I tested a model with glasish material and double layer in the sandbox. I was having some issues with my original model, so, for testing and understanding what happens, I created a simple torus, wrapped by an other tours. (made in blender.)
The mesh of the two-torus-model is flipping around some normals, when I activate alpha mode “combine” or “Pre-multiplied”. These are the only modes where I can control the level of alpha, so I wonder how to prevent the normals to flip.
Thank you for you advice or any hint on how to understand this and maybe what would be a good idea how to create a mesh with two layers of glas-like material. Debug mode set to “normals” gives me following screenshots which demonstrate the flipping. it is not happening when alpha-value is 1 but happens when alpha is lower than 1:
Find the glb-file here: https://virtueller-messestand.de/fileadmin/3dcontent/torus.glb