tranquil222
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OK, I got this technique from The Photoshop Bible. First off, i'm assuming you know what masks and channels are. Here we go. Lets say that you have two channels/masks that intersect each other at some point, as in fig. 1(a photo is not the best example, as I had to guess where the shapes, were, but other applications may be better suited). How do you combine them into one channel? With calculations(which ive finally found this use for it), how can't you? Calculations a little known tool in the Image file menu. In a nutshell, what it does is it takes two channels and combines them into one, and gives you many blend modes and options along the way. Ok, so you have your 2 channels all ready, as in the middle of fig. 1. Go to Image-->Calculations. This box gives you three sections. Your source 1, your source 2, and how you want to blend them. Mixing these can let you simulate the add, subtract and intersect functions. But what really grabs me about this, and I really wouldn't care about this option otherwise is the fact that it gives you THREE more combo options to choose from: inverse&add(IN THE CALCULATIONS BOX: invert top source(check box to the right of option), set blend mode to screen), exclude(just choose difference blend mode), and exclude&inverse(invert the top and choose the exclude blend mode), shown in fig. 2. These combos are not too easy to do otherwise without some manual workarounds, so this technique can really come in handy.
