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This CHOP calculates the slope (or derivative) of the input channels.

If the input CHOP represents position, the slope can be interpreted as speed. By default, the Slope CHOP converts position to speed.

In mathematical terms, the slope is the first derivative of the channel curve. The second and third derivatives can also be calculated. The second derivative can be interpreted as acceleration (and the third derivative could be interpreted as the rate of change in acceleration).

This CHOP can be used in conjunction with the Area CHOP to manipulate speed or acceleration directly. You can calculate the speed or acceleration of a moving object with a Slope CHOP and manipulate it with other CHOPs. Then you can convert the new speed or acceleration curve back to position data with an Area CHOP. You may need to adjust the starting position, since the Slope CHOP removes this information. This can be done with the use of the Constant parameters in the Area CHOP.

The Units option causes the output to be expressed as a change in value per sample, value per frame, or value per second.

## Parameters

## Slope

Type

The type of slope to calculate.

Slope

Calculates the slope of the channels.

Acceleration

Calculates the acceleration of the channels.

Acceleration of Slope

Calculates the acceleration of the slopes of the channels.

Method

The sample pairs used to calculate the slope.

Use Previous And Current Sample

The current sample and the sample before it are used. This is the only method applicable in realtime applications.

Use Current And Next Sample

The current and the next samples are used.

Use Previous And Next Sample

The previous and the next samples are used, producing slightly more continuous slopes than the other methods.

## 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).

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.

## Examples

Slope Example for Slope channel node

This example demonstrates how the Slope CHOP calculates the slope (or derivative) of the input channels.

If the input CHOP represents position, the slope can be interpreted as speed.

By default, the Slope CHOP converts position to speed.

The following examples include this node.

Slope Example for Slope channel node

This example demonstrates how the Slope CHOP calculates the slope (or derivative) of the input channels.

If the input CHOP represents position, the slope can be interpreted as speed.

By default, the Slope CHOP converts position to speed.

AverageSpeed Example for Vector channel node

This example shows you how to calculate the average speed for each particle over a given time interval using CHOP nodes such as Vector, Trim, and Math.

See also |