In addition to alpha (the measure of coverage) you could also include transparency (which is something a measure of how much light passes through, i.e. the actual transparency of glass, as opposed the the coverage of a screen, this is equivilent to insisting on a layer mask to be included for every layer). We could also include luminesence, which is a measure of how much light a pixel produces (as opposed to reflectance, which is all we measure how with rgb).
Not I am equating the following concepts
alpha == coverage (like a metal screen over a window)
layer mask == transparency (like glass, or, incidently, colored glass if you extend your layer mask to include all color channels).
luminesence == the amount of light pixel produces at zero coverage (this is equivlent to additional layers)
-- Dan
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