# Math channel node

Perform a variety of arithmetic operations on and between channels.

This CHOP allows you to perform a variety of arithmetic operations on and between channels. Channels of a CHOP can be combined, and several CHOPs can be combined.

## Unary Operations

Unary operations are performed on individual channels. Unary operations are:

Off

Don’t do anything to the channel.

Negate

Take the negative value of each sample of the channel.

Positive

Make negative values of the channel positive (absolute).

Root

Take the square root of all values in the channel.

Square

Square all the values in the channel.

Inverse

Take the inverse (1/x) of all values in the channel.

## Combine Channels

Operations between channels can be done within an input or between inputs. The operations are done on a sample by sample basis:

Off

Don’t combine the channels.

Sum all the channels.

Subtract

Subtract all the channels from the first.

Multiply

Take the product of all the channels.

Divide

Divide the first channel by all the rest.

Average

Take the average of all the channels.

Maximum

Take the maximum value of all the channels.

Minimum

Take the minimum value of all the channels.

Length

Assume the channels are a vector and compute its length.

## OP

Channel Pre OP

A menu of unary operations that are performed on single channels as they come in to the Math CHOP.

Combine Channels

A menu of operations that is performed between the channels of a single input CHOP, for each input.

Combine CHOPs

A menu of operations that is performed between the input CHOPs, combining several CHOPs into one.

Channel Post OP

A menu of more unary operations that is performed on the channels resulting from the above operations.

Match By

Match channels between inputs by name or number.

When matching by name, the number of output channels is the number of channels from the first input. For each channel of the first input, the channel of the same name is taken from the other inputs. If a channel name cannot be found, then it is omitted. Only the channels whose names occur in the first input will be used to generate the output channels. Channels whose names do not occur in the first input will not affect the output channels.

When matching by number, the number of output channels is the maximum number of channels out of all the inputs. Then for each input, the original sequence of channels is repeated to match the number of output channels.

Match Failure

It is sometimes desirable to produce a warning or an error when channels in the first input fail to be correctly matched to channels in the other inputs. A failure may occur when matching by Channel Number if an input has fewer channels than input 1, or when matching by Channel Name if a channel name cannot be found in input 1.

Align

The alignment to use when aligning channels from different inputs. See the help for the Merge CHOP.

First, add value to each new channel.

Multiply

Then multiply by this value.

## Range

From/To Range

Another way to multiply/add. Converts from one low-high range to another range.

### Common

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

Scope

To determine which channels get affected, some CHOPs have a scope string. Patterns can be used in the 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.

`r[xyz]`

Matches channels `rx`, `ry` and `rz`.

`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 rate of first input to resample others.

Resample At Maximum Rate

Resample to 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 for which time parameters are specified.

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).

Time Slice

Time Slicing is a feature which boosts cooking performance and reduces memory usage. Traditionally, CHOPs calculate the channel over its entire frame range. If the channel does need 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 known as a Time Slice.

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`.

Graph Color

Every CHOP has this option. Each CHOP gets a default color assigned for display in the Graph port, but you can override the color in the Common page under 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.

## Locals

I

The current index.

V

The current value.

C

Current channel index (0 to NC-1).

NC

Total number of channels.

## Examples

MathBasic Example for Math channel node

These examples demonstrate the ability of the Math CHOP to combine multiple channels together.

The first example shows two different methods of controlling the amplitude of a sine wave. The second example shows three different channels combined into one.

The following examples include this node.

ChannelBasic Example for Channel channel node

This is a simple example of using the Channel CHOP along with a Noise CHOP to add some variety to keyframed animation that can still be easily tweaked as keyframe animation. Notice that the object can be manipulated as usual in the viewport transparently. The values will be modified in the Channel CHOP.

DynamicLights Example for Dynamics channel node

This example demonstrates how to use the Dynamics CHOP to extract impact data from a DOPs simulation, and then modify the data to control lights in the scene.

Envelope Example for Envelope channel node

This example illustrates how the Envelope CHOP outputs the maximum amplitude of a channel in the vicinity of each sample of the input. Here, envelope1 and envelope2 try to follow the maximum amplitude of the noisy curve they use as input.

To visualize the channels in this demonstration, please enter the CHOP network and use the default CHOPs desktop.

ExpressionLine Example for Expression channel node

This example demonstrates how to use an expression in an Expression CHOP.

