Hi Noritaka-san,
Interesting question. In case you are wondering, it also skips in my demo but it just isn’t as noticeable.
The way the bendy-bones work is that it will find the shortest path of rotation. This means that when it reaches 180 (+ or -) it will flip in the other direction because that is the shortest rotation.
Now let’s think about what’s happening with twist along the length of the bone.
If you are twisting the top – it will rotate 100%, at the midpoint it will be rotated 50%, all the way down to the other side of the bone, that will be 0%.
So depending on where the owner bone is connected with the armature constraint along the length of the stretchy bone, this will affect the amount of rotation it will skip with that flipping.
eg – if it’s at the top where it follow 100%, it isn’t going to skip any thing.
If it’s at the bottom where it’s 0%, it isn’t going to skip any thing.
But as we approach the mid point of the bone, it will skip up to 50% the closer it gets to that midpoint (or 50%)
I am finding that mathematically very interesting. But I hope it makes sense.
It is a confusing concept in one language, so it is likely to be even more complex if you are trying to grasp this in a second language.
