Expression channel node

Modify input channels by using expressions.

All

Parameters

Local variables

This CHOP allows you to modify input channels by using expressions. Up to six expressions are available. Each input channel is modified by exactly one expression, and the expressions are looped for multiple channels.

The output is the same length and set of channels as the first input, but its sample values are changed according to the expressions.

An expression is applied to each keyframe value or raw sample. There are up to six expressions. If there are more channels coming from input 0, the expressions are recycled. A repeat parameter controls how many channels to apply the first expression to before going on to the second expression.

The power of this CHOP extends when using special expression functions to access the other input CHOPs: ic(input#, channel#, index) This gets the value at the specified index of the specified channel of the specified input CHOP (all 0-relative). Others include:

ic() can use local variables: $C (channel #), $I (index).
oc(chan_idx, samp_idx) gets at the output CHOP’s samples as they are being built. Like in calculus integration, you need a value from the previous output sample to generate the next one.
ics(input#) gets the start index of any input CHOP. similar for ice() and icl().
icr() gives the sample rate.

This CHOP has Sample Rate Match options in the Common page.

Parameters

Group

Channels Per Expr	The number of channels that use the current expression before the next expression is selected.
Num Expressions	The total number of expressions that are defined.

Expr

Expression 1-6

The expressions.

Common

Some of these parameters may not be avaiable 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).

Note

When you change the Units parameter, it does not convert the existing parameters to the new units.

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.

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 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.

Local variables

V	The current value.
I	The current index.
C	The current channel (0 to NC-1). With this variable, channels can be shifted relative to one another.
NC	The total number of channels.

Usages in other examples

Example name	Example for
Lookup	Lookup channel node	Load \| Launch