Humane Rigging: chapter 03, video 04
Why Blender Stack Exchange recommends it
Blender Stack Exchange users link it as a go-to explanation of quaternion rotation when answering questions about bone rotation, gimbal lock and quaternion values.
Recommended by 2 people across 2 different months, 2014 to 2018.
What it is
A 16-minute chapter from Nathan Vegdahl's Humane Rigging DVD series for Blender, covering quaternion rotation. It explains how quaternions differ from Euler rotation, including gimbal lock, interpolation, and how Blender treats quaternion values outside the 0 to 1 range.
What people said
Recommends Mutant Bob · 2014-04 · Blender Stack Exchange · on “What are the benefits of Quaternion rotation over Eulerian rotation?”
There's actually a good video in the Humane Rigging series that was referenced in the IRC channel just now: [this video] . It talks about gimbal lock and interpolation.
Recommends Legoman · 2018-12 · Blender Stack Exchange · on “Bone rotation quaternion issues”
Quaternions operate in a somewhat different way to Euler rotation. The reason the rotation was misbehaving is because Quaternions seem to break down when values are greater or less than 1.0 or -1.0, respectively. Multiplying certain values by -1.0 can reverse the interpolation, essentially solving the issue in this question. A helpful explanation of Quaternion Rotation can be found here: [this video]
Recommends Legoman · 2018-12 · Blender Stack Exchange · on “Bone rotation quaternion issues”
I found another question that linked to a video of a fantastic explanation of quaternions ( [this video] ) that helped with this issue.
Cites Patrick Marchand · 2016-03 · Blender Stack Exchange · on “How can X, Y, Z and W go above one in a quaternion animation on Blender?”
But....Blender does work with quaternions between 0 and 1. There's an interesting video put out by Nathan Vegdahl (that's part of his Humane Rigging series) that discusses quaternions: [this video] If you go to about 10:31 in the video, it talks about quaternions only being valid between 0 and 1. By 11:09, he mentions what Blender does with values greater than 1. Not sure what would prevent the file from being imported correctly.