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I created an IK setup for a humanoid character using the IK Setup Wizard. I then imported simple idle animation (the feet stays planted on the ground and the character body moves up and down slightly to indicate breathing). When I imported the animation the bones in the FK Rig Layer change from reference pose to the position and rotation of the first keyframe in the idle animation. The bones in the IK Rig Layer remained in reference pose, therefore, I select all the bones in the IK rig and click Set IK to FK ( in the tools section). Now both the IK rig and FK rig have the same position and rotation.

Using the FK/IK blend ( in channel section) I played the animation using the FK rig then the IK rig. And what I noticed is when I played the animation using the IK rig there was a slight difference in the animation also the character's feet were no longer planted on the ground, they move up and down off the ground.

I'm fairly new to game creation and animations, therefore, I'm missing a lot of basic knowledge. I'm assuming the problem is not with Umotion Pro but can you explain why this is happening and how to fix it? I need the IK rig to use the child of constraint to pin the left hand of the character to an object but I can't use the IK rig if the animation doesn't play correctly.

Sorry about the long paragraphs and thank you for your time.

1 Answer

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answered by Expert (148k points)
selected ago by
Best answer

thank you very much for your support request.

Instead of using "Set IK to FK" you can also convert an imported animation to IK automatically (using the "Convert FK to IK" switch in the import settings or if your animation is already imported by clicking on Edit --> Convert FK to IK). This is effectively the same as calling "Set IK to FK" on all your frames.

There are 2 reasons why an FK to IK conversion can introduce subtle errors:

  1. 2 IK driven bones have slightly less degrees of freedom compared to the 2 bones being driven by FK. The reason is that the 2 IK bones ave to be on the same 2D plane (you can see this plane when you select the IK handle in Config Mode). That means there can be no bending except into the direction that is defined by the bend plane.
  2. The pole target (that controls e.g. the orientation of the knee) also has to be guessed which can also introduce a subtle error.

I could imagine that because of reason (1), your IK bones are a bit shorter than the FK bones thus probably not reaching the ground when they are completely stretched?

In your case, if you only want to use IK on the hands, why not just keep using FK on the legs? With the FK/IK Blend channel you can decide which IK chain you want to use for your animation and also when to use it (i.e. at which part of the animation).

Please let me know if you have any follow-up questions.

Best regards,

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