constrains rotation so it always points toward a target position.
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Compares two objects and returns information on their relative positions and orientations.
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Compares two objects and returns information on their relative positions and orientations.
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Constraint Object Pretransform
Returns an Object Pretransform.
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Applies an transformation offset after evaluating a constraint.
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Reparent an object.
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Position an object on a path and orient it to the path’s direction.
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Position and Orient an object using point positions from a geometry.
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Combines multiple chops by blending the inputs in sequence.
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Combines two chops by using a single weight specified as a parameter.
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Position and Orient an object using the surface of a geometry.
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Takes translate, rotate, and/or scale channels and transforms them.
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Produces multiple copies of the second input along the timeline of the first input.
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Counts the number of times a channel crosses a trigger or release threshold.
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Creates cycles.
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Delays the input, and can be run in normal or time-sliced mode.
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Removes channels coming from its input.
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Turns data from device inputs into transform data
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Time-warps the node’s first input (source clip) using its second input (reference clip) as a reference.
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Extracts any information from a DOP simulation that is accessible through the dopfield expression function.
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Outputs the maximum amplitude in the vicinity of each sample of the input.
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Fixes discontinuity of rotation data after cracking matrices
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A convenient tool for exporting channels.
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Export Constraints Network on any object
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Export Transforms to Constraints Network of many objects
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Modify input channels by using expressions.
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Only sets the extend conditions of a chop, which determines what values you get when sampling the CHOP before or after its interval.
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(Deprecated) Extracts the current world or local space bone transforms from a geometry object.
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Extracts locomotion from an animation clip.
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(Deprecated) Creates channels from the specified derived transforms, node parameters and CHOP channels for pose-space deformation.
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Reads in channel data from an FBX file.
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Used for controlling other CHOPs.
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Get the state of a chop as it was one frame or time slice ago.
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Imports channels from other CHOPs.
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Imports channels from other OPs.
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Reads in channel and audio files for use by chops.
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Smooths or sharpens the input channels.
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Computes when position channels are stationary.
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Divides the input channels into groups, cooking the contained network for each group.
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Provides more complicated math functions than found in the Math CHOP such as trigonometic functions, logarithmic functions, and exponential functions.
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Turns input values for the gamepad or joystick device into channel outputs.
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Uses a geometry object to choose a sop from which the channels will be created.
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The engine which drives Inverse Kinematic solutions using the Handle object.
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Sample and hold the value of the first input.
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Solves inverse kinematics rotations for bone chains.
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Returns an identity transform.
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Converts rows and/or columns of pixels in an image to CHOP channels.
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Treats its multiple-inputs as keyframes and interpolates between them.
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Generates channels for bone objects based on a bone chain and an end affector.
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Returns an invert transform of the input.
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Creates a jiggling effect in the translate channels passed in.
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Turns key presses into channel output.
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Adds lag and overshoot to channels.
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Mix weighted layers of keyframed animation from multiple Channel CHOPs to a base Channel CHOP.
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Provides a variety of functions to limit and quantize the input channels.
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Converts channels of all its input chops into binary channels and combines them using a variety of logic operations.
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Uses a channel in the first input to index into a lookup table in the second input, and output values from the lookup table.
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The MIDI In CHOP reads Note events, Controller events, Program Change events, and Timing events from both midi devices and files.
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The MIDI Out CHOP sends MIDI events to any available MIDI devices.
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Perform a variety of arithmetic operations on and between channels.
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Takes multiple inputs and merges them into the output.
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Outputs X and Y screen values for the mouse device.
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Turns input values for the Connexion space mouse into channel outputs.
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Post multiplies all the input transformations.
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Similar to the Pipe In/Out CHOPs in Network mode.
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Makes an irregular wave that never repeats, with values approximately in the range -1 to +1.
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Used as a place-holder and does not have a function of its own.
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Compares two objects and returns information on their relative positions and orientations.
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Creates channels representing the transforms for a chain of objects.
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Generates sounds in two ways.
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Marks the output of a sub-network.
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Filters an audio clip, and then applies other audio effects.
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Produces translate and rotate channels to move Objects according to the positions of particles in a POP Network.
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Filters audio input using one of four different filter types.
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Translates english text into a series of phonetic values.
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Pipes data from custom devices into a CHOP, without needing the Houdini Developers' Kit or knowledge of Houdini internals.
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Transmit data out of Houdini to other processes.
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Attempts to extract the fundamental pitch of a musical tone from the input audio.
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Store a transform pose for later use by evaluating the input.
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Computes the difference between two poses.
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Takes translate, rotate, and/or scale channels and transforms them using the pretransform of the given object.
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Generates pulses at regular intervals of one channel.
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Renames channels.
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Reorders the first input CHOP’s channels by numeric or alphabetic patterns.
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Resamples an input’s channels to a new rate and/or start/end interval.
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Takes all its inputs and appends one chop after another.
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This time-shifts a CHOP, changing the start and end of the CHOP’s interval.
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Reorganizes a list of channels.
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Calculates the slope (or derivative) of the input channels.
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The rendering engine for producing 3D audio.
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Calculates the frequency spectrum of the input channels, or a portion of the channels.
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Edit the channel data by using direct manipulation of cubic or Bezier handles in the graph of the CHOP.
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Creates vibrations influenced by the input channels, as if a mass was attached to a spring.
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Caches the input motion in the node on command, and then uses it as the node’s output.
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(Deprecated) Stashes the bone transforms and pose-drivers for use by the Pose-Space Deform SOP and Pose-Space Edit SOP nodes.
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Preserves the shape of channels and the sampling rate, but resamples the channels into a new interval.
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Allows for the simplification of complex networks by collapsing several CHOPs into one.
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Control the flow of channels through a CHOPnet.
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This converts an input node in Current Frame mode to a Time Range mode by re-cooking it multiple times.
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This time-shifts a CHOP, re-cooking the node using different time.
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Takes translate, rotate, and/or scale channels and transforms them.
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Contains a VOP network that can manipulate transform data.
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Combines a chain of translate, rotate, and/or scale channels.
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Adds an audio-style attack/decay/sustain/release (ADSR) envelope to all trigger points in the input channels.
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Shortens or lengthens the input’s channels.
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This function is a sub-set of the waveform CHOP.
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Performs vector operations on a set or sets of channels.
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The Voice Split CHOP takes an audio track and separates words out into different channels.
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The Voice Sync CHOP detects phonemes in an audio channel given some audio phoneme samples and pro…
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Time-warps the channels of the first input (the Pre-Warp Channels) using one warping channel in the second input.
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Creates a waveform that is repeated.
