Monday, October 20, 2008

Poll Results: FK or IK?

Thanks everyone who voted.


Just so you know, this poll was purely for my own curiosity, since the last 3 animators I chatted with preferred IK arms (great animators btw). I never want to start a war by saying one method is better than the other. I see advantages for both, esp. for specific situations.

With that disclaimer out of the way, my personal preference, if a situation allows for either, is FK arms (with rotations that follow world space). For me, it feels faster to pose and edit. I'm sure proponents of IK arms would argue otherwise. Translating in all 3 axes always feel slightly slower to me as I'm tumbling the camera around more. Also, I feel that if the IK's are following world space translations, editing anims could be a bit trickier once things are offset.

OTHER FK PRO'S (in my humble opinion):
  • You get some of the arcs for free. Just remember that you should always check them and add keys if necessary.
  • You get some free noise as well because the arms are translating with the torso. And with rotations set to follow world space, you don't get TOO much noise that is often magnified by fast moving chest rotations.
  • With FK, it's easier to make it feel like the body and shoulders are driving the arms. With IK, you could definitely do it--you just got a work slightly more to avoid the marionette look.


  • The big one, of course, is whenever the character's interacting with the environment or specific targets. For these situations--IK all the way! Just watch out for the magnetic hands and feet syndrome. Maybe slide them a bit before the complete stop.
  • Character flopping around like ragdolls (for deaths, knocked down anims, etc). Because IK's are "separate" from the torso, it's easier to get the whole body looking loose and floppy.
  • Stabs--I'd probably use IK for these. Just to note, though, I have used FK for these in the past when I'm...uh...too lazy to switch. I just add keys in the end and track paths and contact points. And while it's not ideal, a good amount of the 2 character attacks that I've done (like the alien stuff and the Kull beast take-downs) were all FK for the humans. If slippage is obvious then it's my bad and I probably should have been more adamant about switching to IK. If it's not that noticeable, then I guess I can say that it supports the idea that what ultimately matters is the pose on screen--forcing more keys to get the right pose whenever necessary.


I think with IK, you have more control when it comes to fine tuning the arcs of the hands, but you may need more keys to get those arcs in the first place. Some of my friends use IK set-ups that follows the Root/COG controller. This could be interesting to try. Another method that might be interesting as well is animating with IK's that translate with the top chest controller but doesn't rotate with it.

I do like to switch back and forth often, even when posing, so FK/IK matching tools for the rig is a must. Luckily, this is fairly standard with rigs these days. And if a rig doesn't come with it, it's probably the first thing I'd ask for.

So in the end, as I have always believed, use what YOU like because I'm sure the only ones who talk about FK vs. IK are us crazy animators. I'm sure everyone else who watch our movies and play our games could care less about these debates:) Also, I think real speed will come from knowing what you want with the animation at hand. And why do I always talk about speed? It's not because I want to call it a day early (though that can be nice!), but if I can get to 80% faster I'll have more time to milk that last 20%.

Wow, sorry if this was a bit long winded:) I totally thought this post was going to be a quick one.


Tristan Sacramento said...

For the quick and dirty get-it-in animations, i've been using ik hands that follow the chest. This is to get my poses and timing out quick. Keep my keyed frames to key poses and some inbetweens, get the timing and the feel right, then when it feels good, go back and switch them to fk. And if its a good rig, have the arms at the shoulder follow world coordinates. I agree with you tho that it really also depends on what your animation is doing. Having a versitile rig really helps.

TJ Phan said...

Very interesting! Awesome man, thanks for chiming in. And make sure you hook me up with some cool tasks once I join the team over there!

And if anyone else has ideas to add to the mix, definitely feel free! I love hearing this stuff.

jeff said...

it depends on the animation and the rotate order you have. i use fk with a global local control as much as possible (snWorldSpaceTool.mel makes working with any kind of switch easy), but if a character is rotating 360, world space arms aren't too much fun.

in the end, you're stamping down a lot of keyframes so I'm not sure how much it matters...having the arms in world space allows spine tweaking without losing your animation in your arm, or having to counter rotate agains them..

TJ Phan said...

Yeah, world space fk arms do get a bit tricky when the character's flipping and rolling all over the place. For these anims, I sometimes block with arms following local space, then convert them to global to finish up. To be honest though, I usually just keep them following global space from start to finish (maybe I secretly like gimbal lock).

I think when the characters rotating around a lot, it's tricky any which way. I still think I'd prefer to add more keys in the end to finesse arcs than to counter animate the arms everytime I rotate the chest.

And thanks for pointing me to snWorldSpaceTool.mel a while back--I love that script!

Anonymous said...

I love it! Very creative!That's actually really cool.