I was thinking about effects in games the other day and I was reminded of the Gasmask effect from Metro 2033. Once you put the gasmask on it blurred a bit in the corners and could ice up and even get cracked.

I assume that something like that is done using a shader. I have been experimenting a bit with game development, so far mostly playing with existing rendering engines and adding physics support etc.

I would like to learn more about this sort of effect. Can someone give me a simple example of a shader that would alter the entire scene like this. Or if not a shader then an idea on how it would be done. Thanks.

Edit : Include screenshot of the metro 2033 gasmask effect.