Photoshop CS Timesaving Techniques For Dummies [Electronic resources] نسخه متنی

Technique 14.)

Color channels

Color channels are actually grayscale images that store specific color information. Each channel can contain up to 256 shades of gray — black and white are at the opposite ends of this grayscale spectrum. Channels can be viewed with the Channels palette, which is shown in Figure 52-1.

Figure 52-1: Color channels are listed first in the Channels palette.

In Figure 52-1, notice that the individual color channels (Red, Green, and Blue for an RGB image, or Cyan, Magenta, Yellow, and Black for a CMYK image) are listed first along with a composite color channel that shows all the color channels mixed together. Figure 52-2 shows an RGB image of colored pencils separated out into its individual color channels.

Figure 52-2: An RGB image is made up of three color channels.

Every pixel in an image can contain up to four color channels depending on the color mode that the image is set in.

By default, color channels are viewed in grayscale in Photoshop. For an experiment, you can take a look at the color channels in their respective color by choosing Edit/Photoshop>Preferences>Display & Cursors, and then selecting the Color Channels in Color check box in the Preferences dialog box.

You may be wondering how a grayscale image fits into this channel scheme. Any image with less than 256 colors is displayed as a single channel. So, a grayscale image contains only one channel.

Table 52-1 shows how many channels exist per color mode.

Multichannel mode is a bit of an oddball because it can have from one to four channels depending on the color mode conversion. If an RGB image is converted to Multichannel mode, it still has three channels. A CMYK image that’s converted to Multichannel mode still has four channels. Likewise, when a color channel is deleted from an RGB or CMYK image, the image is automatically converted to Multichannel mode.

Alpha channels

Just like color channels that store the color information for an image, alpha channels are also grayscale images. However, alpha channels are used to store layer masks or selections. They’re located at the bottom of the list in the Channels palette. (Refer to Figure 52-1.)

You can view an alpha channel at any time in two ways: as an overlay superimposed on the image or load it as a selection.

Using any shade from the grayscale spectrum (including black and white), you can paint on an alpha channel to enlarge or reduce the protected (selected or masked) areas. (If you want to find out how to work with and edit layer masks, turn to Technique 49.)