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Alphas Made Transparent (page 2)

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Another misconception about video that supports alpha channels is that the alpha areas automatically become transparent when you import them into your editing system. Some will automatically detect the right setting and some won't. Don't be alarmed when you import an alpha image that isn't immediately transparent.

With many applications, when you import an alpha element, you'll notice that the transparent area is filled in black - that's standard. What you have to do is key out the alpha information, the same way you would do a luminance key or chromakey. However, do not make the mistake of doing a luminance or chromakey to remove alpha information. Sure, it may work (more or less), but it defeats the purpose of using an alpha in the first place and the quality will certainly suffer. Using one of these other keying methods will not only remove the alpha information, but it will also remove colors from the image that are supposed to be there. Using the incorrect keying method for your images will either make them look very jagged at the edges, or they'll have holes throughout them where colors have been mistakenly removed. Although the correct method of keying alphas will vary slightly, depending on your editing system, the results will be the same - a clean image, with transparent areas usable to composite with your video clips.

Sidebar: Transparent GIF

While the CompuServe GIF format supports transparency, it does not support variable transparency. Instead, the GIF uses binary transparency. This means that instead of being able to control the degree of transparency, a GIF image's pixels can be set either to an on or off value (totally transparent or opaque). By contrast, file formats that support alpha channels allow 256 possible transparency settings - totally transparent, totally opaque and variable degrees of gray between these two extremities.

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