# Dark backface polygons

**URL:** <https://community.cesium.com/t/dark-backface-polygons/25308>\
**Category:** Cesium ion\
**Created:** [July 11, 2023, 10:33pm UTC](https://community.cesium.com/t/dark-backface-polygons/25308 "2023-07-11T22:33:24Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![AndreasRemograph](https://sea2.discourse-cdn.com/cesium/user_avatar/community.cesium.com/andreasremograph/32/14203_2.png) [@AndreasRemograph](https://community.cesium.com/u/AndreasRemograph)\
**Post date:** [July 11, 2023, 10:33pm UTC](https://community.cesium.com/t/dark-backface-polygons/25308/1 "2023-07-11T22:33:24Z")

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Hi,

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?

/Andreas

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**Author:** ![Ankit\_Trehan](https://sea2.discourse-cdn.com/cesium/user_avatar/community.cesium.com/ankit_trehan/32/14038_2.png) [@Ankit\_Trehan](https://community.cesium.com/u/Ankit_Trehan)\
**Post date:** [July 12, 2023, 12:24pm UTC](https://community.cesium.com/t/dark-backface-polygons/25308/3 "2023-07-12T12:24:55Z")

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Hi,

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”
3. …

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

Thanks,  
Ankit

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**Author:** ![AndreasRemograph](https://sea2.discourse-cdn.com/cesium/user_avatar/community.cesium.com/andreasremograph/32/14203_2.png) [@AndreasRemograph](https://community.cesium.com/u/AndreasRemograph)\
**Post date:** [July 13, 2023, 1:17pm UTC](https://community.cesium.com/t/dark-backface-polygons/25308/4 "2023-07-13T13:17:32Z")

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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](https://community.cesium.com/t/erroneously-semi-transparent-pixels-in-alpha-textures/25307)