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Vector math has several useful operations for use with channels, such as projections, dot products and distance/length calculations. This can be used for determining whether objects are facing each other, how far they are apart, or the angle between them.
This CHOP separates input channels into vectors based on the Vector Mask parameter, and applies a vector operation to each set individually. Two inputs can be used for binary vector operations. If the second input does not have as many vectors as the first, Vector cycles through the second input’s vectors.
Channels that are not assigned to a vector mask will not be processed or present in the output.
Some vector operations produce a scalar result. In this case, the vector channels are removed and replaced with a single channel, whose name depends on the operation performed (such as 'dist', 'dot' or 'angle').
Additionally, an optional test can be done, comparing the vector operation results to a constant value or other vector channels. The result of this test can be used to remove vector channels or to produce zero/one channels representing where these vectors fail or pass the test. The Test tab contains the parameters for the vector test.
Specifies which vector operation to perform on the vectors.
Magnitude of A
Computes the magnitude of A and replaces the vector channels with a single scalar channel, 'len'. If more than one vector is processed, the wildcard replacement is appended to 'len' (ie, (tx3,ty3,tz3) would produce len3). Only one input is required for this operation; the second input is ignored.
Reduces the vector to a unit-length vector. Only one input is required for this operation; the second input is ignored.
Distance between A and B
Computes the straight-line distance between A and B, as if they were points. This replaces the vector channels with a single channel named 'dist'.
A dot B
Computes the dot product of a vector from input 1 (A) with a vector from input 2 (B). If more vectors exist in A than B, the B vectors cycle back to the first vector. This replaces the vector channels with a single channel named 'dot'.
Normalized A dot B
Computes the dot product of A and B, but normalizes A and B first. This replaces the vector channels with a single channel named 'dot'.
Angle between A and B
Computes the angle between the two vectors, A and B, in degrees. This replaces the vector channels with a single channel named 'angle'.
A cross B
Computes the cross product of A and B. Both vectors must have exactly
three components. The cross product is the vector that is orthogonal to both A and B.
Scalar Project B onto A
The scalar projection of B onto A determines the length of the vector produced when B is projected onto A. This replaces the vector channels with a single channel named 'proj'.
Vector Project B onto A
Projects the vector B onto the vector A, so that the result is parallel to A, and the magnitude is proportional to the angle between A and B.
Adds vector A to B.
Subtracts vector A from B.
Subtracts vector B from A.
Passes vector A through as-is. Generally only useful when used with Perform Vector Test to compare vectors.
A set of patterns which describe the members of the vectors. A wildcard
* can be used to match more than one vector. A single vector will
contain one channel from each pattern with the same wildcard replacement
tx* ty* tz* would process vectors like
Vector Mask B
An optional parameter which sets the vector mask for vectors in the second input (B vectors), in case they happen to represent a different vector quantity (such as N, P or V). If disabled, the Vector Mask is used.
Vector Mask C
The vector mask for the third comparison input, similar to the above two. If disabled, the Vector Mask is used.
For two-input vector operations, this parameter specifies which frame range to use if the inputs' frame ranges differ, and how to sample the inputs.
Extend to Min/Max
Find the earliest start and latest end, and extend all inputs to that range using the extend conditions.
Stretch to Min/Max
Find the earliest start and latest end, and stretch every input channel’s start and end to that range.
Shift to Minimum
Find the earliest start and shift all input channels so they all start at that index. All channels are extended to the length of the longest one.
Shift to Maximum
Find the latest end and shift all input channels so they all end at that index. Extend all channels to the length of the longest one.
Shift to First Interval
Shift all input channels to the start of the first channel and sample all inputs using the first input’s range.
Trim to First Interval
Trim all input channels to first channel’s range.
Stretch to First Interval
Stretch all input channels to the first channel’s range.
Trim to Smallest Interval
Trim all input channels to the smallest start/end interval. The start and end values may not come from the same channel.
Stretch to Smallest Interval
Stretch all channels to the smallest start/end interval. The start and end values may not come from the same channel.
A comparison test can be run after the initial vector operation, which can remove vectors that fail the test or produce a channel containing the samples which passed or failed for each vector.
Perform Vector Test
Enables the vector test on the result of the vector operation, which compares the result to a constant value or another input.
Result Vector Test
Describes how the vector is compared, with one of the inequality operations listed.
The vector result is compared to a constant (Vector Constant) or vector/scalar channels from one of the three inputs.
This parameter is the constant when Test Against is set to Constant. It will either have all fields enabled (if the vector result is a vector quantity) or only the first field enabled (if the result is a scalar).
The output of the vector test can be the original vectors, modified vectors, or an on/off channel containing the test results.
Keep Original Vector A
If the test passes, the original A Vector is kept in the output. If the test failed, the vector channels are removed. This is useful for eliminating points or directions that fail the test in order to thin out the number of channels for faster downstream cooking.
Keep Result Vector
If the test passes, the result of the vector operation is kept in the output. If the test failed, it is removed.
A single channel is created called 'test' for each vector, with the suffix of the vector appended to it. This channel is 1 for samples where the test passed, and 0 for where it failed. This can be used for controlling a Blend or Switch CHOP.
When determining whether to keep or discard a channel (when Test Result is Keep Original Vector or Keep Result Vector), the test may pass for some samples and fail for others. Since a channel can only be entirely removed or entirely kept, vector channels that pass on some samples and fail on others must be resolved.
All Samples Pass
All samples in the vector test must pass the test in order for the vector channels to be kept.
One Sample Passes
Only one sample needs to pass the vector test in order for the vector channels to be kept.
A Fraction of the Samples Pass
A specified fraction (from 0.0 to 1.0) of the samples must pass the test in order for the vector channels to be kept.
If Pass If is set to 'Fraction of the Samples Pass', this sets the fraction of samples which must pass in order for the test to be considered successful. Zero means none need to pass, while 1.0 means that all samples must pass.
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.
The current channel (0 to NC-1).
The total number of channels.
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.
The following examples include this node.