I experimented with a simple Unity program with 1 camera, with a tracked pose driver. The result looks like this:

When given a Euler angle of XYZ format:

• pitch up <=> X decrease
• pitch down <=> X increase
• yaw right <=> Y increase
• yaw left <=> Y decrease
• roll clockwise <=> Z decrease
• roll counter-clockwise <=> Z increase

all evidences seems to indicate that Unity Euler angle uses a LDB (left-down-back) coordinate system, with both right-hand rotation and right-hand axes order.

However, when dealing with quaternion, many documents imply that a left-hand system was used.

Are these documents stating the truth? If so, this will cause a lot of trouble in conversion between different protocol. What’s the purpose of this divergence?