I looked into the shader code yesterday and tried out to do some normalmaps for textures myself.

What striked me when I saw the first generated normal maps in the game is that they looked much more "extreme" than they did in the preview in crazybump. The side of the bricks that faced the light where so light they were almost white, the other side were black. And that even though I made the normalmap in crazybump quite flat and so did crazybump's preview look like.

So I looked into the code to find out the reason for that. However, I had big difficulties to understand the code because most variable names did not speak for themselves:
lipx, lopx, npx, spx, mpx, ompx, bright, bright2, omi, mi...
Peter, I think it would be great to write-out the variables more to make it clearer to other programmers whats-what in that code. I didn't do it myself because I didn't understand the code and I don't want to fiddle around in your code without your consent as long as your work on it is not finished yet.
One thing I didn't understand for example are the two normals - I tried to leave out the normal2 completely and got the same result.

Anyway, eventually I found the places in the code that created this behaviour:

line 135-136: the brightness for a pixel is
light-level at that point * skalar product of the pixel's normal and the light  direction * 2
So if fully lighted, the landscape looks like as if you would have turned up the brightness in gimp by 100%. The notion of a diffuse map is to show the texture as if it is fully lighted. So I removed the *2 and suddenly the landscape looked pretty dark. But this is not the fault of the shader but of the diffuse textures because they currently not only have the height map baked into them but apparently also a diffuse shading as if looked on on a cloudy day*. So it is the textures that have to be changed, not the shader. Have a look at the preview in crazybump - as long as you don't enable specularity, the previewed texture is never lighter than the diffuse map.

But even by dimming the by-two light down, the effect of the normal map was still extreme. The reason I figure is an adjustment in the shader. In the crazybump preview and often in 3D games when you run around in rooms and along corridors, the lights are located at an angle (45°?) in front of the texture.
But clonk is a 2D world, so the question is where to put the light in the z-direction. If it was on the same level as the landscape, it would look completely odd. So you did already move the light a bit into the z-direction. I think the spot in the code where this can be adjusted is line 59-60. I found 0.5 to give a much more similar effect to what I would expect where the normal map only hints to the texture having a 3D-structure instead of being the main visible feature. What do you think?

Also, one last point:
As crazybump seems to be our program of choice, I suggest to keep track of with which parameters in crazybump the normalmap has been created so that we can create them with the same parameters again when the texture is changed or when we want to slightly adjust the normalmap.

* In fact, I did create most of these textures on cloudy days because otherwise the sun would have left some ugly shadows on the textures which should not at all be on a diffuse/texture map