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Chapter 13. Super-imposing Clips > Using the Black and White Alpha Matte Keys

Using the Black and White Alpha Matte Keys

When the alpha channel is premultiplied, the transparency information is stored in all four channels of the image. This causes the edges of the opaque part of the image to blend into a background color. (For more about premultiplied alpha channels, see Chapter 2.) Use Black Alpha Matte or White Alpha Matte with images that contain an alpha channel premultiplied with a black or white background.

The alpha channel used by a Premiere title is premultiplied with a background color. Chapter 11 advises you to make the background of title clips black or white if you plan to superimpose the title over other clips. That way, the black alpha matte or white alpha matte keys will be most effective. If you're keying Premiere titles, don't forget another crucial step you learned in Chapter 11: In the Title Window Options, make sure Opacity is unchecked when you create the title. Otherwise, the title won't generate an alpha channel you can use for keying purposes.


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