FK/IK blend ?

[michael]

just a question for character animators...

do you guys and gals uses the blend at all...or do you just switch back and forth?

I don't do any character animation so I really don't know what the advantages are in having a blend - switching back and forth I can see but my experiences in Maya with FK/IK blend lead me to believe that it's kind of a hassle...

any thoughts?

[will]

not that i have used it in a production environment , but i have been told by those that do that having the ik/fk blend is very helpful in particular situations.

the example given to me was a character that is falling in the air and then lands on the ground, like a character skydiving. i guess the speed of the transition makes the blend useful. just a guesstimation.

[edward]

I'll add my two bits from what I've learned but beware that I don't have production experience either.

The premise behind IK/FK blending is that it's more natural and easier to key arcs using rotations rather than translates. Not only is it more natural, it also takes a lot less keys on rotates (of bones) to do arcs than to get good looking arcs by keying translates (of IK goals). When only given IK, I've once even went so far as to draw the actual curve geometry and then keying the translates of the IK goal along the curve.

The most common setup I've seen is to use IK/FK blending for arms. Here, the arms are in FK mode probably most of the time because of their swinging nature and only in IK mode when the animator needs it to be sticking to something. Although most people use only IK for legs because they need to stick to the ground most of the time, actions like kicking are easier to do with FK. Another situation I've seen is where a character jumps up onto monkey bars, goes across and comes back down. When on the ground, you have FK on arms and IK on legs, but then the situation is reversed when going through the monkey bars where the arms are IK and the legs are FK.

There's two drawbacks to using IK/FK blending though. First, more management of keys is required. Not only do you need to manage the IK translate keys, the FK rotate keys, but also the keys on the IK/FK blend channel. When you want to shift a pose, you need to make sure to move the keys on the blend as well as the translates/rotates together.

The second problem is that when you switch from FK to IK, your solution can pop. First note that even when in IK mode, there's usually more than one potential solution to how your bone chain touches the goal. So what happens is that when you switch from FK to IK, even though your FK solution touches your goal, there's no reason for the IK solver to choose the same solution. And if it chooses a different solution, you get popping. There's two ways to address this problem. The common way is to give a transition period where it blends from FK to IK over a couple of frames. This "eases in" the popping that you would otherwise get. This probably works most of the time but I've also heard of situations where the animator hand keyed every single frame in the transition period. The second way to address the problem is to key your rest angles when you go from FK to IK. This will ensure that your IK solution matches your FK solution because you're telling it to use the FK solution as the preferred angles. The drawbacks to this are: 1) those might not be the rest angles you want, and 2) even more key management as you have another 3 keys per bone you need to worry about.

This is looking at IK/FK blending at a very low-level view though and you might be able to hide some of that complexity with higher-level tools.

[michael]

thanks guys...

I do understand the use of the blend, I guess I should have phrased it better, I was really just refering to the transition between FK and IK and asking what the advantages were to having a blend rather than a simple switch...

the poping issue is the one reason I can think of that can be cured(?) by having a blend

I'm trying to think of a nice solution to get the switch to the point where it can be used without resorting to the need for a blend, so the animator can just choose what he/she wants and keep going.

I guess what I wanted to know is if animators like having the abillity to blend