While changing the shading normal is a great way to add additional detail to a model, there are some problems with it.
One of them is that at grazing angles and/or strong changes to the normal, the reflected ray can end up pointing into the actual geometry, which results in a black spot.
This patch helps avoid this by automatically reducing the strength of the bump/normal map if the reflected direction would end up too shallow or inside the geometry.
Differential Revision: https://developer.blender.org/D2574
d6e769d32e