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Chapter 13. Super-imposing Clips > Using the Alpha Channel Key

Using the Alpha Channel Key

Photoshop, Illustrator, and After Effects can all create images with a straight alpha channel—that is, the alpha channel contains all the transparency information. Use the alpha key for images that contain a straight alpha channel. For more about straight alpha channels versus premultiplied alpha channels, see Chapter 2.

To use the alpha channel key:

1.
Select a clip in a superimpose track, and choose Clip > Video Options > Transparency (Figure 13.38).

Figure 13.38. 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 of the Transparency Settings dialog box, choose Alpha Channel (Figure 13.39).

Figure 13.39. Choose Alpha channel from the Key type menu.


Areas are keyed out based on the straight alpha channel (Figure 13.40). No slider controls are available.

Figure 13.40. Areas are keyed out based on the straight alpha channel.


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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