Houdini 20.5 Nodes Channel nodes

Channel Wrangle channel node

Runs a VEX snippet to modify channel data.

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Since 16.0

This is a very powerful, low-level node that lets experts who are familiar with VEX tweak channel data and attributes using code.

This node corresponds to the Channel VOP, but uses a textual VEX snippet instead of a VOP network.

This node runs the snippet for every channel (or group of channels depending on the Iterate parameter) in the input channels. This node also runs over all the samples based on the first input channel range. The snippet can edit the input channels by changing the bound parameters. It can access information from other channels or geometry using VEX functions.

Note

If you create a Transform VOP node using the Tab menu, it creates a Channel VOP and sets it to work with transform channels and constraints.

Context

This node runs the VEX snippet in the CHOP context and the following global variables are available.

Globals

float V

Value of the current sample. This variable should be set to the new value by the function. The variable is initialized to the value of the first input’s channels.
int I Read only

Index or sample number of the current channel. Time dependent only in Current Frame mode.
int IN Read only

Index or sample number of the current channel. Non Time Dependent.
int S Read only

Index of the start of the current channel. This is the index of the first sample.
int E Read only

Index of the last sample (end sample).
float SR Read only

Sample rate for the current channel.
int L Read only

Length of the channel (total number of samples).
int C Read only

Channel number for the current channel. When processing multiple channels, this is the index of the channel currently being evaluated.
int NC Read only

Total number of channels the CHOP will affect.
string CN Read only

Name of the current channel.
float FF Read only

Frame number as a float corresponding to the sample being evaluated. Time dependent only in Current Frame mode.
float FFN Read only

Frame number as a float corresponding to the sample being evaluated. Non Time Dependent.
float Frame Read only

Frame number as a float corresponding to the evaluation time. Forces Time Dependent.
float T Read only

Time in seconds corresponding to the sample being evaluated. Time dependent only in Current Frame mode.
float TN Read only

Time in seconds corresponding to the sample being evaluated. Non Time Dependent.
float Time Read only

Time in seconds corresponding to the evaluation time. Forces Time Dependent.

Syntax

The VEX snippet parameter lets you enter a snippet of VEX code to run on the input geometry. See VEX snippets for basic information on the syntax available in the snippet parameter. See the VEX chapter for general information on the VEX language.

VEX variables

You can create temporary VEX variables. Normal VEX variables do not have a @ prefix. For example, the following code swaps px and py.

float temp = @px;
@px = @py;
@py = temp;

VEX structures for Constraints

CHOP contraints make use of VEX structures defined in $HFS/houdini/vex/include/chop_constraints.h.

chopTRS

Holds transform channels within a structure.

struct chopTRS
{
    vector t; // tx ty tz channels
    vector r; // rx ry rz channels
    vector s; // sx ry rz channels

    void fromIdentity();
    void fromMatrix(const matrix m; const vector pivot; const int trs; const int xyz);
    void fromMatrix(const matrix m);
}

chopConstraintContext

Holds the current evaluated transform channels within a structure. This abstracts the global variables 'C' and 'I', so you don’t have to worry about the current time and the current channel index.

struct chopConstraintContext
{
    vector t; // tx ty tz channels
    vector r; // rx ry rz channels
    vector s; // sx sy sz channels

    void init();
    void init( vector t0; vector r0; vector s0; const string prefix0 );

    void fromMatrix(const matrix m;
                   const vector pivot; const int trs; const int xyz);

    void fromMatrix(const matrix m);

    chopTRS fetchInput( const int i );
    float fetchInput( const int i; const string name; int result );
    float fetchInput( const int i; const int index; int result );

    matrix fetchInputMatrix( const int i );

    int isConnected( const int i );
    int numInputs();

    // Evaluate a channel input by name
    float chinput( int i; const string name; int ret );

    // Evaluate a float parameter on the current CHOP node
    float chf( const string parm );

    // Evaluate an integer parameter on the current CHOP node
    int chi( const string parm );

    // Evaluate a vector parameter on the current CHOP node
    vector chv( const string parm );
}

Here’s an example of how to blend the translations of the 4 inputs.

