4 answers · asked @ video mark 11:56 · Lesson: IK Constraint Demo · Course: Learn How to Rig Anything in Blender | Fundamentals of Rigging

What is the correct direction of the shin bone's axis relative to the knee pole?

Forgive me if I'm mishearing or misunderstanding, but at around 11:48, I though you said you want the x-axis of the shin bone pointing at the knee pole, so you rotate the bone until this is so. But then, as soon as you select it, it snaps so that the shin's x-axis is pointing along the global Y and the knee pole's x-axis pointing along the global Z. Am I mishearing? Or can you break down a little more what is happening here?

Thanks. I'm enjoying the course!

  • crew

    Hi Ryan, 

    I'll have to take a look at the video again to see what you mean and get back to you after the weekend.

    (just wanted you to know that I've seen this message and you haven't been abandoned)

  • crew

    Hey Ryan,

    OK I think I know what you are talking about.  

    Generally you want the axes of the Thigh and the Shin to be the same.  (for example Z axis pointing backwards).

    And when you have multiple chains sitting on top of each other, like an FK set of leg bones and an IK set of leg bones, you want them all to be aligned the same way.  This is something not shown in the course but imagine you want to switch from FK to IK using a custom bone property that turns on/off the relevant constraints to do just that.

    The reason you want all the bones to be aligned is so they don't twist awkwardly when you switch. (I hope that makes sense)

    Now what's happening in the video is that the IK constraint settings is trying to point the X axis towards the pole vector but the armature is setup in edit mode to have the -z point that way.  So when you jump from edit mode to pose mode you can see the constraint jump.  (I call this 'mode shifting' but I don't know if it even has a name)

    So if I change the offset of the of the IK settings to negative 90 that will aim the -z axis towards the vector target.

    Then when you jump from edit mode to pose mode, the knee is still pointing in the correct direction.

    Hope that helps.

  • Ah, I see better now what's going on. Thank you. I think my confusion was mostly due to my not visually understanding the angle of the bone. In the video you manually adjust the angle of the shin bone a bit, then pause, and then snap it to 90 by just typing it in numerically. On the screen I was viewing it on, it looked like it was already "pointing" in the right direction at the end of your manual adjustments right before you paused. So then after you typed in 90, it looked like it snapped past that, and I was like "huh?"

    Thanks again for the expanded explanation and knowledge drop.

  • crew

    Ah yes, I see your confusion.

    I started rolling it in the direction (CTRL R) and I wanted to see if it was positive 90 or negative 90 I needed to roll.

    Once I saw that it was the positive direction, I started typing the number and hit enter.

    This is something I do quite often.