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Creating an Alpha Channel

Creating an Alpha Channel

Probably the most powerful option in Photoshop is its capability to create alpha channels. An alpha channel is a separate channel that contains transparency information. Standard images have three channels, one for each primary color: Red, Green, and Blue (or RGB). The fourth channel, the alpha channel, doesn’t contain any color information, just transparency information. It’s very handy when you want to incorporate text or a company logo into your sequence.

  1. Open the image or file you want to create an alpha channel for. Make sure it is cropped or resized to the proper size.

  2. Select the Magic Wand tool and make sure the Anti-Aliased option is on. Do this from the options bar (see Figure C.9).

    Figure Figure C.9. Make sure the Anti-Aliased option is turned on.

  3. Make sure the Tolerance is set to a value higher than 25 but not more than 40. A value of 32 works well.

  4. Click on the logo if it has consistent color. If it does not, select the background.

  5. Choose Similar from the Select menu or Shift-click any areas that are not within the selection (see Figure C.10). To deselect any areas, use the Option key (Macintosh OS) or the Alt key (Windows) as you click.

    Figure Figure C.10. Make sure everything you want to be invisible or transparent is selected.

  6. Depending on your selection, choose Modify from the Select menu and choose Expand or Contract. If you selected the logo with the Magic Wand, choose Contract. If you selected the background, choose Expand. Regardless of which option you choose, expand or contract the selection by one pixel (see Figure C.11).

    Figure Figure C.11. You can expand or contract your selection by as many pixels as you want.

  7. Choose Save Selection from the Select menu (see Figure C.12).

    Figure Figure C.12. Choose Save Selection to define the alpha channel.

  8. The Save Selection dialog box will open. Click OK to make a new channel.

  9. In the Channels Palette, you’ll see the alpha channel you just created in channel 4.

  10. Save your file to a file format that supports alpha channels, such as Photoshop files, PICTs, and TIFFs.



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