// Declare the context
// This is done automatically on transform wrangles
chopConstraintContext c;

// Fetch transform inputs
chopTRS c0 = c->fetchInput(0);
chopTRS c1 = c->fetchInput(1);
chopTRS c2 = c->fetchInput(2);
chopTRS c3 = c->fetchInput(3);
matrix m0 = c->fetchInputMatrix(0);
matrix m1 = c->fetchInputMatrix(1);
matrix m2 = c->fetchInputMatrix(2);
matrix m3 = c->fetchInputMatrix(3);

// Compute transform
@t = c0.t + c1.t + c2.t + c3.t;
@t *= 0.25;
@r = c0.r;
@s = c0.s;

Parameters

Code

Channel Names

Space separated names of the channels to create when the first input isn’t connected. Supports the same patterns as the Scope parameter.

Channel Count

Repeats the Channel Names multiple times when the first input isn’t connected.

Iterate

How to iterate over channels and samples.

Over Channels and Samples

Iterates over all the samples of a channel one channel at a time. Use the V global variable to read and write the current channel value.

Over Samples and Channels

Iterates over channels one sample at a time. Use the V global variable to read and write the current channel value.

Over Samples and Every Nth Channels

Iterates over channels one sample at a time, but you can evaluate a group of channels together. Use the Iterate Step to define the size a channel group. Use the Iterate Names with a @ prefix to read and write the current values.

Over Samples and Every Translate/Rotate/Scale

Iterates over transform channels one sample at a time. Access the translation vector using @t, the rotation using @r and the scale using @s. A struct chopConstraintContext is also bound to the c variable. Use c->fetchInput(1) to return the second input transform as a struct chopTRS. Use c->fetchInputMatrix(1) to return the second input transform as a matrix.

Over Clip

Runs the snippet a single time with C and I variables set to zero. Use this to if you want to analyze all the channels using a snippet. You can’t write to the global variable V or to any channel data. You can read and write clip attributes using chattr and chsetattr.

Over Channels

Runs the snippet one time per channel with the I variable set to zero. Use this to if you want to analyze channels individually using a snippet. You can’t write to the global variable V or to any channel data. You can read and write channel attributes using chattr and chsetattr.

Over Sample

Runs the snippet one time per sample with the C variable always set to zero. Use this to if you want to analyze the all channel values for a single sample using a snippet. You can’t write to the global variable V or to any channel data. You can read and write sample attributes using chattr and chsetattr.

Iterate Step

Sets the number of channels to regroup together when Iterate is set to Every Nth Channels.

Iterate Names

A space separated string representing the bind names when Iterate is set to Every Nth Channels. Use an iterate name prefixed by @ to read and write to a channel.

VEXpression

A snippet of VEX code that will manipulate the channels. You can use @‹channel_name› syntax to access bound channels or V to write to the current channel sample.

Evaluation Node Path

VEX functions like ch() usually evaluate with respect to this node. Enter a node path here to override where the path search starts from. This is useful for embedding in a digital asset, where you want searches to start from the asset root.

Enforce Prototypes

Requires that you declare @ bindings in snippets as prototypes before using them. This applies to both attributes (for example @Cd) and “convenience” bindings such as @ptnum and @Frame. For example:

// Declare bindings
int @ptnum;
float @Frame;
vector @Cd;

// Use bindings after declaration
int pointnum = @ptnum;
float red = @Cd[0] / @Frame;

Automatic binding with the @ syntax can be convenient, but as your scene becomes more complex there is the risk that a typo in an @ binding will silently just bind a non-existent attribute.

Channel

Align

The alignment option to use.

Extend to Min/Max

Find the earliest start and latest end, and extend all inputs to that range using the extend conditions.

Stretch to Min/Max

Find the earliest start and latest end, and stretch every channel’s start and end to that range.

Shift to Minimum

Find the earliest start and shift all channels so they all start at that index. All channels are extended to the length of the longest one.

Shift to Maximum

Find the latest end and shift all channels so they all end at that index. Extend all channels to the length of the longest one.

Shift to First Interval

Shift all channels to the start of the first channel and sample all inputs using the first input’s range.

Trim to First Interval

Trim all channels to first channel’s range.

Stretch to First Interval

Stretch all channels to the first channel’s range.

Trim to Smallest Interval

Trim all channels to the smallest start/end interval. The start and end values may not come from the same channel.

Stretch to Smallest Interval

Stretch all channels to the smallest start/end interval. The start and end values may not come from the same channel.

Range

Specifies the range of data to generate.

Use Full Animation Range

All of the animated range.

Use Current Frame

Only the sample at the current frame.

