Coordinate System and Units about gltf HOT 2 OPEN

See #1640
And #2139
And #2149

We're in the process of trying to find a better wording/image. So even though this is a duplicate, we could as well leave this issue open for now...

from gltf.

@helviett There is a bit of confusion with the wording because of the mixing of directions and sides (forward vs front and back vs rear).

In OpenGL, cameras face -Z. By convention, it is common for objects to face towards the camera (so they face +Z). In GLTF, this convention is codified as part of the standard. Meaning:

• The forward direction is -Z, the back direction is +Z. This implies the left direction -X is and right direction is +X.
  • These directions are used for things like cameras, lights, etc.
• The front side is +Z, the rear side is -Z. This implies the left side +X is and right side is -X.
  • These sides are used for oriented 3D assets like characters and many props.
  • For Blender where +Z is up, -Y is front and +Y is rear.
• For non-oriented assets, like maps, use the cardinal directions instead: +X east, -X west, +Z south, -Z north.
  • For Blender where +Z is up, +Y is north and -Y is south.

from gltf.