I want to confirm my understanding of
Both control if a mesh appears one-sided or two-sided, in different ways.
The geometry builders have the sideOrientation option. They use this function:
It will duplicate vertices and triangles for the DOUBLESIDE case to physically generate two sides.
This means that even with backFaceCulling on both sides will be rendered in this case.
With backFaceCulling off the FRONTSIDE case will also lead to rendering of both sides. But here it may instead be the result from the shader now ignoring the normal direction.
backFaceCulling off and DOUBLESIDE may actually render each side twice.
Is that about right ?
Finally, the material.sideOrientation property is about which side is considered FRONT or BACK. It has the same name but is really about something else though related.