When using normal map for static lighting, UE4 Editor will generate the normal data (normal map) for a material using rendertarget, and export it to Lightmass. It seems that the rendertarget will clamp negative component (R.G.B or A) of its texels to zero, so every normal in the map will only point to the non-negative quadrant, causing wrong bake result. How can i solve it, or it is a bug?