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The Feedback CHOP allows you to get the state of a CHOP as it was one frame or time slice ago. This allows you to connect CHOPs in circular loops without incurring the dreaded 'Infinite Recursion' error, because it simply copies its input without cooking it first.
POPs do this inherently, and some CHOPs like Lag and Filter look back in time (iterate) internally to the chop, but sometimes the output of a chain of CHOPs is needed as the input of the same chain one frame later.
For example, if you need the position or speed of an object from a frame ago in order to compute its position, displacement or speed at the current frame, you would select a chop containing those values and feed it into the Feedback CHOP. It will then output it a frame or time slice later.
Since its input is not recooked, it must be forcibly updated by following it with an exported CHOP to a displayed object.
Specifies how the input should be copied.
Previous Channels at Previous Time
All in the past.
Previous Channels at Current Time
Some at current time.
Last Sample at Current Time
Only last sample at current time.
If on, adds a
dt channel whose value is the elapsed time since
the last cook. It is expressed in units determined by the Units
parameter of the Common page.
Some of these parameters may not be available on all CHOP nodes.
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:
Matches a single channel name.
chan3 tx ty tz
Matches four channel names, separated by spaces.
Matches each channel that starts with
Matches each channel that has
foot in it.
? matches a single character.
t? matches two-character channels starting with t.
Matches number ranges giving
[xyz]matches three characters, giving channels
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.
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).
When you change the Units parameter, it does not convert the existing parameters to the new units.
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.
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
You can leave the Export Prefix blank, but then your CHOP track names need to be absolute paths, such as
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.
In this example, we demonstrate how a feedback node can be used to get the state of a CHOP as it was one frame or time slice ago.
The following examples include this node.