GeometryMethods Example for Geometry channel node

This example demonstrates using the different methods of the Geometry CHOP - Animated and Static.

HoldLight Example for Hold channel node

This example uses the Hold CHOP in conjunction with the Dynamics CHOP to hold a light at the position of an impact from a DOPs simulation until a new impact occurs.

Lookup Example for Lookup channel node

This example demonstrates how to use the Lookup CHOP to play animation based on an event, or trigger.

MathBasic Example for Math channel node

These examples demonstrate the ability of the Math CHOP to combine multiple channels together.

The first example shows two different methods of controlling the amplitude of a sine wave. The second example shows three different channels combined into one.

WaveGrid Example for Wave channel node

This example demonstrates how to warp geometry using a Wave CHOP.

DensityViscosity Example for FLIP Solver dynamics node

This example demonstrates two fluids with different densities and viscosities interacting with a solid object.

ParticlesAttract Example for POP Attract dynamics node

This example demonstrates how to use the POP Attract node to get a group of particles to follow the motion of an animated sphere. POP Interact and POP Drag nodes are also used in the example to control the interaction between particles and their distance from the sphere.

BillowyTurbine Example for Pyro Solver dynamics node

This example uses the Pyro Solver and a Smoke Object which emits billowy smoke up through a turbine (an RBD Object). The blades of the turbine are created procedurally using Copy, Circle, and Align SOPs.

ChannelSOPColorExample Example for Channel geometry node

This example demonstrates using CHOPs to drive geometry color values via the Channel SOP.

RampParameter Example for Parameter VOP node

This example shows how to control the particle colours using the temperature attributes from a pyro simulation using a Ramp Parameter VOP node.

# Channel nodes

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

• Imports an animation clip from an agent primitive.

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

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

• 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.

• 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.

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

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

• Creates channels from the value of its parameters.

• Contains a VOP network that can manipulate channel data.

• Runs a VEX snippet to modify channel data.

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

• Create up to forty new channels.

• Combines two or more chops by using a list of weights specified as parameters.

• Returns an Object Local Transform.

• Returns an Object Parent Transform.

• Returns an Object World Transform.

• Constrains rotation so it always points toward a target position.

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

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

• Returns an Object Pretransform.

• Applies an transformation offset after evaluating a constraint.

• Reparent an object.

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

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

• Combines multiple chops by blending the inputs in sequence.

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

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

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

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

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

• Creates cycles.

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

• Removes channels coming from its input.

• Turns data from device inputs into transform data

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

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

• Fixes discontinuity of rotation data after cracking matrices

• A convenient tool for exporting channels.

• Export Constraints Network on any object

• Modify input channels by using expressions.

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

• Extracts the current world or local space bone transforms from a geometry object.

• Extracts locomotion from an animation clip.

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

• Reads in channel data from an FBX file.

• Used for controlling other CHOPs.

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

• Imports channels from other CHOPs.

• Imports channels from other OPs.

• Reads in channel and audio files for use by chops.

• Smooths or sharpens the input channels.

• Computes when position channels are stationary.

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

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

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

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

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

• Sample and hold the value of the first input.

• Solves inverse kinematics rotations for bone chains.

• Returns an identity transform.

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

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

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

• Returns an invert transform of the input.

• Creates a jiggling effect in the translate channels passed in.

• Turns key presses into channel output.

• Adds lag and overshoot to channels.

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

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

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

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

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

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

• Perform a variety of arithmetic operations on and between channels.

• Takes multiple inputs and merges them into the output.

• Outputs X and Y screen values for the mouse device.

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

• Post multiplies all the input transformations.

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

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

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

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

• Creates channels representing the transforms for a chain of objects.

• Generates sounds in two ways.

• Marks the output of a sub-network.

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

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

• Filters audio input using one of four different filter types.

• Translates english text into a series of phonetic values.

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

• Transmit data out of Houdini to other processes.

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

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

• Computes the difference between two poses.

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

• Generates pulses at regular intervals of one channel.

• Renames channels.

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

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

• Takes all its inputs and appends one chop after another.

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

• Reorganizes a list of channels.

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

• The rendering engine for producing 3D audio.

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

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

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

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

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

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

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

• Control the flow of channels through a CHOPnet.

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

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

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

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

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

• Shortens or lengthens the input’s channels.

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

• Performs vector operations on a set or sets of channels.

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

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

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

• Creates a waveform that is repeated.