Master Photoshop Transparency
One of Photoshop’s greatest powers lies in its ability to preserve complex transparency. By employing masks, both in layers and embedded into the saved files as alpha channels, this transparency data can travel seamlessly into the nonlinear editing (NLE) or motion-graphics environment.
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Learn about layer masks, alpha channels, and selections — http://www.peachpit.com/articles/article.aspx?p=1617518