What is Kinematics?

KINEMATICS

The character's skeleton consists of a hierarchy of bones. Bones can be animated using two methods: FK (Forward kinematics) and IK (Inverse Kinematics).

With the help of Forward kinematics, you move and rotate the bone or controller. This is useful for creating arc movements, for example, using FK, you can easily animate hand movements while walking.

With the help of inverse kinematics, you can move the IK Handle to animate the skeleton. Ik Handle controls all the bones between the root and final bone of the IK chain. This is useful when it is necessary to accurately position one object relative to other objects. Most often IK is used in the legs when walking, or when the hand is fixed at a certain point (for example, when the door is opened).

IK Spline is used to create soft deformations (indispensable for setting the back, tails, etc.).


FK

With FK, you rotate or move individual joints to pose and animate your joint chains. Moving a joint affects that joint and any joints below it in the hierarchy.

IK

With IK, you move an IK handle to pose an entire joint chain. An IK handle is an object you can select and move that affects the joints it is assigned to.

IK SPLINE

Use IK Spline for posing and animating long, sinuous joint chains like those for tails, tentacles, snakes, necks, spines, and similar objects.

IK/FK blending

You can pose and animate the joints of a joint chain using both FK and IK. This is called animation blending. See Blend IK and FK animation. This blending between FK and IK animation is possible because of the IK handle’s Ik Blend attribute. Ik Blend lets you switch between posing and animating with pure FK or pure IK, as well as control the blend between the two kinds of animation. See Ik Blend.

Posing and animating a joint chain with both FK and IK changes the way the joints and bones are displayed in your scene view. A joint chain with FK and IK is drawn using three default or user defined colors. The blue joint chain represents the skeleton with pure FK animation, the brown joint chain represents the skeleton with pure IK animation, and the magenta joint chain represents the resulting animation blend.

Also, you can customize the colors of the IK/FK joint chains and the size of the IK/FK joints. See Set the display of IK/FK animated joint chains.

In addition to blending IK and FK animation over multiple frames, a blend can occur over a single frame. If you want to animate an instantaneous switch between IK and FK, such a planted, stationary hand quickly rising into the air, you should blend IK and FK over a single frame. Whereas if you want to animate a motion that eases in or out of IK or FK, such as a character jumping up to grab a horizontal bar, you should blend IK and FK over several frames. Blending IK and FK over several frames is also useful when you want the prevent jerking in your animation.

Blend IK and FK animation
Example of IK and FK blending

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