Normal maps: Get normal x/y/z from color

hi!
i think the way you get the normal vector from the pixel color of a normal map is

Code:

---

nx=128-red
ny=128+green
nz=256-blue

---

can someone please tell me whether this formula is correct?
thanks :)

Real-Time Normal Map DXT Compression

thats amazing. i felt over this link while searching but i didn't find it much useful...

edit: whats also bothering me is how to calculate the world space normal from the surface normal + the normal from the normal map.
adding them only works on flat surfaces pointing up...