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I'm attempting to define the "skeleton" for a robot arm animation. I added the first bone, and it looks perfect. However, when I add the second one, I encounter the following issue:

enter image description here

The Roll is set to 0°, but the bone appears to be rotated along its axes.

What am I doing wrong?

I considered that it could be the rotations of the spheres I refer to for moving the cursor, used as a snap. I reset all the rotations of the spheres, but it doesn't seem to have any effect.

I'm using Blender 4.0

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2 Answers 2

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You're not doing anything wrong, other than mistaking what "roll" is. It is does not indicate any specific alignment with world axes, or armature axes, or anything else. It is a number without much absolute meaning-- its meaning is primarily relative.

I believe that when Blender makes a bone, it considers it in its armature's axes, then damped tracks to the position of the tail, for a roll of 0 degrees. (If that's not it, then it's close.) "Damped track" is referred to, some places, as XZ swing. It's not rotating in some set of orthogonal axes in some space-- it's rotating in the axis that gives it the shortest path to the orientation it needs:

enter image description here

If I need different axes, I can tune the roll to eye. But when I need precise axes, I use a "recalculate roll" operation (in armature edit mode.) I'm likely to recalculate either to cursor (pointing the bone's +Z axis at the cursor) or, in conjunction with numpad view hotkeys, by view axis (pointing the bone's +Z axis at the viewport's eye.) Here, if I wanted the bone more aligned with world axes, I would probably numpad 7 to adopt a top-down view, then recalculate roll to view axis.

I don't pay any attention to the actual number of the roll. The number is an artifact of how we choose to measure it. The thing that matters is where the bone's axes are pointing.

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  • $\begingroup$ Yes, you're right. My issue with the bone rotation (x, y, z) was elsewhere and did not depend on the visual rotation of its axes. I highly appreciate your effort to provide me with a comprehensive written and visual response. Thanks! $\endgroup$
    – Seraphim's
    Jan 8 at 13:25
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If you just want to get two bones aligned straight, there’s a couple of ways.

First you can stretch the one bone out to the desired length while your locked into an axis view you want to move it on. Then it cannot move off that axis while moving or transforming it. Then just Right Click on the bone and hit Subdivide.

Alternatively you can grab the bone while locked into axis view, hit Eon your keyboard and move the new bone out to where you want it.

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