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The Handle CHOP is the "engine" which drives Inverse Kinematic solutions using the Handle object. The role of the Handle CHOP is to generate rotation values for the bones which will bring their attached handles as close to their respective targets as possible.
The use of this methodology is best described with an example:
Increase the frame range to 10000. Create a Null object called 'target' and animate a few random positions for it to travel to.
Create a three bone IK chain with No Kinematics. Append a Handle object to the last bone.
In the Handle object set the Target menu to Target. This is the null object you just created.
The system is now specified. To actually start the IK enter a CHOP pane.
Place a Handle CHOP. In the Source field type in the names of the bones that were earlier created.
In the Viewport, you can use the Select state to select the bones you created. Then in the Handle CHOP click the Grab Source Form Selection button. The names are entered automatically for you. You don’t have to worry about being too picky when selecting objects this way, since non-bone objects are ignored.
Click the Export button on the Handle CHOP and click Play. You should now see the two-bone system chasing the animated null.
You can place any number of bones with any number of (possibly zero) Handles attached to the system. The following parameter description will now give a more detailed explanation of the functionality:
Creates rx ry rz channels for every bone listed.
Grab Source From Selection
Select bones in the viewport and select this to automatically enter bone names above. Non-bones are ignored.
Enter groups of bones which are to rotate as a combined unit. (Eg, a shoulder or pelvic branch).
Grab Fixed From Selection
Select bones in the viewport and select this to automatically enter fixed bone names above.
Specifies the accuracy of the solver.
Specifies a frame in which the bones are reset to their default rest angles.
Specifies the number of iterations to solve at the initialization frame.
Max Angle Change
Specifies the maximum change in degrees the solver is allowed to move each bone per frame. Use this parameter if the solution is too drastic.
When ON, only the current frame is solved. When OFF a range of frames is solved. Turn this option off to cache and or edit the entire solution.
Specifies how the Start/End fields are evaluated below.
Specifies a range of frames to solve for when Single Frame option is turned Off.
The sample rate of the channels.
The left extend conditions.
The right extend conditions.
The default value for extend conditions.
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.