The Calculations command is an old school way to work with channels and color (it’s been around since before Photoshop had layers. It’s also a great way to combine complex color channel data to build an alpha channel. This can then be converted into a layer masks for a nondestructive way to hide pixels in a photo.
[blip.tv http://blip.tv/play/hocKg5aBdQI.x?p=1 width=”600″ height=”380″]
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Disclaimer: This is just one way to work with masks.