As I understand, there is no cage, like in 3ds Max, there is low poly model work like a cage itself, and this is nice, but…
As we know, there is two type of raycasting from cage, relative to the vertex normals, when it hard or soft edge. It nice illustrated on polycount wiki http://wiki.polycount.com/NormalMap/ (Working with Cages section).
I will attach illustrations here:
And as I can see, Blender cages uses only the first methods, that incorrect.
This make sense, because my cage mesh have a hard edges.
And when I set shader to smooth, rays starts to work correctly, like on the second image.
But appears another problem, if I bake a normal map, cage surface starting to affect resulting normal map, instead of hi-poly object, as result, map is incorrect.
p.s. Not very simple issue, so not sure that I’ve got to explain a problem with my english. Hope someone understand me. 