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SonyDennis wrote on 7/10/2002, 4:45 PM
An alpha channel is a per-pixel opacity level that lets you composite an image over another. If visualized in grayscale, black means transparent, white means opaque, and grays are in-between. The Video Preview has an Overlay Mode called "Alpha as grayscale" just for this purpose.

Uncompressed AVI files can store alpha channels, but DV does not. However, you can compute one on-the-fly using the Chroma Keyer FX. It computes an alpha channel based on color, which is useful for blue-screen or green-screen work.

///d@
Tyler.Durden wrote on 7/10/2002, 6:17 PM
To expand on the subject...

The alpha information is embedded into the image (or video) and is not visible, But its effects can be seen when there is transparency in an image.

For example: the "Hot" Sample Text in Vegas has alpha information to make everything but the letters and shadows transparent. If you could see the alpha info (like using the "alpha as greyscale" preview mode described above), the alpha for the letters would be solid white (telling the software to make them opaque), the alpha for the shadows would be grey (telling the software to make them semi-transparent), and the background would be black (telling the software to make it totally transparent).

If you have a still image with alpha, you can see the alpha info the same way, or in a program like Photoshop by clicking on the "channels" tab in the info panel.

HTH, MPH