Alpha channels are channels in the image that store transparency information in the form of masks. You can use alpha channels to show and hide particular parts of the image without erasing the image itself.
Basic Alpha Channel Creation
The easiest way to create an alpha channel to store a selection in is to make the selection first. With any selection active in the image, use the menu command Select, Save Selection to open the Save Selection dialog box (see Figure 7.18).
Figure 7.18. The Destination area of the dialog box is where you choose both the document and the channel in which to save the selection. You can also select a name when creating a new channel.
The Document pop-up menu lists all files open in Photoshop that have exactly the same pixel dimensions; it also includes the option to create a new document. (Open files that are a different size than the document in which you made the selection do not appear in the list.) Selecting New creates a new document with the same dimensions and resolution as the original. The document consists of only the single alpha channel and is in Multichannel mode. Although the new image contains no color channels and holds only the saved selection, you can convert the document to any color mode (after first converting it to Grayscale). The name selected in the Save Selection dialog box is applied to the channel, not to the new document.
Remember that when a pixel is 50% selected, any adjustment or filter is applied at half the intensity compared to a pixel selected at 100%. This behavior also holds true for deleting, painting, and other steps performed on a selection.
The Channel pop-up menu lists all available alpha channels in the image selected in the Document field. You can create a new channel or add to, subtract from, or intersect with an existing alpha channel. When you're working with an existing channel, the Name field is grayed out.
With a selection active in the image, you can also create a new alpha channel through the Channels palette (see Figure 7.19). Clicking the Save Selection as Channel button at the bottom of the palette creates a new alpha channel and bypasses the Save Selection dialog box. (The new channel is named Alpha 1 or the next available number.)
Figure 7.19. A new channel added with the Save Selection as Channel button can be renamed by double-clicking the name in the Channels palette.
The third technique for adding an alpha channel from an existing selection is through the Channels palette menu. Select the command New Channel (not New Spot Channel). You have an opportunity to name the channel and decide the channel's options (see Figure 7.20).
Figure 7.20. The Color Indicates and Color options determine the appearance of the mask when it is activated over one or more channels.
Quick Mask Mode
Photoshop offers a convenient way to edit selections through the use of maskswithout ever having to look at the Channels palette. Quick Mask mode takes you directly to a red overlay that is active and ready to be edited as an alpha channel. Simply press Q on the keyboard or click the button in the Toolbox to enter Quick Mask mode (see Figure 7.21). After editing the mask, press Q again or click the opposite button to exit Quick Mask mode.
Figure 7.21. The button to the left of the cursor returns you to Standard mode.
In terms of the actual content of a newly created alpha channel, there is no difference among the Select, Save Selection menu command, the Channels palette Save Selection as Channel button, and the palette menu command New Channel. Each generates the same channel from the same selection.
When you're in Quick Mask mode, a temporary channel is created in the Channels palette. Just as a work path disappears when it's no longer needed, so, too, is a quick mask deleted when you exit Quick Mask mode. After you exit Quick Mask mode, a selection is active in the image, and you can, of course, save that selection as a regular alpha channel.
In Quick Mask mode, the Channels palette menu also offers the Quick Mask Options command. You can change the overlay's opacity or color and even decide whether it shows or hides the masked areas. You can also open the dialog box by double-clicking the Quick Mask button in the Toolbox.
Alpha Channels and File Formats
Remember to paint in shades of gray when editing a mask. Using colors is deceivingonly the color's luminosity is used.
Alpha channels can be stored in several file formats. Photoshop's native format (PSD) supports multiple alpha channels, as does Photoshop's large image format (PSB). TIFF images in RGB or CMYK mode can support multiple alpha channels, as can PDF files created in Photoshop. The RAW file format can support numerous channels, but it can present problems of its own. For example, when reopening a RAW file, you must know the exact dimensions of the image, the correct number of channels, and whether the color data was interleaved when saved.
A single alpha channel can be saved with BMP, PICT, and Pixar files. The PNG-24 file format can generate an alpha channel to store transparency in an image, but that information cannot be accessed directly or edited. Rather, it is updated if you change the transparency of the image.