Forward And Backward Reaching Inverse Kinematics (FABRIK) is an IK solver that operates on a chain of bones of arbitrary length as long as there are at least of 2 bones links in the chain.
Overview
You can add the FABRIK blueprint node to the AnimGraph of your Animation Blueprint. Once added, you can integrate the FABRIK node into your AnimBP using your objects Component Pose.
You can use the FABRIK node's Effector Transform input pin to connect a transform variable to implement a relative or absolute bone transform to the chain. You can use a relative transform to reference a different bone from the same skeleton for the transform, or transform the chain without a reference using an absolute transform.
In order to determine the strength of the applied Skeletal Control, you can set an alpha value on the Alpha input pin, either manually on the Alpha input pin or with a dynamic variable in the AnimGraph. Larger alpha values apply more Skeletal Control, while lesser values apply less.
Properties and Settings
Inside the Details panel for the FABRIK node you will find additional settings which you can be use to adjust how the solver functions:
In the End Effector section, you determine the target location and rotation.
The End Effector properties, found in the FABRIK node's details panel are very similar to the TwoBone_IK node properties.
| Property | Description |
|---|---|
| Location, Rotation, Scale | Coordinates for target location of tip bone. If the EffectorLocationSpace property is set to bone, this is the offset from Target Bone to use as target location (can also be set as a Pin on the node). |
| Effector Transform Space | Set the position of the bone in SkeletalMeshComponent's reference frame. |
| Effector Transform Bone | If the Effector Transform Space property is set to bone, this properties defined bone will be used. |
| Effector Rotation Source | Control the Rotation (maintain component space, local space or match end effector target rotation). |
In the Solver section, you define the chain of bones to use, from the Root to the Tip. The Tip will try to reach the end effector location.
| Property | Description |
|---|---|
| Tip Bone | Define a bone as the Tip Bone from the Skeleton Tree. |
| Root Bone | Set a reference to the skeleton's Root Bone from the Skeleton Tree. |
| Precision | Here you can set a value that will operate as the Tolerance for final tip bone location delta from Effector Location input pin. The lower the tolerance, the more precise the tip bone's location will be in relation to the End Effector target, but will result in higher performance cost for the evaluation. |
| Max Iterations | Here you can set the maximum number of iterations allowed. You can tune this value in order to control performance cost of the evaluation to ensure high volume usage will not reduce your project's frame rate. |
| Enable Debug Draw | Here you can toggle drawing of axes to debug joint rotation. |
In the Settings section, you set the strength applied.
| Property | Description |
|---|---|
| Alpha | Current strength of the Skeletal Control (can also be set as a Pin on the node). |
| Alpha Scale Bias | Set the Minimal and Maximal input scale values. |
| Node | Can be used to reset the Effector Rotation Source if it is set to something other than Local Rotation Relative to Parent. |
