What happens to the existing normals of an object after you apply a normal map to it??

Noblahblah

To clarify what i mean by 'existing normals', in blender you can visualize the normals of faces,vertices etc as little arrows shooting out from them, these exist even when you have never even imported a normal map much less applied it to the object .Do they get replaced, added , multiplied, something else entirely ? I realize this could be a very stupid question if so i apologize.

Eric Chadwick

Usually, vertex normals do influence the normal map. This wiki page attempts to explain how it works:

http://wiki.polycount.com/wiki/Normal_Map_Technical_Details#Tangent_Basis

Noblahblah

After reading the article it looks like, correct me if i am wrong here, that vertex normals get completely ignored if using object space normal maps? And that deformations aren't really supported with object space normals, weird thing is i could have sworn skyrim uses object space normals on characters, even UE4 which is supposed to be state of the art doesn't seem to support that.

rollin

The important part here is that there is _some_ shader who is responsible for deciding how the normals look like. The normal map is just an input. You can do with this input whatever you want in the shader. 
So to answer your question: the normals will be altered!
This can be anything from adding, subtracting, replacing, etc. 

So practically you could even input an object space normal map for vertex deformed meshes if you do the correct math in the shader. 
The common case is to add an offset to them (thus tangent space normal maps)

Eric Chadwick

Dive deeper!

http://wiki.polycount.com/wiki/Normal_Map_Technical_Details#Tangent-Space_vs._Object-Space