Dark backface polygons


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?



Can you please provide us with a workflow of how you are using Cesium ion for your use case which will help us better understand what you are seeing. Something like the following:

  1. Upload a glTF tileset to ion
  2. Pick “3D Capture” under “What kind of data is this”

If possible could you also provide us with sample data and/or screenshots that show what your desired output would be.


Sorry, now that I’m testing separate models I can see that it actually seems to work as expected. I was probably changing more than one thing at once and drew the wrong conclusions.

The real problem is probably alpha textures which accumulated make a darker appearance, and I have asked about that in a separate question here: Erroneously semi-transparent pixels in alpha textures