Facade Grunge

I should start off noting that the building was never as dirty in real life as it appears in my renders; I've found it necessary to exaggerate the amount of grunge and dirt to help sell the scale, age and overall realism of the buildings. Adding in a sky to the background definitely helps with the scale as well, as you can see in the embarassing comp in the second to last post.

The brick facade is mapped with a layered shader, consisting of a bitmapped texture for the brick and several procedural textures for the grunge. I wanted to make the grunge easy to manipulate, especially since I knew I would need to use it on every building, but would of course need to vary it up so it did not look duplicated. Managing three bitmaps (the brick's color, bump and specular) for a single building was enough for me, and I did not want to make my render times explode by using bitmapped grunge.

In my initial textured test render, I felt that the grunge on the brick looked simply overlaid evenly; it wasn't sitting in the grooves of the grout even with the bump map directed into the grunge shaders:
I decided to create a "grime mask" to pump into my grunge shaders, so that dirt and mildew would appear bolder and more built up in the grout:
This was different from the bump and spec maps, in that the contrast was reasonably high, and the grout tone was consistent, as was the tone between different bricks.

Here's a closeup render of the brick with this new mask added into the shader:
The green mildew is especially more apparent in the crevices, as one might expect it to be in real life. It's such a subtle change, but it goes a long way. I'll need to dial down the grunge a little bit because I feel that it's a bit too strong now, but I'm overall very satisfied with it.