Detect Back Faces

Hey there,

I have a shader cutting geometry using alpha test, and I am trying to fill this gaps.
I tried various methods, but nothing kinda worked.

I’ve got another idea which would use a postprocess material to lerp between the sceneColor and a customColor to fill those gaps, but I miss the alpha to perform this lerp operation. As an alpha for this operation I thought to use the TwoSided sign to check if the pixel I am rendering is a backface, but it doesn’t seem to be possible in a postprocess material.
Is there a way that I can know if a pixel is a backface in the postprocess material?
If not, can I write to a custom postprocess input in the object shader so I can use it later in the postprocess one?

Thanks!