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Chapter 13. Superimposing 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. Use Black Alpha Matte or White Alpha Matte with images that contain an alpha channel premultiplied with a black or white background.

To use the black alpha matte and white alpha matte keys:

1.
Select a clip in a superimpose track, and choose Clip > Video Options > Transparency (Figure 13.43).

Figure 13.43. Select a clip in a superimpose track, and choose Clip > Video Options > Transparency.


The Transparency Settings dialog box appears.

2.
From the Key Type pull-down menu, choose one of the following options (Figure 13.44):

Figure 13.44. Choose Black Alpha Matte if your clip’s alpha is premultiplied with black; choose White Alpha Matte if your clip’s alpha is premultiplied with white.


Black Alpha Matte— if a clip has an alpha premultiplied with black

White Alpha Matte— if a clip has an alpha premultiplied with white

Areas are keyed out based on the premultiplied alpha channel (Figure 13.45).

Figure 13.45. The alpha channel is keyed out.


3.
Preview the key in the sample frame of the Transparency Settings dialog box.

4.
Click OK to close the Transparency Settings dialog box and apply the settings.


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