The Constraint Path CHOP computes a position on a path using a distance on the path or a parameteric coordinate. It can also optionally compute a rotation to make the object look at the direction of the path. The up vector for the lookat can be given as a vector point attribute on the path, as a up vector parameter or as channel in input2.
The path geometry to follow. Works with Bezier and Nurbs curves. It also works with Polygon curves but will result in jumpy direction vectors if the Along Path mode is used. It doesn’t support Subdivided Polygons yet. Distance based parametrization modes will use the 'arclength' primitive attribute to speed up computation if it is present. You can use the Measure SOP in Perimeter mode to compute this attribute.
How to map the Position value on the path.
A value ranging from 0 to 1 where 0.5 is the middle based on the curve length.
A value ranging from 0 to 1 that depends on the number of knots on the curve.
Distance from Start
A world unit distance on the path computed from the start of the path.
Distance from End
A world unit distance on the path computed from the end of the path.
Distance Point Attribute
A user distance point attribute on the path.
The attribute name representing the Distance point attribue on the Path Geometry. The attribute must be a vector with 3 components, but only the x component is used.
Where to sample the path. See Parametrization.
Look At Mode
Define how to compute the Look At direction vector.
The constraint won’t change the previous rotation.
Use the path tangent for the Look At vector.
Direction Attribute from Path
Use a vector attribute value on the current position for the Look At vector.
Look Up Mode
Define how to compute the Up vector.
Use the Up Vector parameter value.
Up Vector Attribute from Path
Use a vector attribute value on the current position for the Up vector.
Use the path tangent for the Up vector.
Look At Axis
Define which of the 6 axis (X-,Y-,Z-,X+,Y+,Z+) is oriented along the Look At direction.
Look Up Axis
Define which of the 4 allowed axis act as the up direction. The allowed axis change based on the Look At Axis. You can’t use the same axis to define the look at and up directions.
The attribute name representing the Direction vector on the Path Geometry.
The attribute name representing the Up vector on the Path Geometry.
Roll angle in degrees around the Lookat At Axis.
A tolerance used when converting parametriztion. Lower the value to get more precise results.
The alignment option to use.
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 channel’s start and end to that range.
Shift to Minimum
Find the earliest start and shift all 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 channels so they all end at that index. Extend all channels to the length of the longest one.
Shift to First Interval
Shift all channels to the start of the first channel and sample all inputs using the first input’s range.
Trim to First Interval
Trim all channels to first channel’s range.
Stretch to First Interval
Stretch all channels to the first channel’s range.
Trim to Smallest Interval
Trim all 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.
Specifies the range of data to generate.
Use Full Animation Range
All of the animated range.
Use Current Frame
Only the sample at the current frame.
The range is specified from the Start and End parameters.
Use Value Animation
The range is taken from range of keys available in the evaluated Value parameters.
The start time of the channel range.
The end time of the channel range.
The sample rate of the CHOP.
Number of Threads
The number of separate threads to use to evaluate the channel samples. The default is no threading.
Since Houdini evaluates the VEX program for each sample in the input geometry, it can benefit greatly from threading on a multi-processor or multi-core machine when the CHOP nodes have many samples.
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.