Upon request I decided to test my theory of weather or not I could get scratches to appear in glass when light shines on it.
Its all very simple, and I have yet to perfect teh technique but it will work and is photorealistic-capable.
As you can see, in the image below, I have applied a basic texture is slapped togetehr in photoshop.
I set the “Map to” to Spec and sometime Ref. Feel free to experiment. Make sure to deselect “col” = color.
It should work fine, but I suggest playing with the Spec and Hard value, I find that 1600 and 50 Hard can be good, but I recommend experimenting with these.
This is showing what happens under extreme light and extremely high spec and low hard.
The previous Image is just to demonstrate it workes, I do recomend that you UV map the scrashing, to reduce scratching on the inside of the galss, however I did not, and it seems to work fine.
Here is a subltle light, the far end of the glass is showing loud and clear, and teh near end is showing subtle scratches.
Here is even more subtle conditions, as you can see the scratch ing is at the very top based on where the light is.
Here is more normal light conditions, although I did not have time to perfect anything.
Now, my images are not that great, but it will tell the intermediate blenderman how too accomplish scratching. Now you may notice the scratches are not taht realistic, but that is because of my poor textureing in photoshop, the more realistic you can get with the texture the more realistic your scratches will be.
Here is the one I made in PS