How to achieve smooth tangent space normals?

For smooth surfaces (no edges) I do it like this:

create space for per vertex
```
double N[3]; //normal
int cnt;
```
per vertex init
```
N={0.0,0.0,0.0}
cnt=0;
```
compute per face normal

normal must be normalized length=1.0 !!! add this Normal to all vertexes used in face and increment cnt to all vertexes used in face
per vertex normalize
```
N/=cnt; // N = average normal from all vertex - neighbour faces
```
be aware of cnt=0 for unused vertexes (division by zero)
per vertex N contains the Normal you want

now compute T,B vectors for TBN matrix (per vertex) as you do now
output image is smooth

My earth preview (with atmospheric scattering, bump mapping and more…) is here

hope it helps