Area
channel node
Calculates the area under a channel’s graph, which is the same as calculating the “integral” of a channel, or “integrating” the channel.
This CHOP calculates the area under a channel’s graph, which is the same as calculating the “integral” of a channel, or “integrating” the channel.
It uses the graph of each input channel and calculates the area between the graph and the horizontal 0-value line. It finds the area between a Start and End index, which is by default the entire chop range.
The area is calculated by adding the channel values for every sample, starting with the sample at the Start index. Negative values reduce the area. The area is converted to the Units by dividing by samples per Unit. The cumulative values are put in the output channels.
This CHOP is particularly useful for calculating a point’s position from its velocity (speed) or acceleration. If the input is a velocity, a First Order integral will return the position. If the input is an acceleration, a Second Order integral will return the position, and a First Order integral will return the velocity.
The first input contains the channels to be integrated.
The second input is used to reset the area to zero. At samples where the second input is zero or less, the area is reset to zero. A Wave chop passed into the second input causes the Area to be zero for half a cycle.
The third input is an optional start/end reference. If connected, it will override the parameters in the Range page and integrate the first input’s channels between the start and end of the reference input.
Parameters
Area
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Order |
Determines the order of the integral to use. |
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First Constant |
Constant to add to the entire result after integrating once. |
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Second Constant |
Constant to add to the entire result after integrating twice. |
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Third Constant |
Constant to add to the entire result after integrating three times. |
Interval specifying the range to integrate over
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Unit Values |
Determines whether the start and end parameters listed below are absolute or relative to the channel’s start and end. |
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Start |
The start of the range over which to compute the area. |
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End |
The end of the range over which to compute the area. |
Common
Some of these parameters may not be avaiable on all CHOP nodes.
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Scope |
To determine which channels get affected, some CHOPs have a scope string. Patterns can be used in the scope, for example The following are examples of possible channel name matching options:
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Sample Rate Match |
The Sample Rate Match Options handle cases where multiple input CHOPs’ sample rates are different.
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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. |
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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. |
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Unload |
Causes the memory consumed by a CHOP to be released after it is cooked and the data passed to the next CHOP. |
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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 Note
You can leave the Export Prefix blank, but then your CHOP track names need to be absolute paths, such as |
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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. |
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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
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C |
Current channel index. |
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NC |
Total number of channels. |
Example files
Area with one input
Area with three inputs
$HFS/houdini/help/examples/nodes/chop/area/Area03.cmd
This example contains a demonstration of the Area CHOP using all three inputs.
A single Wave CHOP is used in the first and second input as a source and as a range modifier. Then another Wave CHOP is used in the third input to modify the length of the output channel of the Area CHOP.