Handle channel node

The “engine” which drives Inverse Kinematic solutions using the Handle object.

See also: Export, Fetch, InverseKin, Object

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.

Tip

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:

Parameters

Handle

Source	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.
Fixed	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.
Iterations	Specifies the accuracy of the solver.
Init Frame	Specifies a frame in which the bones are reset to their default rest angles.
Preroll	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.

Channel

Single Frame	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.
Unit Values	Specifies how the Start/End fields are evaluated below.
Start/End	Specifies a range of frames to solve for when Single Frame option is turned Off.
Sample Rate	The sample rate of the channels.
Extend Left	The left extend conditions.
Extend Right	The right extend conditions.
Default Value	The default value for extend conditions.

Common

Some of these parameters may not be avaiable on all CHOP nodes.

Scope

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:

`chan2`	Matches a single channel name.
`chan3 tx ty tz`	Matches four channel names, separated by spaces.
`chan*`	Matches each channel that starts with `chan`.
`foot`	Matches each channel that has `foot` in it.
`t?`	The `?` matches a single character. `t?` matches two-character channels starting with t.
`r[xyz]`	Matches channels `rx`, `ry` and `rz`.
`blend[3-7:2]`	Matches number ranges giving `blend3`, `blend5`, and `blend7`.
`blend[2-3,5,13]`	Matches channels `blend2`, `blend3`, `blend5`, `blend13`.
`t[xyz]`	`[xyz]`matches three characters, giving channels `tx`, `ty` and `tz`.

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.

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.

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.

Unload

Causes the memory consumed by a CHOP to be released after it is cooked and the data passed to the next CHOP.

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 geo1:tx, and the prefix was /obj, the channel would be exported to /obj/geo1/tx.

Note

You can leave the Export Prefix blank, but then your CHOP track names need to be absolute paths, such as obj:geo1:tx.

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.

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.

Usages in other examples

Example name	Example for