Sector-masked screenspace shaders

[Caligari87]

So I was thinking about fog and such, and had an idea: An optional "sector mask" for screenspace shaders, applied only to an on-screen area occupied by a certain sector. Ideally the shader would also get access to the mask texture, so that could be modified as well.

gif example, made in GIMP:

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This could be useful for any number of things, including pseudo-fog effects with soft boundaries. Even better would be if shaders additional masks were available, such as height (gradient from lowest to highest point), floor, ceiling, walls.

Another example, using a fade to simulate how a heightmask and some "cloud" noise could be used to make a toxic "floor fog"

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I imagine masks like this would be useful for things like water effects too.