Use Start/End

The range is specified from the Start and End parameters.

Use Value Animation

The range is taken from range of keys available in the evaluated Value parameters.

Start

The start time of the channel range.

End

The end time of the channel range.

Sample Rate

The sample rate of the CHOP.

Number of Threads

The number of separate threads to use to evaluate the channel samples. The default is no threading.

Since Houdini evaluates the VEX program for each sample in the input geometry, it can benefit greatly from threading on a multi-processor or multi-core machine when the CHOP nodes have many samples.

Common

Some of these parameters may not be available on all CHOP nodes.

Scope

To determine the channels that are affected, some CHOPs have a scope string. Patterns can be used in Scope, for example * (match all), and ? (match single character).

The following are examples of possible channel name matching options:

chan2

Matches a single channel name.

chan3 tx ty tz

Matches four channel names, separated by spaces.

chan*

Matches each channel that starts with chan.

*foot*

Matches each channel that has foot in it.

t?

The ? matches a single character. t? matches two-character channels starting with t.

blend[3-7:2]

Matches number ranges, giving blend3, blend5, and blend7.

blend[2-3,5,13]

Matches channels blend2, blend3, blend5, blend13.

t[xyz]

[xyz]matches three characters, giving channels tx, ty and tz.

Sample Rate Match

The Sample Rate Match options handle cases where multiple input CHOPs’ sample rates are different.

Resample At First Input’s Rate

Use the rate of the first input to resample the others.

Resample At Maximum Rate

Resample to the highest sample rate.

Resample At Minimum Rate

Resample to the lowest sample rate.

Error if Rates Differ

Does not accept conflicting sample rates.

Units

The units of the time parameters.

For example, you can specify the amount of time a lag should last for in seconds (default), frames (at the Houdini FPS), or samples (in the CHOP’s sample rate).

Note

When you change the Units parameter, the existing parameters are not converted to the new units.

Time Slice

Time slicing is a feature that boosts cooking performance and reduces memory usage. Traditionally, CHOPs calculate the channel over its entire frame range. If the channel needs to be evaluated every frame, then cooking the entire range of the channel is unnecessary. It is more efficient to calculate only the fraction of the channel that is needed. This fraction is the Time Slice.

Unload

Causes the memory consumed by a CHOP to be released after it is cooked, and the data passed to the next CHOP.

Export Prefix

The Export Prefix is prepended to CHOP channel names to determine where to export to.

For example, if the CHOP channel was named geo1:tx, and the prefix was /obj, the channel would be exported to /obj/geo1/tx.

Note

You can leave the Export Prefix blank, but then your CHOP track names need to be absolute paths, such as obj:geo1:tx.

Graph Color

Every CHOP has this option. Each CHOP gets a default color assigned to it for display in the graph, but you can override the color with the Graph Color. There are 36 RGB color combinations in the palette.

Graph Color Step

When the graph displays the animation curves, and a CHOP has two or more channels, this defines the difference in color from one channel to the next, giving a rainbow spectrum of colors.

See also

Channel nodes

  • Acoustic

    Design audio filters and sound materials for the spatial audio system.

  • Agent

    Imports an animation clip from an agent primitive.

  • Area

    Calculates the area under a channel’s graph, which is the same as calculating the integral of a channel, or integrating the channel.

  • Attribute

    Adds, removes or updates attributes of the input chop.

  • Audio In

    Receives audio input from the analog audio ports or the digital port.

  • Band EQ

    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.

  • Beat

    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.

  • Blend

    Combines two or more chops in input 2, 3 and so on, by using a set of blending channels in input 1.

  • BlendPose

    Performs multi-dimensional, example-based interpolation of channels.

  • Channel

    Creates channels from the value of its parameters.

  • Channel VOP

    Contains a VOP network that can manipulate channel data.

  • Channel Wrangle

    Runs a VEX snippet to modify channel data.

  • Composite

    Layers (blends) the channels of one CHOP on the channels of another CHOP.

  • Constant

    Create up to forty new channels.

  • Constraint Blend

    Combines two or more CHOP inputs using a list of weights specified as parameters.

  • Constraint Get Local Space

    Returns an Object Local Transform.

  • Constraint Get Parent Space

    Returns an Object Parent Transform.

  • Constraint Get World Space

    Returns an Object World Transform.

  • Constraint Lookat

    Constrains rotation so it always points toward a target position.

