UNDERSTANDING CHANNELS

In Photoshop CS2, an image can have up to 56 total channels, up from 24 in older versions of Photoshop. (The total does not include the composite channel, RGB, CMYK, or Lab, which doesn't actually hold color data.) There are three types of channels:

• Component color channels: Component color channels contain the basic color information for an image. They are used with RGB, CMYK, and Lab color modes. The single channel in a Grayscale mode image can also be considered a component color channel. Component color channels store the color information for all artwork on layers in the image.

• Spot channels: Spot channels are used in CMYK images to hold special color information. A spot channel represents ink that will be printed independently of and in addition to the component inks (cyan, magenta, yellow, and black). Spot colors and their channels are independent of the layers in the image and are defined only in the spot channel.

• Alpha channels: Alpha channels do not hold color information. Rather, they are saved selections and are referred to as masks. Black, white, and shades of gray are used to store the selection information. A hard-edged selection made with the Rectangular Marquee tool would appear by default as a white rectangle on a black background. A heavily feathered selection made with the same tool would appear as an area of white blending through gray to black. Alpha channels can also be used to store transparency information for a variety of file formats.

Figure 5.18 shows a Channels palette containing an image's CMYK channel and includes a spot channel using a metallic Pantone color and an alpha channel to mask out portions of the image. You can use the palette menu or buttons at the bottom of the Channels palette to add these channels.

Figure 5.18. The Channels palette contains the image's CMYK channel, a spot channel, and an alpha channel.

Spot Color Channels

Spot color channels are primarily used with CMYK and Multichannel documents. They are designed to provide a channel for additional inks to be used in commercial printing. The location in the image where the ink should be applied is stored in the spot channel. A separate printing plate is generated, and an additional run through the press is required. This usually increases the cost of the print job. (Remember that Photoshop CS2 can now work with as many as 56 channels.)

If you need to ensure an exact match for a corporate logo, you might want to use a spot color. Another typical use is extending an image's color range beyond what can be produced by using CMYK inks. For example, you can add neon and metallic colors to an image with spot channels. Spot channels are also used to identify areas of an image over which a varnish will be applied.

Spot channels don't interact with layers, so adding a type layer in a spot color is out of the question. Instead, you create a type mask in the spot channel and fill the selection with black.

A spot channel is added to the imageand to the Channels paletteby using the Channels palette menu command New Spot Channel. In the dialog, you click the color swatch to open the Color Picker.

Remember that conversion to CMYK color mode often happens late in a workflow. You can add the spot color channels at any time; they're maintained when you do your final preprint color conversion. In addition, you can use a spot channel as an interim step in an RGB image. You create the spot channel and apply the color. Because the spot color is in a separate channel, it is protected while you continue editing and adjusting the rest of the image. Later, you can use the Channels palette menu command Merge Spot Channel to integrate that channel into the RGB channels.

Color Channels: Grayscale at Heart

The key to working directly in the Channels palette is remembering that each channel is nothing more than a grayscale image. You can treat a channel as a single-layer grayscale image and use the same tools and commands that you would use on the layer of a grayscale image. Each pixel can be any of 256 different "gray" or brightness values. When considered with the other component color channels of the image, that value is actually the proportion of the channel's color.

To edit an individual channel, you click it in the Channels palette. By default, the image appears in grayscale, with only the values of the selected channel visible. Changes made are applied to only that active channel.

You can also make two or more channels active at the same time by Shift-clicking them. When multiple color channels are active, the image appears in color, using a blend of the selected colors (see Figure 5.19).

Figure 5.19. The Green and Blue channels are active in the Channels palette, so the window displays the image in a combination of greens and blues rather than in grayscale.

You can also work with one or more channels active and all channels visible. By clicking in the left column, next to the composite channel, or next to any channel of your choice, you can make the channel visible without being active. Likewise, you can hide the active channel by clicking the eyeball icon. At least one channel must be visible at all times.

Photoshop's Display & Cursors page of the Preferences window offers the option of showing individual channels in color. When one channel is active, you see a monotone image rather than a grayscale image in that channel's color. Although this may be a handy reminder of what channel is active, it can be very difficult to see detail in the Yellow and Cyan channels when they're shown in color.

Filtering and Adjusting Individual Channels

You can apply Photoshop's filters to channels individually. You might do this to control the impact of a filter, to produce a special effect, or to fix a problem that occurs in only one or two channels.

Digital camera images often have a lot of noise. When you examine the channels individually, you often find that the noise is primarily or exclusively in the Blue channel. Instead of applying a Gaussian Blur or Dust & Scratches filter to the entire imagewhich results in a general softening of detailyou can filter only the noisy channel, retaining detail in the other channels.

Using the Channel Mixer to Create Perfect Grayscale from Color

Using the Channel Mixer is an excellent way to create grayscale images from color pictures. To see for yourself, try this:

1.	Open a copy of an RGB image.
2.	If it has multiple layers, flatten it.
3.	Click twice on the leftmost button at the bottom of the History palette. This makes a pair of copies of the image. Zoom out, if necessary, and position the windows so that you can view all three images onscreen at the same time. (Choose the Windows, Arrange, Tile menu command.)
4.	Use the Image, Mode, Grayscale menu command to convert one image.
5.	Click another copy of the image. Use the Image, Adjustments, Desaturate menu command followed by the Image, Mode, Grayscale command on this second copy of the image.
6.	Switch to the third copy and use Image, Adjustments, Channel Mixer to open the Channel Mixer dialog.
7.	Click the Monochrome check box.
8.	Set the Red, Green, and Blue fields to 40 and change the Constant slider to 7.
9.	Concentrate on an area of extreme highlight or extreme shadow. Slowly move the sliders back and forth, juggling the amounts of each channel you add or subtract. Watch how you can target certain tonal ranges, depending on the content of each channel.
10.	As you work, compare this image with the other two. Look for a balance in your mix that makes the image "pop" and provides the best tonal range.
11.	When the image looks perfect, click OK and then select Image, Mode, Grayscale.

You can even make selections and work on different areas on an image according to their needs. Generally speaking, for a normal key image, you want the total of the sliders to equal about 100. When doing the math, triple the valuepositive or negativeof the Constant slider.