Animation is the process of creating and editing the properties of objects that change over time. These properties can be adjusted and set using keyframes, which are markers that specify the property values of a node at a particular time or frame.
For example, you can keyframe the joints and IK handles of a character’s arm to create an animation of its arm moving. You can set keyframes by selecting a node(s) and pressing ‘S’ key on the keyboard.
Curves can be formed from two or more keyframes (particularly cubic interpolation) to create “in-betweens”.
Animating 3D shapes and meshes can be relatively straightforward, however character animation is more complex. A lot of motion could be happening across many different bones. The character could be moving up and down, side to side, rotating in several axes, and different parts of the body might be moving in different ways at different times.
Characters typically have limbs that can be tricky to animate solely with forward kinematics (FK). Inverse kinematics (IK) is an alternative method to animating limbs or body parts such as tails.
So what is forward and inverse kinematics?
TLDR: Use IK when there is a force acting on it (e.g. foot on ground, hands pushing against wall). FK for everything else.
Let's take the example of an arm. To animate an arm using forward kinematics (FK), you rotate the upper arm away from the shoulder, then rotate the forearm, then the hand and so on. This animation starts by adjusting from the root to the leaves.
Inverse kinematics (IK) is a method of animating that reverses the direction of the chain. Rather than work from the root, it works from the leaves backwards.
To animate the arm using IK, you move a goal (e.g. IK handle) that positions the hand itself. The upper and lower arms are rotated by the IK solution which follows the hand, called an end effector, toward the goal (IK handle). This allows moving the hand to any given position without worrying about how to rotate the upper and lower arms.
IK is particularly useful for planting feet onto the ground or making a character touch an object with the hand. In many cases, complex limb motions can be posed and animated more quickly with IK than FK.
Many IK solutions operate on a simple three bone chain e.g. thigh, knee, foot.
The reverse foot operates on a four bone chain (thigh, knee, foot, toe) and is typically used for foot animation such as foot-to-ground placement and locking. Reverse foot offers control over the heel and toe to create animations such as tip toes, foot bends and more. It is usually easier to perform foot-to-ground placement with the reverse foot than a standard three bone IK chain.
In industry-standard software such as Maya and Blender, the reverse foot handle can be quite tricky to set up, especially for artists who are not used to rigging character models. However, in PixCap this is as simple as 5-7 clicks (see Reverse foot section).