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rigid knee and pole vector (6 posts)

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  1. pataz

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    Posted 6 years ago
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    Hi Javier, and every body.

    The question I have might appear to you as very silly, but well, it is like that. I am professor of 3D in the unique university that gives animation in Costa Rica. Since in this very little country 3D animation is quite new, there is really nobody with true knowledge, with some exceptions of course. Anyway, nobody knows how to rig well, and despite the fact it is very not my speciality, I have to explain some rigging basics to my students.

    And this my question: I showed my students how to rig a knee with the pole vector control, and also how to rig a rigid knee, like Javier do in his pdf tut. But some of my students decided to do it easier: They don't even care about the pole vector, they just let the ik handle as it is, and they put a Twist attribute on the foot Ctrl. And, as far as I've seen, the knee follows the direction of the foot without the need of any constrain, and the twist attribute works well too. Seeing this, I can't answer correctly to the correction: why do we need a pole vector?

    You will certainly find an obvious answer to this, please releive me!
    thanks

    Sarah

  2. Javier "Goosh" Solsona (admin)

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    Posted 6 years ago
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    Hey Sarah

    You can certainly use a twist attribute. Depending how it's setup it can work fairly well. Also, depending how it's setup you can get the knee to flip when the character is placed in certain poses.

    The pole vector gives you a nice visual representation where the knee should be aiming.

    There is no "one" way to rig. There are tons of good solutions out there. And often one, is not necessarily better than another.

    Hope that helps

    Javier

  3. BrianKenny

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    Posted 6 years ago
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    Using the twist is the method taught by the Cg Toolkit book: Art of Rigging Volume 1

    I personally prefer this method over Pole Vectors myself. Though with adjustments on either setup, both of these can be used to accomplish the same thing. End of the day, depends on what the animator wants to use: attribute slider or physical object to control knee.

    That being said, some problems can occur when the leg joints are angled (which is 99% of the time, only test legs are aligned strait on the Y axis)

    These problems can be overcome in various ways but the default setup in Art of Rigging can cause the following effects in a normal human biped rig:

    Basic setup zeroes out the Pole Vector values on the IK, causing the knee to shift. Solution: make a duplicate of the leg so you know its original position and can readjust the twist leg to match after setup is done.
    The knee can wobble slightly when rotating the foot joints in a reverse foot setup. Solution for stopping this varies on skeleton.

    By default it follows the knee but sometimes you can actually not want that. Might want to give your students a slight challenge of being able to blend that affect on and off through another custom attribute.

    Once those issues are addressed then, yes, I personally like the twist setup better than pole vectors. Though those can also be adjusted to also have the knee follow or not follow the foot control.

  4. pataz

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    Posted 6 years ago
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    Thank you very much for your answers!!! I thing I get it... but know I still don't get 1 thing: What so I win doing the rigid knee of Javier's tutorial? It has the same twist attribute, and knee follows the foot, but constraining the pole vector to the hip... That is the part I don't understand. Why constrain the pole vector if, at the end, we will only use the twist attribute?

  5. Javier "Goosh" Solsona (admin)

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    Posted 6 years ago
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    Ok.. it's been a looong long time.. But I think that technique is to stabilize the system a bit more. I think it makes it more robust and less prone to break when putting the character in extreme poses.

    I wouldn't be surprised if there are better techniques out there now or even better IK solvers

    J

  6. pataz

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    Posted 6 years ago
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    ok ok. That's what I wanted to know. Thank you very much.

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