  • Constraint Object

    Compares two objects and returns information on their relative positions and orientations.

  • Constraint Object Offset

    Compares two objects and returns information on their relative positions and orientations.

  • Constraint Object Pretransform

    Returns an Object Pretransform.

  • Constraint Offset

    Applies an transformation offset after evaluating a constraint.

  • Constraint Parent

    Reparent an object.

  • Constraint Path

    Position an object on a path and orient it to the path’s direction.

  • Constraint Points

    Position and Orient an object using point positions from a geometry.

  • Constraint Sequence

    Combines multiple chops by blending the inputs in sequence.

  • Constraint Simple Blend

    Combines two chops by using a single weight specified as a parameter.

  • Constraint Surface

    Position and Orient an object using the surface of a geometry.

  • Constraint Transform

    Takes translate, rotate, and/or scale channels and transforms them.

  • Copy

    Produces multiple copies of the second input along the timeline of the first input.

  • Count

    Counts the number of times a channel crosses a trigger or release threshold.

  • Cycle

    Creates cycles.

  • Delay

    Delays the input, and can be run in normal or time-sliced mode.

  • Delete

    Removes channels coming from its input.

  • Device Transform

    Turns data from device inputs into transform data

  • Dynamic Warp

    Time-warps the node’s first input (source clip) using its second input (reference clip) as a reference.

  • Dynamics

    Extracts any information from a DOP simulation that is accessible through the dopfield expression function.

  • Envelope

    Outputs the maximum amplitude in the vicinity of each sample of the input.

  • Euler Rotation Filter

    Fixes discontinuity of rotation data after cracking matrices

  • Export

    A convenient tool for exporting channels.

  • Export Constraints

    Export Constraints Network on any object

  • Export Transforms

    Export Transforms to Constraints Network of many objects

  • Expression

    Modify input channels by using expressions.

  • Extend

    Only sets the extend conditions of a chop, which determines what values you get when sampling the CHOP before or after its interval.

  • Extract Bone Transforms

    (Deprecated) Extracts the current world or local space bone transforms from a geometry object.

  • Extract Locomotion

    Extracts locomotion from an animation clip.

  • Extract Pose-Drivers

    (Deprecated) Creates channels from the specified derived transforms, node parameters and CHOP channels for pose-space deformation.

  • FBX

    Reads in channel data from an FBX file.

  • Fan

    Used for controlling other CHOPs.

  • Feedback

    Get the state of a chop as it was one frame or time slice ago.

  • Fetch Channels

    Imports channels from other CHOPs.

  • Fetch Parameters

    Imports channels from other OPs.

  • File

    Reads in channel and audio files for use by chops.

  • Filter

    Smooths or sharpens the input channels.

  • Foot Plant

    Computes when position channels are stationary.

  • Foreach

    Divides the input channels into groups, cooking the contained network for each group.

  • Function

    Provides more complicated math functions than found in the Math CHOP such as trigonometic functions, logarithmic functions, and exponential functions.

  • Gamepad

    Turns input values for the gamepad or joystick device into channel outputs.

  • Geometry

    Uses a geometry object to choose a sop from which the channels will be created.

  • Gesture

  • Handle

    The engine which drives Inverse Kinematic solutions using the Handle object.

  • Hold

    Sample and hold the value of the first input.

  • IKSolver

    Solves inverse kinematics rotations for bone chains.

  • Identity

    Returns an identity transform.

  • Image

    Converts rows and/or columns of pixels in an image to CHOP channels.

  • Interpolate

    Treats its multiple-inputs as keyframes and interpolates between them.

  • InverseKin

    Generates channels for bone objects based on a bone chain and an end affector.

  • Invert

    Returns an invert transform of the input.

  • Jiggle

    Creates a jiggling effect in the translate channels passed in.

  • Keyboard

    Turns key presses into channel output.

  • Lag

    Adds lag and overshoot to channels.

  • Layer

    Mix weighted layers of keyframed animation from multiple Channel CHOPs to a base Channel CHOP.

  • Limit

    Provides a variety of functions to limit and quantize the input channels.

  • Logic

    Converts channels of all its input chops into binary channels and combines them using a variety of logic operations.

  • Lookup

    Uses a channel in the first input to index into a lookup table in the second input, and output values from the lookup table.

  • MIDI In

    The MIDI In CHOP reads Note events, Controller events, Program Change events, and Timing events from both midi devices and files.

