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All the transparency operations in Premiere seem to be about eliminating darker elements from a clip on the superimpose tracks to create transparent areas where the underlying clip shows through. I'm trying to figure out a way to reverse this, for example to make the sky transparent in a superimpose track while maintainng the outlines and colors of the cityscape below. There should be a way to create a negative image of the clip on the superimpose track, then make the now-dark sky transparent (the alpha channel?), and then switch the image back to positive but I'm not quite sure how to do that and all the books I look at seem to assume that Il only want to cut out relatively darker areas of the overlying image. Any suggestions would be appreciated. Thanks