[x3d-public] X3D, Blender, and which way is up.

[vmarchetti at kshell.com]

I've seen that the email thread starting with the minutes of the last Blender-X3D support call had a subthread about the orientation conventions that glTF, Blender, and X3D use.

I recommend Michalis' answer at https://web3d.org/pipermail/x3d-public_web3d.org/2024-November/020882.html.

One confusing aspect is that not only do Blender and glTF disagree about which axis is forward, they disagree about what the word "forward" means.

For Blender, the word "forward" refers to the axis that the viewer is looking along when they are viewing the front of the model. 
The viewer is translated from the model in the -Y direction, and they are looking in the +Y direction. That, +Y,  is Blender "forward"

glTF defined "forward" by the model.Looking at the first sentence of the of section 3.4 of the glTF specification. https://registry.khronos.org/glTF/specs/2.0/glTF-2.0.html#coordinate-system-and-units, 

> glTF uses a right-handed coordinate system. glTF defines +Y as up, +Z as forward, and -X as right; the front of a glTF asset faces +Z.


glTF says the "forward" refers to the direction the asset, a.k.a. model is facing and the spec shows this example picture . In Blender terminology this model is oriented "-Z forward"





The X3D standard has the same orientation convention as glTF, by virtue of the X3D default placement of the Viewpoint at a Z=+10 position and looking in the -Z direction. That is why in Blender terminology the X3D convention should be considered -Z forward. The code in the Blender extensions server was changed to this default for X3D import and export on Nov 4 2024 and this default is in release 2.4.0 of this Blender extension.

Vince Marchetti



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