Normal Maps

1. Bump map
   Use grey texel to record bump, and converts bump to the vertex normal, Bump maps require additional information describing how the bump intensity range maps to global space distance units, in order to convert to normal map.
   
   
   
      Bump map elevation
2. Object space normal map
   More cheap computation than tangent space normal map, but only support rigid mesh, can't support deformed mesh, and isn't suited for tiling and symmetric model.
3. Tangent space normal map
   A little expensive one. It supplies deformed mesh and all results that object space normal map and bump map can be. So this solution is more flexible for artists pipeline using.

     Left: tangent space. Right: bump. 

    Have the same result on the cube

reference:  Understanding Normal Maps, http://www.pixologic.com/docs/index.php/Understanding_Normal_Maps#Object_Space_Map_Uses
http://tech-artists.org/wiki/Normal_mapping

Calculation tangent space:
http://jerome.jouvie.free.fr/opengl-tutorials/Lesson8.php
http://www.terathon.com/code/tangent.html