$nocull
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$nocull is a material shader parameter available in all 
Source games. It disables backface culling, resulting in triangles showing from both sides. It is used to allow for infinitely thin dual sided objects without duplicating the triangles with reversed vertex order. Lighting will still only be calculated from the "front" side of the face, and be copied to the reverse side, potentially creating an effect where light bleeds through to the other side like a thin sheet hung up in direct sunlight, so use with caution.
Note:Has no effect on world brushes (so tie each one to func_detail). Works on displacements.
Bug:Cannot be used with $translucent on models. Use $alphatest instead.
Confirm: Does it work with $alpha?
VMT Syntax and Examples
$nocull <bool>
LightmappedGeneric { $basetexture glass\window001a $nocull 1 }
VertexLitGeneric { $basetexture models\items\medkit $nocull 1 }
VertexLitGeneric { $basetexture models\props_2fort\blue_window001_glass $alphatest 1 $nocull 1 }
