In the 3D Tiles tileset we created here, some polygons are double-sided, i.e. without backface culling, by setting doubleSided in their glTF materials. They indeed appear double-sided, but the backsides seem to be much darker, regardless of the light direction.
I found the text about the doubleSided flag below in the glTF specification, does that mean that they should react to lighting just like their front-facing friends by inverting the normals? And if so, is it correctly implemented in Cesium?
When this value is true, back-face culling is disabled and double sided lighting is enabled. The backface MUST have its normals reversed before the lighting equation is evaluated.
If I understand this correctly, I have been able to do that myself before in a GLSL shader, inverting the normals if the gl_FrontFacing flag is false. Perhaps this is already implemented but I have to activate it somehow?