How does an ik bone constraint use the pole target?

[twisted]

Here is a armature of 6 bones: Bone, Bone.001, … Bone.005. Each being the parent of the next. Plus 2 control bones that I’ve coloured red.

Bone.005 has IK Bone Constraint applied to it, referencing the 2 control bones and with a chain length of 5.

As in the screenshot, Bone.005 is getting pulled around by the CTL bone. My problem is with the POLE bone.

I thought that purpose of the pole bone was that after Bone.005 has been pulled to some position by CTL, the 5 bones in the chain will adjust such that the chain will get pulled as near to the pole as any ‘slack’ in the chain will allow. I can imagine doing that with a length of string fixed at begin & end.

However, my mental image of that process is wrong. The available 5 bones in my chain could be pulled closer to the pole than they in fact are. I don’t think it’s an issue with the Pole Angle. Viewing from the top, the movement of the chain is all in the direction of the pole, not bend over sideways

What would be the best way to think of the influence of the pole?

[VVruba]

From blender manual:

Inverse Kinematics Bone Constraint

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To me, it looks like the pole simply decides in which direction the bending will happen. The bending in their pole example doesn’t produce an even parabola since they chose to rotate one bone in the armature for whatever reason, which makes it a bit confusing.

If I’m honest I can’t say much on this topic since I’ve always used rigify because I’m lazy and didn’t need a specialized rig.

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Edit:
This article might also help you. Here is a snippet:

So, what is a Pole Target? Pole Targets are a Component of an IK chain that allows us to control the direction that the chain bends in , they are commonly used when posing and animating Legs and Arms as we need to make sure they bend in the right direction.

[NP5]

You might see more the effect of the pole looking top down like the last image. Moving it side to side. Poles are about pointing direction I suppose, not a curvature adjustment.

[VVruba]

When you said “pointing direction” it made me think of “north pole” and “south pole”.

[twisted]

Seems the IK Bone Constraint just doesn’t do what I thought it should!

I regularly use it for simple knee joints, that’s not my issue. It’s predicting what will happen in a more general situation, as in my initial screenshot, where things get difficult.

Yes, the pole will ‘control the direction that the chain bends in’, but I think that still leaves many ways the chain could adjust itself to changes in the control bones.

Probably down to some deep mathematics inside the IK Solver:
Armature>Object Data Properties>Inverse Kinematics

I’ll mark the question as solved since the influence of the pole is to give direction. How to predict how the bones end up getting adjusted is I guess a different question.

[VVruba]

Well, Blender is open-source. In theory, you can go and look at the code responsible for IK, whether you’ll take away anything from that is another matter…