material with bumps only (transparent diffuse)

Hi,

I’d like to do something similar as the leak-patterns in the sewer example, except with normals rather than diffuse texture. I thought it would be as simple as having a completely transparent diffuse texture and just a normal map, but apparently that makes the normal map invisible as well - any tips?

So, I’d like the object (=plane) be transparent so it would show the color of the underlying object while being affected by the normal map.

Uh, anyone?

Is this possible with any shader?