Monday, April 26, 2010

Do You Have Any Special Techniques Using Inverse Kinematics (IK) in "Freehand Animation" When the Hands are Free From Other Objects?



First, it is important to understand some of the differences between IK and FK. When you are animating an arm in FK, you first move the shoulder, then the elbow, and then the wrists. That's more or less what we do when we move our own arms. So, all and all, FK seems a bit more similar to the way we move, with the movement starting on the shoulder. The process of animating an IK arm is very different; instead of starting the movement from the shoulder, you will work in the inverse direction. You position the wrist wherever you want it to go, and the rest of the arm kind of goes with it. Both ways of animating are perfectly fine, and, for example, when animating a character dancing, walking, or running, you can certainly achieve good results with both IK and FK.


The important thing here is to keep in mind that a realistic movement of the arms will mostly start on the shoulder. This is somewhat easier to achieve with FK, because you will will work from the shoulder down. However, it can be a little tricky with IK – the very method of animating with IK, positioning the wrist first and having the rest of the arm following it, will make it look like the hands are always leading the movement. Your job as an animator is to make it feel like the shoulder is leading, even though you are positioning the wrist first.


Another important point is that when you are animating a human body, all the body parts need to feel interconnected. When you move the spine, the arms will also move. When you raise a hand up in the air, the spine will move as well. Again, this is somewhat easier to achieve with FK. If you have the arms in FK and you move the body node the whole arm – including the wrist - will go with it, so we perceive the arms as being connected to the rest of the body. With IK arms, the movement of the spine will not automatically affect the position of the wrist. You will move the body node and the wrist will be stuck in place. Because of that, sometimes we can have the feeling that the arms (wrists) are independent from the body.


So, if you are animating with the arms in IK, you have to try your best to convey the feeling that the movement is rooted on the shoulder, and it is not the hand that is leading the movement. Basically, you have to fight the IK, and animate the wrist and the elbow just like a real arm moves. You will need to position the hands making sure you move the elbow in a way that will sell the idea that the root of the movement is in the shoulders. It is really, really important to pay special attention to the elbow and also the clavicle and chest when you are working in IK. My advice is that you study the movement in question, in your own body and also looking at reference, making sure you understand what is the relationship of shoulder/elbow/wrist for the particular scene you are animating. Then try to get the same relationship going on your shot, even though you are positioning the hands first.


The other important point is to not leave the hands stuck in space while the rest of the body is in movement – a very common error for beginners. That's what we call an “IK-ish look” – for example, let's imagine that a character steps to his right side and reaches out for a glass of water. You will have the shift of weight, the spine/torso moving towards the right, the step, and then the right wrist moves towards the glass of water... but if you forget about the left wrist... the left wrist will be stuck in space as if it is glued to the air. If this happens even for a couple of frames, it will look unnatural; people never have an experience where their whole body moves but their hand gets stuck in mid-air! So, if you have the arms in IK, you have to fight the “IK-ish look” by selling the idea that the movement of the hands is rooted on the shoulders; by making sure the the wrist feels connected to the rest of the body; and by not having the wrists stuck in place while the rest of the body moves around.

Guest Blogger Raquel Rabbit

21 comments:

Unknown said...

I find that using a combination of both can work well. I like to get as far as I can using FK, then switch to IK to fix up any odd motions or wobbles. Sometimes I'll end up switching between IK and FK a bunch of times per scene, depending on the complexity of the motion. In the end, I guess it doesn't really matter which technique you use, as long as you get natural looking results.

Anonymous said...
This comment has been removed by the author.
Anonymous said...

John, how exactly are you fixing odd motions and wobbles with IK if you've switched off the FK? This seems like a great way to work, I don't understand how it's possible with binary switches...

Unknown said...

Currently I'm working in MotionBuilder, the IK/FK rig in there is nice and easy to use. My workflow like such: I get all my animation to the point where you are more or less happy with it on FK. I'm Motionbuilder, the IK and FK rigs are always following each other. So at this point, wth IK still active, I bake all the animation on the IK rig, which basically transfers the motion from the FK to IK. I can now turn the FK off and IK on and I am left with identical motion. In this way you can flip back and forth between FK and IK, retaining all of your animation. As long as you are comfortable dealing with a lot of keys, I find the method quite useful. Unfortunately I have yet to find a similar rig on Maya.

Wei Xing said...

Good article, thanks!

Bruno Andrade said...

nice article, I always use FK, unless I have to pick something up then I switch, but I`m yet to practice more with Ik and see how it goes. Something comes to mind, how can one achieve overlaping motion using IK arms? Like when you want to break joints a little?

Ken Music said...

Yeah, when I was working at Tippett last year I used this method of switching back and forth between IK and FK several times.

I find IK to be especially good when blocking in the arms where the wrists are close to the body (as they are when holing something close to the chest) and arms are bent. These poses are really hard to get with FK and you are constantly fighting the FK rig, counter animating rotations to get poses. With IK one simply moves the wrist and pole vector (or inverse twist) and you're done. Sometimes I would block in IK and then switch to FK so that I could make body adjustments and not have to tweak the IK poses.

At Tippett, the way I would do it was by using the IK/FK switching script we had there. I would block in IK the main poses, then step pose by pose matching the FK poses with the scrip. Then I would switch to IK off and presto! Most times the FK would give me the arcs for free so polish was easier as well.

Jerzy Perez said...

I translated this article to spanish and put some visual examples, you can see it in my blog:

http://jerzy3d.blogspot.com/2010/09/st1behaviorurlieooui-style-definitions.html

hope it will be helpful!
cheers,
j.

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