What's "forward" about "Forward Kinematics"?

[robcat2075]

I'm writing a script for tutorial and I'm trying to briefly acknowledge why the word "forward" is in "Forward Kinematics" (FK)

 

How about this... this line happens after a a demonstration of putting a chain of bones in a dinosaur tail that is animated in FK...

 

The practice of controlling bones with parent-child hierarchies is often called "Forward Kinematics", or FK, because the movement of any bone in the chain goes forward to affect the remaining bones in the chain.

 

 

From there we go to show why Inverse Kinematics is also needed and so on.

 

I want to introduce the term because it is a common one but i don't want it to be arbitrary jargon.

 

However, I've never seen a cogent explanation of how "forward" came to be part of that term except to say that it's the opposite of "inverse", which is really isn't. <_>

 

Has anyone seen a better, shorter explanation?

[Tore]

I guess both IK and FK could be said to control bones with parent-child hierarchies. I found this explanation in the Softimage Wiki, that maybe could be of use:

 

About Forward and Inverse Kinematics

--------------------------------------------

Chains are manipulated using inverse kinematics (IK) and forward kinematics (FK). The method used depends on what part of the chain you manipulate.

 

• IK is a goal-oriented way of animating: you move the effector (end) of a chain in place and Softimage calculates the angles at which the previous joints in the chain must rotate in order for the bone or end of the chain to reach its goal. This process is called solving.

 

IK is an intuitive way of animating because it’s how you traditionally think of movement. For example, when you want to grab an apple, you think about moving your hand to the apple (goal-oriented).

 

It’s also a quick way of creating poses which you can then animate, keying either the effector’s translation or the bones’ rotation values. For more information, see Animating with Inverse Kinematics.

 

• FK allows for complete control of the chain’s behavior because you rotate each joint’s position. Only the angle of the selected joint is affected; all other joint angles are preserved. Positioning a skeleton’s hand to grab an apple means rotating each joint in the arm, from the shoulder to the wrist and fingers.

 

However, using FK allows you to create many types of movements that may not be possible to animate with IK alone.

[robcat2075]

Positioning a skeleton’s hand to grab an apple means rotating each joint in the arm, from the shoulder to the wrist and fingers.

 

that might possibly be re-written as

 

"Positioning a skeleton’s hand to grab an apple means rotating each joint in the arm, working forward from the shoulder to the wrist and fingers."

 

Still... a somewhat doubtful use of the word "forward".

 

I wonder who coined the term "forward kinematics"?

[Tore]

After all there is some kind of logic to it. In FK the energy/kinetics moves from the joint FORWARD to the goal, whereas the energy in IK runs in the INVERSE direction, from the goal backwards to the joint...if I have understood it correctly :-)

[Rodney]

I wonder who coined the term "forward kinematics"?

 

Dale Forward.

 

(Just kidding! Stranger things have happened though.)

 

 

Here' my try:

Forward kinematics- Everything forward of the moving joint is affected by the movement.

(Example: Grip your upper arm with your opposing hand and by moving the upper arm, move the lower arm and hand)

The motion of each part is related to the motion of the linked parts. That way you simply have to animate the starting and ending joints, and the ones in between will adjust themselves to create a more natural movement.

 

Inverse kinematics- Everything in the heirachy is (independently) affected by any moving joint.

Each piece has to be animated separately.

(Example: Regular arm)

[Simon Edmondson]

My understanding may be faulty but, as an effort...

 

Forward Kinematics

The movement of one bone affects the position of all bones forward of it in the heirachy within the world space, without changing their position relative to each other.

 

Inverse Kinematics

A change in the world position of the bone at the end of the heirachy, will affect the relative positions and the world positions of the bones that precede it.

 

for example

If a bicep is rotated with FK, the relational positions of the bones in front of it stays unchanged, while their world position is altered.

 

In IK

If a hand is moved in the world space the relational positions of the bones which precede it will change as well as their position in the world space.

 

 

tuppence worth.

simon

[largento]

It seems to me that I've heard the term forward used in animation for animating without key frames. Just starting and going forward. As opposed to key frame animation.

 

FK would be comparable to stop motion, which would be forward animation.

 

I just woke up, so maybe I'm wrong on this, but I thought that was the reasoning behind the name.

 

[edit]

 

I was confusing it with straight ahead animation, so I guess that doesn't work. Shouldn't try to think before morning coffee. :-)

[robcat2075]

Just for the record, here's the Wikipedia definition of "Kinematics"

 

Kinematics is the branch of classical mechanics that describes the motion of points, bodies (objects) and systems of bodies (groups of objects) without consideration of the causes of motion...

 

The study of kinematics is often referred to as the geometry of motion.

[robcat2075]

Let's see...

 

Forward = "progressive"

 

Kinematics = "motion of objects"

 

 

 

Forward Kinematics = progressive motion of objects

[largento]

I think Rodney's explanation is probably the best way to use the term "forward."

 

You could have a visual with a character and a background image with an "x" or target that you show the two ways of getting a finger to that mark. The first, rotating all of the joints from shoulder to fingertip and then just grabbing the finger and putting it in place. If you do this from the shoulder being screen left and the target being screen right, this will visually read as forward and backward.

 

It will make IK seem preferable, but then maybe you can follow up with a quick video comparison of the animation showing how the movement of the FK has arcs and looks more natural.

[NancyGormezano]

I have two cents worth of understanding, maybe...not sure...but I susspexs:

 

Forward kinematics: mathematical ordering of the computation (solving) of the position, orientation of the bones going DOWN the hierarchy. ie bicep first, then forearm, then hand.

 

Whereas the inverse computation or inverse kinematics (going UP the hierarchy) is that the hand postion, orientation is used to compute the position, orientation of the forearm, which is then used to solve the position, orientation of the bicep.

 

I suspect the terms are probably related to the order in which the matrices get multiplied.

[Rodney]

Forward = "progressive"

 

Hmmm.... let's try to go literal here for a moment (but with a bias toward the word 'forward kinetics' in our definitions.

 

Forward: fore (front) ward (subordinate to)

 

Kinematics: the study of how a given force will mechanically move (step by step) a set, series or sequence of objects.

 

I use the word 'step' here because that is what we find with 'gress'; the root word in 'pro-gress'.

Although, "progressive" might not be quite as precise as "progression". That's more gut feel than anything though.

It seems to me that 'progressive' is a looking back from a forward perspective (perhaps from the end of the causal chain?) whereas 'progression' is more concerned with a view of a source (or if you prefer... the location where force is/was applied).

 

So in a sense we take your definition of kinematics:

Kinematics is the branch of classical mechanics that describes the motion of points, bodies (objects) and systems of bodies (groups of objects) without consideration of the causes of motion...

 

And add in a consideration of the initial force.

In other words change the word 'without' to 'with' and you have a decent working definition of 'forward kinematics'.

 

Following this logic we can then determine that 'inverse kinematics' primarily focuses not on the origin of the force but on the end point.

 

Nancy said:

I suspect the terms are probably related to the order in which the matrices get multiplied.

This makes sense to me especially if we consider that in IK a hand may stay in place even though a forearm or arm or shoulder or body is in motion.

With FK this wouldn't be the case because the hand will be affected by anything upstream in the hierarchy.

 

I think... I'm looking at that right.