When animating in blender we will often come across IK and FK, they are the two most common methods for handling the posing and animation of our armatures, though the two can often be hard to distinguish. So, what is the difference between IK and FK?
IK and FK are two ways of handling motion where multiple joints are involved. In the case of an arm, FK explains a robotic control from the shoulder to the hand, one joint at a time. While IK works by positioning the hand and having the chain of joints back to the body adjust automatically thanks to constraints.
A more technical way to describe it would be something like this.
FK can be thought of as a Parent -> Child structure where the Child Bone's Position and Rotation is a combination of its Parent's Position and Rotation plus the Child Bone's position and rotation meaning position and rotation changes propagate forward, IK is Child -> Parent, it starts at the IK bone and averages out the positions and rotations of each bone in the chain.
In the rest of the article we'll go over more details about IK and FK, as well as when you should use either one.
While the example above may explain what the difference between IK and FK is, it can be a bit difficult to really visualize this difference with just a description, so let's look at an example starting with FK.
As mentioned above, FK has a Parent to Child structure where the Child Bone inherits its base Position and Rotation from its Parent Bone.
Another word we'll often seen used to describe this Parent to Child structure is chains, chains are simply the term used to describe a series of bones connected either via FK or IK.
In the case of FK, the chain starts at the parent and propagate forward.
Here, the chain goes from left to right with the left most bone being the top-level parent bone.
We can see how rotating a bone in the FK chain affects each subsequent bone in that chain. All bones going forward from bone two inherit its position and rotation. Since bone 3 is a child of bone 2 it inherits its position and rotation and as a result bone 4 inherits bone 3's position and rotation etc. until the end of the chain is reached.
We can think of IK like the inverse of FK hence the name Inverse Kinematics, but they are not a direct opposite of each other.
For example, IK bones can affect the rotation of the parent bones in their chain up to a defined chain length and will average out the positions and rotations of the bones in the chain to create a smooth bend.
For IK, the chain starts at the IK bone and propagates from child to parent until the IK chain length is reached.
In this example I have set the final bone on the right as the IK target, we can see that the IK chain is averaging out the positions and rotations along its length to create a smooth bend. Whenever we move the IK bone the chain will follow.
While the concepts of IK and FK may be similar as we can see their results can widely vary.
Next, we'll go over FK and when it's best to used it as well as some of its uses.
FK can be especially useful for when we want to move our bones using the Parent - Child structure. Each child can only inherit position and rotation from the parent bone. FK can have a wide variety of uses, mainly for situations where IK won't work or would be unnecessary to set up.
Here's a small sample of what FK is commonly used for:
In general FK gives us direct control over how our armature is posed, this can be very handy if for example we are setting up multiple poses for our model that we plan on reusing. For example we may want to create a set of facial expressions for a character or have a set of hand gestures. Since we'd likely be reusing these poses a lot, being able to fine tune them is very useful.
In this example I used FK for the fingers of the hand to be able to pose them manually to make sure the hand was grabbing the sphere properly, FK can be very useful in situations like this as it allows us much more control over how the armature is posed, though animating complex large armatures with only FK can be a tedious process that may be better suited for a IK - FK mix.
Finally, we'll go over when you should use IK and give some examples of how it is used.
One of the many and common uses of IK is to have a chain of bones snap to a point or reach for it. To give a few examples of how IK is used this technique can be used to snap feet to the group, and have characters reach out to grab objects as well as allowing characters to rotate.
Here are some examples when IK is used:
This is just a small sample of some uses for IK as it can have a wide range of uses since it can be controlled by modifiers to allow for a variety of effects.
In these examples I used IK to pose the arm portion of the Armature and then set the IK target to the red sphere so that the arm reaches and bends to be able to keep contact with it.
In general IK can speed up the animation process quite a bit as well as posing, in general IK just allows for a much more dynamic animation process and is great for getting the general pose or movement of our models set up, though IK can be a bit finicky sometimes or just not look the way we want it and in that case FK may be a better option.
IK and FK are both very valuable tools when posing and animating, they both allow for a much more efficient workflow and can be combined to create a wide variety of affects. In general, they both have many applications and are something 3D artists should know about as even outside of animation they can be very useful.
Thank you for your time.