sorry i dont know how this is called but..
ive read that some game sutilize a shader that makes the textures and envirometn models shaby.
say, you have a wall, that is textured with some crick or wood texture or whatever. you can now eihter equip a bump mapping shader or you can exuip that shabby shader. the shader cuts some edges or carves them into the block or model, so taht it looks as if its shabby, old or broken.
ive included a picture to let you see what i mean:



the upper half of it is totally damaged and stuff.
i hope you got my point..
thx alot