Creating an Alpha Channel

[ LiB ]

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.

    graphic/apcicon09.gif


  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.

    graphic/apcicon10.gif


  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.

    graphic/apcicon11.gif


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

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

    graphic/apcicon12.gif


  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.

Avid Xpress Pro uses a film-derived alpha channel configuration. The black portions in the alpha channel represent the opaque areas and the white represents the transparent portions. If your alpha channel is the opposite , you can choose Invert Alpha when you import the image into Avid Xpress Pro.

Importing an Image or Graphic that Contains an Alpha Channel

When it comes time to import a logo or graphic with an alpha channel, you'll need to make sure a certain setting in the Import Settings options is turned on. Prior to importing the image, open the Import Settings from the Project window. Make sure that the Use Existing or Invert Existing options are selected (see Figure C.13). If you need help adjusting import settings, refer back to Chapter 2, "Capturing and Importing". If the alpha channel was built specifically for Avid Xpress Pro, choose the Use Existing option.

Figure Figure C.13. How the alpha channel is imported into Avid Xpress Pro is up to you. From the Import Settings dialog box, you can invert the alpha channel if needed.

graphic/apcicon13.gif


Once an image is imported into Avid Xpress Pro, it will be displayed in the bin with the effect icon. Once placed on a top track in the Timeline, it will display which parts of the graphic are transparent (these areas will be black) and which parts are not (see Figure C.14).

Figure Figure C.14. The Timeline shows how the effect is applied.

graphic/apcicon14.gif


To see the alpha channel, nest into the effect and view the top matte layer. It will look similar to the alpha channel in Photoshop (see Figure C.15).

Figure Figure C.15. The alpha channel can be viewed in Avid Xpress Pro, but it cannot be edited. Notice the small padlock icon next to the track in the Timeline.

graphic/apcicon15.gif


I've barely scratched the surface on how useful Adobe Photoshop is. If you simply used it to resize images, the time you'd save would cover the price of the software. It truly is an amazing piece of software. For more information about Photoshop, visit Adobe's site at www.adobe.com .

[ LiB ]


Avid Xpress Pro Power.
Avid Xpress Pro Power!
ISBN: 1592001513
EAN: 2147483647
Year: 2003
Pages: 165
Authors: Steve Julin

flylib.com © 2008-2017.
If you may any questions please contact us: flylib@qtcs.net