Hi there, I’ve been having a problem recently in 4.27 where emissive materials are producing light in all directions when baking static lighting with GPU Lightmass. This is causing issues when baking the lighting as emissive light is being produced in all directions rather than just the direction of the faces.
As you can see when I rotate this emissive plane it produces light on the walls even when it’s facing away from it. If anyone was a solution to this issue I’d be very grateful, thanks! 
Solved! - Basically GPU Lightmass treats backfaces as Opaque (basically when it bakes light it bakes as if all of the materials are 2 sided) so anywhere that’s emissive is also producing emissive on its backface. So to resolve the issue I setup my emissive like this with 0 going into A to make the backfaces appear black and going into B is my emissive node setup. You can keep “Two Sided” disabled in the material too so it only renders as one sided but when baking the lighting with GPU Lightmass it detects that the backfaces should be black and therefore doesn’t add emissive lighting on the backfaces 
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