Introducing Channels


Each channel is a grayscale image that stores specific color information. Each channel can contain up to 256 shades of gray. Channels are available on the Channels palette, as shown in Figure 15-1.

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Figure 15-1: The Channels palette displays color information, saved selections, and layer masks.

The individual color channels — Red, Green, and Blue — are listed first (depending upon the color mode you are working in, a CMYK image contains Cyan, Magenta, Yellow, and Black channels). The color channels store the color information for an image. Then come any alpha channels that have been added. An alpha channel can contain a saved selection or layer mask. A thumbnail showing a channel’s contents appears to the left of the channel name.

To see the separate color channels and how combine to create a color image, turn to Color Plates 5-2 and 5-3. Color Plate 5-2 shows an RGB image broken down into its Red, Green, and Blue channels. In addition, the color plate shows how the image looks when two channels are combined. Color Plate 5-3 shows the same image from 5-2 converted to CMYK mode and broken down into the Cyan, Magenta, Yellow, and Black channels. Color Plate 5-3 also shows how the color in a CMYK image is created as the channels are put together.

Color Plate 15-2 shows how you can modify channels to change the look of an image. In Color Plate 15-1, I first adjusted the highlights and midtones of the red channel in an RGB image. Next, I applied the Sponge filter to the Blue channel, and then I applied the Watercolor filter to the Green channel.

Figure 15-2 shows the three color channels that make up an RGB image.

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Figure 15-2: This image of two girls is made up of red, green, and blue channels.

Getting to know alpha channels

Just like regular channels that store color information, alpha channels are represented as grayscale images that store layer masks or selections. What sets alpha channels apart from other channels is the fact that they store selections that can be activated at anytime. (Regular channels store color information only.) This means that you can always load a saved selection into an image, reactivating the selection.

Alpha channels also can be used to contain layer mask transparency and opacity information because layer masks hide or reveal areas of a layer.

You can paint on an alpha channel to change the selected areas or alter the visible areas of a layer mask. (I cover working with and editing layer masks later in this chapter in “Layer Masks.”)

When working with an alpha channel that is a saved selection, its thumbnail in the Channels palette appears as black and white as shown in Figure 15-3. By default, the white area represents the selected area of the image that can be edited. The black area represents the protected part of the image that cannot be edited. Alpha channels appear at the bottom of the Channels palette.

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Figure 15-3: The protected areas of the image are black and the selected (editable) areas are white.

Working with the Channels palette

Using the Channels palette, you can select, show, hide, and delete channels.

  • To select a channel: Click the channel’s name to make it active. The channel becomes highlighted. You can now make changes to the active channel. For instance, you could adjust the levels or curves of the channel, or add special effects such as applying a filter.

  • To select more than one channel at a time: Hold down the shift key and click the channels you want to select.

  • To hide a channel: Click the eye icon in the column to the left of the channel. To view the channel, click the eye icon again.

  • To rename a channel: Double-click the channel’s name, type in a new name, and then press Enter (Return on a Mac).

  • To view the color channels showing their colors (red, green, and blue, for instance), choose Edit→Preferences→Display & Cursors. In the Preferences dialog box, select Color Channels in Color.

  • To create a new alpha channel: Click the Create New Channel button at the bottom of the Channels palette. The channel is automatically named Alpha 1 (or Alpha 2 if you already have an Alpha 1).

  • To duplicate a channel: Select the channel you want to duplicate in the Channels palette. Then choose Duplicate Channel from the Channels palette menu. In the Duplicate Channel dialog box, enter a name and select whether the new channel appears in the current document or a new document, and then click OK. If you want to create a grayscale image using a channel, duplicating a channel and saving it as in a new document is a good way to go. For more about creating grayscale images by selecting a color channel, turn to Chapter 5.

  • To move an alpha channel up or down in the stacking order: Drag the channel up or down the stack to its new location. Alpha channels have a stacking order just like layers do in the Layers palette. (Notice that you cannot reposition an alpha channel up with the color channels because the color channels always appear at the top of the stacking order.)

    Warning

    To set the color display for masked and unmasked areas in an alpha channel: Double-click the channel you want to change. In the Channel Options dialog box, you can select how the masked (protected) and selected areas are colored. By default, the setting for coloring the masked and selected areas is Color Indicates: Masked Areas (Figure 15-4). As this chapter has discussed, this sets the selected areas to white and the protected areas to black. If you select Color Indicates: Selected Areas, you make the selected areas black and the white areas protected. Don’t get confused! This chapter is written using the default setting.

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    Figure 15-4: The Color Indicates: Masked Areas selection is the default. It sets an alpha channel to show the selected (editable) areas as white and the protected (unselected) areas as black.

  • To delete a channel: Drag it to the trash icon at the bottom of the channels palette. If you delete a color channel, the image is automatically converted to Multichannel mode (for details about color modes, turn to Chapter 5).

Remember

If you’re working with a large image, keep in mind that channels add to image file size. It’s always good to do some housekeeping when working on a big project. If you find that you don’t need a saved selection anymore, just delete the alpha channel that’s storing the selection.




Photoshop CS For Dummies
Photoshop CS For Dummies
ISBN: 0764543563
EAN: 2147483647
Year: 2006
Pages: 221

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