Why are the default axes on the default bone a 90° rotation from the world axes?
Hi Bee,
The fact that the Y-Axis of a Bone is along its length is not very surprising; it's the same as the Y-Axis of an Edge (Mesh in Edit Mode).
An Armature comes in, with its Axes aligned to the World, so nothing special there either.
The rest is a 'mystery' (or probably, just a choice).
Why is a single Bone standing, when Adding an Armature? (Because its common use is for characters?)
Why is its Head down and its Tail up? (Because the system was made by an Australian?)
In lesson 03 of this chapter, Wayne says something about how Euler XYZ is the most natural rotation mode for an animator. It's obviously the most syntactically memorable. I wonder if that's the determining factor in tail-Y bone orientation/nomenclature.
Let me just break out a thesaurus to see if I can comment on that. Hmm, wait one sec, I might just have to break out my Enigma machine.
Ok, so I think you're asking if Blender was designed so that the length of the bone was the Y, because Euler XYZ is the most common rotation order.
It's possible. XYZ is alphabetical and in mathematics it is often taught in that order first.
The middle axis will hit gimbal lock when it reaches +- 90, and Y axis is more often than not the axis that is generally animated the least.
Ton was the one who decided which way all the axes would point many decades ago, and I think it's possible that was part of his reasoning.
There's only 1 person who can answer that for sure though.
I know that Euler XYZ is the most natural for an animator because it is the default in every software that I know of. (Blender is odd in the fact that it allows the user to use Quaterions for bones by default)
I wonder whether the full answer to this is tied up in the politics of left-handed vs. right-handed coordinate systems. But really, at this point I'm surely projecting something into my question that isn't there in the answer. Thank you for the insights on my question.
It has nothing to do with left/right hand. That actually has to do with why world z is up instead of y. I'm stepping away from that debate.
As for your question, it was an arbitrary decision. Basically, you start rigging characters in front view and having the length(y axis) easy to start extruding from front view is the reason. Even Ton has said it wasn't well thought out at the time.