Inverse Kinematics Professional

There are 2 approaches on how to modify the orientation of bones in UMotion.

The traditional forward kinematics approach lets you directly modify the orientation of every bone. This is a simple and straight-forward way to define poses. While animating you often have situations where you want to place e.g. a hand of the character at a certain position. To achieve this in forward kinematics it is necessary to rotate every bone of the arm until the hand is in the correct position. It is hard and time consuming to do a precise placement of the hand.
Inverse kinematics allows modifying the end point of a bone chain directly. For example, the hand position can be modified directly and all bones of the arm are updated accordingly. This results not only in a faster workflow but it also enables new possibilities like sticking the hands to a world position.

Rig Layers

Most of the time it depends on the situation if forward kinematics or inverse kinematics is more suitable. UMotion allows you to seamlessly switch between both approaches during an animation. This is possible due to the so called rig layers:

There is one rig layer for forward kinematics and one for inverse kinematics. It's like the animated GameObject has two skeletons - one that is affected by forward kinematics and one that is affected by inverse kinematics. Each inverse kinematics constraint has a switch that can be used to smoothly blend between the two skeletons.

Explanation Of Terms

The IK handle is the joint/transform that has the Inverse Kinematics Constraint attached. It controls the rotation of a chain of bones whereby the end of the chain is the IK target.

The pole axis is the line between the start of the first bone in the IK chain to the start of the target bone.

Inverse Kinematics - Naming Definitions

Inverse Kinematics Goal

The IK constraint's goal is to rotate all bones of the chain in such a way, that the IK target's position is the same as the position of the IK handle. So by moving the IK handle in Pose Mode (all Constraints are disabled during Config Mode) the whole IK chain will update automatically.

The IK solver used by the IK constraint is a Rotate Plane Solver. This type of solver is incredible easy to use and produces robust results (no jitter, sudden bone movements). The algorithm projects the bone chain and the IK handle onto a 2D plane (shown in blue in the screenshot below) and applies the IK solving algorithm in 2D space. Elbows, knees, etc. are only bending in the direction the arrow of the IK plane is pointing to. The bone chain is then rotated in such a way that it points into the IK handle's direction thus making the IK target position match the IK handle's position.

Inverse Kinematics Plane

When configuring an IK constraint the IK plane should always point into the direction the elbow, knee, etc. should bend. The surface of the plane defines the space in which the bones will be bended or stretched by the IK algorithm.

The IK handle can't be a child of a joint/transform in its chain.

If the parent of the IK handle is set to be the hips, then for example, the hands will stay at the same place relative to the hips. If the parent is set to be the animated GameObject's root, then the hands won't move when the character's hips are moved. This way it is possible to pin the hands at a certain location (useful for climbing animations etc.).

Only one Inverse Kinematics Constraint can be added per joint/transform.

IK Pinning

The Child-Of Constraint can be used to extend the Inverse Kinematics Constraint with IK pinning functionality. When an IK handle is pinned, it keeps its current position even if the rest of the rig is moved. When an IK handle is not pinned it moves with the rest of the rig.

Setup