FK vs. IK and Arm Posing -- Opinion Needed

Support Basics & Interface
#1

Note: This refers “Rigging a Hand and Foot” from the Blender 2.3 documentation.

The arms on my model are both IK chains and are difficult to pose.

For example, to place a hand in a certain position requires moving the hand IK to the final position. The hand assumes an accurate position but then the wrist rotation and elbow placement can be wrong. To rotate the hand requires rotation of the shoulder IK which rotates the upper arm --> lower arm --> hand. However, the elbow can be moved.

My first solution, after frustrating myself for an hour last night working with an IK chain, was to break the movement into pieces. First bring the hand in front of the torso in side view. Then place the hand in front of the face in front view. Then rotate the shoulder IK in top view. Needless to say, this is very slow.

Reading recent posts has led me to question my use of IKs as the animation is essentially constructed by keyframing. I suspect that using FK chains would allow more precise placement of my model and reduce animation time.

Can more experienced users please post their opinions on the matter? Which is the preferred method when complex motions must be animated for humanoid characters: FKs or IKs? What are the advantages of choosing one over the other?

Lastly, if FKs are preferred, how can I convert my existing arms from IK to FK? Is deletion and re-rigging of new arms necessary? I spent quite a bit of time on rigging the hands (learning by doing) and am reluctant to begin again.

Thanks, Bob

#2

I must admit that I just use FK most of the time :-|. My animations are pretty short (2 min.) so I find by the time I have rigged a character using IK so that it works exactly as I want it to I would have completed the animation using FK. If I were to create a longer animation however, I would probably take the time to create a rig or modify an existing one since animating does go faster once you have a “good” IK rig.

IMHO I personally find the current IK system a little difficult to deal with.

GreyBeard

#3

You can go from IK to FK by selecting the bones in edit mode and clicking off the IK button next to the bone’s names. You then have to be careful not to move the bones in edit or pose mode unless you want it that way. If you accidentally move a bone it will stretch the attached skin ofcourse. IK can be useful sometimes but most of the times like graybeard mentioned FK is the way to go. Seems more natural, like posing those rubber bending stick figures that we got for toys.

#4

When you ask the question, “Which is the preferred method when complex motions must be animated for humanoid characters: FKs or IKs?”, I would say … neither one!

Complex motions are best described in terms of Actions, which are assembled together using the NLA window. For example, if you want your humanoid to Grin and ScratchHisNose while WigglingHisEars, the easiest way to do such an (oddball) thing is to define each of these individual activities as actions … using simpler mechanisms such as keyframes and RVKs, IKs or FKs … and then blend those actions together into a performance using the NLA.

The low-level facilities are used to describe simple motions. Complex motions are then achieved by blending those simple actions together.

#5

Hiya

IK should be much easier to pose than FK :slight_smile:

Correct

Wrong, there should not be a shoulder IK, the Hand position is made using the IK_bone of the hand. The hand orientation is done with FK! You grab the Palm bone and rotate it!

No, believe me, IK is vital, expecially it the hand/foot must stay still while the body moves!

Keep blending

Stefano

#6

I concur with s68. IKs are the way to go.

The main points you have to remember for IKs is they animate best when modelled at half joint flex. That’s in the manual.

This means that you position the leg half bent. It doesn’t really have to be 100% accurate because you can use FKs in conjunction with IKs to help the animation.

You should also remember to reset the bone roll angles before animation.

The main reason for IKs is exactly what s68 said - when you move a character, you want the feet to stay put so you don’t get awkward sliding on the ground. The setup in the manual is the one you want.

#7

You’re right. Acting on intuition, I place a bone at the shoulder which can rotate the entire chain. Are you saying that I should remove the bone and rely upon rotation of the hand bone during placement?

You did not address the matter of the placement of the elbow. The first time I placed the hand in front of the face, the elbow moved inside of the rib cage. Much work was required to learn that the arm positioning had to proceed in stages so that the elbow would be correctly placed.

I will examine the results of deleting the shoulder IK and relying upon rotation of the hand bone in the positioning process.

Thank you for your advice!

Bob

#8

Use a bone that’s offset from the elbow bone, and constrain the elbow to it with a lock track constraint. The elbow will always point that new bone.

Check out this rig for a sample:

http://www.harkyman.com/finalrig.blend