  • MIDI Out

    The MIDI Out CHOP sends MIDI events to any available MIDI devices.

  • Math

    Perform a variety of arithmetic operations on and between channels.

  • Merge

    Takes multiple inputs and merges them into the output.

  • Mouse

    Outputs X and Y screen values for the mouse device.

  • Mouse 3D

    Turns input values for the Connexion space mouse into channel outputs.

  • Multiply

    Post multiplies all the input transformations.

  • Network

    Similar to the Pipe In/Out CHOPs in Network mode.

  • Noise

    Makes an irregular wave that never repeats, with values approximately in the range -1 to +1.

  • Null

    Used as a place-holder and does not have a function of its own.

  • Object

    Compares two objects and returns information on their relative positions and orientations.

  • ObjectChain

    Creates channels representing the transforms for a chain of objects.

  • Oscillator

    Generates sounds in two ways.

  • Output

    Marks the output of a sub-network.

  • Parametric EQ

    Filters an audio clip, and then applies other audio effects.

  • Particle

    Produces translate and rotate channels to move Objects according to the positions of particles in a POP Network.

  • Pass Filter

    Filters audio input using one of four different filter types.

  • Phoneme

    Translates english text into a series of phonetic values.

  • Pipe In

    Pipes data from custom devices into a CHOP, without needing the Houdini Developers' Kit or knowledge of Houdini internals.

  • Pipe Out

    Transmit data out of Houdini to other processes.

  • Pitch

    Attempts to extract the fundamental pitch of a musical tone from the input audio.

  • Pose

    Store a transform pose for later use by evaluating the input.

  • Pose Difference

    Computes the difference between two poses.

  • Pretransform

    Takes translate, rotate, and/or scale channels and transforms them using the pretransform of the given object.

  • Pulse

    Generates pulses at regular intervals of one channel.

  • ROP Channel Output

  • Record

  • Rename

    Renames channels.

  • Reorder

    Reorders the first input CHOP’s channels by numeric or alphabetic patterns.

  • Resample

    Resamples an input’s channels to a new rate and/or start/end interval.

  • Sequence

    Takes all its inputs and appends one chop after another.

  • Shift

    This time-shifts a CHOP, changing the start and end of the CHOP’s interval.

  • Shuffle

    Reorganizes a list of channels.

  • Slope

    Calculates the slope (or derivative) of the input channels.

  • Spatial Audio

    The rendering engine for producing 3D audio.

  • Spectrum

    Calculates the frequency spectrum of the input channels, or a portion of the channels.

  • Spline

    Edit the channel data by using direct manipulation of cubic or Bezier handles in the graph of the CHOP.

  • Spring

    Creates vibrations influenced by the input channels, as if a mass was attached to a spring.

  • Stash

    Caches the input motion in the node on command, and then uses it as the node’s output.

  • Stash Pose

    (Deprecated) Stashes the bone transforms and pose-drivers for use by the Pose-Space Deform SOP and Pose-Space Edit SOP nodes.

  • Stretch

    Preserves the shape of channels and the sampling rate, but resamples the channels into a new interval.

  • Subnetwork

    Allows for the simplification of complex networks by collapsing several CHOPs into one.

  • Switch

    Control the flow of channels through a CHOPnet.

  • Time Range

    This converts an input node in Current Frame mode to a Time Range mode by re-cooking it multiple times.

  • Time Shift

    This time-shifts a CHOP, re-cooking the node using different time.

  • Transform

    Takes translate, rotate, and/or scale channels and transforms them.

  • Transform VOP CVEX

    Contains a VOP network that can manipulate transform data.

  • TransformChain

    Combines a chain of translate, rotate, and/or scale channels.

  • Trigger

    Adds an audio-style attack/decay/sustain/release (ADSR) envelope to all trigger points in the input channels.

  • Trim

    Shortens or lengthens the input’s channels.

  • VEX Waveform

    This function is a sub-set of the waveform CHOP.

  • Vector

    Performs vector operations on a set or sets of channels.

  • Voice Split

    The Voice Split CHOP takes an audio track and separates words out into different channels.

  • Voice Sync

    The Voice Sync CHOP detects phonemes in an audio channel given some audio phoneme samples and pro…

  • Warp

    Time-warps the channels of the first input (the Pre-Warp Channels) using one warping channel in the second input.

  • Wave

    Creates a waveform that is repeated.