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fk ik switch vs no flip knee... (7 posts)

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  1. greatbear

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    Posted 6 years ago
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    Hello,

    I've been playing around with a no flip knee - where you point the pole vector in the x direction then use the twist to re-align it corrextly, so the pole vector is at 90 degrees and acts like a axle. The animator then uses the twist attribute to align the knee instead of a pole vector constraint.

    When setting up an ik fk switch (no blend), if I am not mistaken, to align the ik chain with the fk chain you snap the ik handle to the ankle joint and a pole vector controller to a null node parented to the first joint (or just parent the pole vector controller to the first joint in the fk chain). So to do this the ik needs a pole vector controller

    The only problem is to do this the ik chain needs a pole vector controll. Is it possible to set up a no flip knee and have a pole vector constraint on the ik handle so it can be aligned to the fk chain??

    Many thanks!

  2. BrianKenny

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    Posted 6 years ago
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    There are other ways around this using the no flip knee while still being able to Ik Fk swap without blending; though it does require some math. This other way is to find the value of the triangle angle between the IK and FK knee joints and the set your no flip attribute to match.

    Recently I had to go through this myself for my own no flip knee setup.

    the first two points of the triangle are the IK and FK knee joints.
    the third point of the triangle is the flat plane intersection of the two joints and the ik handle line (the one going from the Thigh to the Ankle)

    after that use the distance tool to find the length of the sides of the triangle (should be an Isosceles triangle as the length of the IK and Fk points to the third point should be the same)

    After that, use cosine to find the angle.
    if you script it out then the command to convert the decimal of the cosine equation is
    acosd

  3. greatbear

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    Posted 6 years ago
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    Nice one

    Cheers mate, I'm with you. To get the distnce from the knee joints to the point on the ik line I was thinking of creating a locator and point constraining it to the thigh(hip) joint and the ik handle and then parenting a distance node from the knee to that locator?

  4. BrianKenny

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    Posted 6 years ago
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    I tried that as well during my project to get the third point and, while it got me close to what I wanted (usually off just a few degrees), it wasn't exact and didn't create a perpendicular intersection with the ik handles line.

    The problem was that, while that was the mid section of the Thigh and Ik handle, the Knee joint is rarely in the perfect center of those two as well so the intersection of the Ik handle line would not be at a 90 degree angle needed to find the angle difference of the IK and Fk Knee joints.

    The way I found to get the Ik handle triangle point is to create an extra 2 joint chain from the Thigh to ankle. As these are joints, the extra Thigh joint auto oriented itself down the Ik line along the X axis.

    Orient constrain the extra end joint with the Extra thigh parent joint so its orientation is same as the Extra thigh, so now we have both the extra joints pointing down the Ik line on its X axis. The extra end joint will only have the X translation value now and can freely be moved up and down the Ik line.

    After that, just point constrain the extra end joint to the IK and FK knees so it will snap to between both of them. delete the point constraint you just made and zero out the Y and Z translations and it will snap back to the Ik line but with the trans X value at what you need for the 90 degree intersection.

  5. greatbear

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    Posted 6 years ago
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    Thanks very much for that Brian, really appreciate it. I've gone through your instructions and I think it worked for me. I've posted a couple of images on my blog to show what i've come up with:

    http://fmp-characters.blogspot.com/2011/04/fkik-switch.html

    Now for the easy bit of finding those angles...

    Cheers

    GB

  6. BrianKenny

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    Posted 6 years ago
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    Happy to help

    You had good timing with your question as I, myself, had only worked this out just a couple weeks ago.

  7. greatbear

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    Posted 6 years ago
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    It's all about timing!

    Thanks again!

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