Alpha Mapping

My model has a number of glass or perspex windows surrounding the cabin, and to represent these I made plane meshes cover the gaps of where this glass would be. In Photoshop I added a an alpha channel in the channels tab and flooded it with white. White is opaque and so will not be transparent on the model. I then selected where the glass UVs were and used the paint bucket again to flood it grey. Black is fully transparent and so moving up and down the grey scale in the alpha channel produces dark to light semitransparent textures, which when applied to the model look like tinted, or if you wish scratched, glass.

Once this was saved as a 32-bit Targa image file (32 allows the alpha instead of the 24 or 16) I applied it and my glass was looking good.