Other Channel Functions

In addition to viewing and editing channels using any of the techniques discussed in future chapters of this book, you can choose commands from the Channels palette menu and select icons along the bottom of the palette (labeled back in Figure 4-15). The following items explain how the commands and icons work.

You'll notice that I say "see Chapter 9" every so often when explaining these options, because many of them are specifically designed to accommodate masks. This list is designed to introduce you to all the options in the Channels palette, even if you'll need more background to use a few of them. After I introduce the options, we'll revisit the ones that have a direct effect on managing the colors in your image.

♦ Palette Options: Even though this is the last command in the menu, it's the easiest, so I'll start with it. When you choose Palette Options, Photoshop displays four Thumbnail Size radio buttons, enabling you to change the size of the thumbnail previews that appear along the left side of the Channels palette. Figure 4-19 shows the four thumbnail settings — nonexistent, small, medium, and large.

Have you ever wondered what those thumbnail icons in the Palette Options dialog box are supposed to show? They're silhouettes of tiny Merlins on a painter's palette. How do I know that? Switch to the Layers palette and choose Palette Options and you'll see them in color. But how do I know they're specifically Merlins? Press Alt when choosing Palette Options to see the magician up close. We're talking vintage Easter egg, here — circa Photoshop 2.5.

♦ New Channel: Choose this command to add a mask channel to the current image. The Channel Options dialog box appears, requesting that you name the channel. You also can specify the color and translucency that Photoshop applies to the channel when you view it with other channels. I explain how these options work in the "Changing the red coating" section of Chapter 9. An image can contain up to 24 total channels, regardless of color mode.

Figure 4-19: The Palette Options command lets you select between four thumbnail preview options and a Merlin.

Figure 4-19: The Palette Options command lets you select between four thumbnail preview options and a Merlin.

Tip You can also create a new channel by clicking on the new channel icon at the bottom of the Channels palette. (It's the one that looks like a little page.) Photoshop creates the channel without displaying the dialog box. To force the dialog box to appear on screen, Alt-click the page icon.

* Duplicate Channel: Choose this command to create a duplicate of the selected channel, either inside the same document or as part of a new document. (If the composite view is active, the Duplicate Channel command is dimmed, because you can only duplicate one channel at a time.) The most common reason to use this command is to convert a channel into a mask. Again, you can find real life applications in Chapter 9.

Tip You can also duplicate a channel by dragging the channel name onto the new channel icon. No dialog box appears; Photoshop merely names the channel automatically. To copy a channel to a different document, drag the channel name and drop it into an open image window. Photoshop automatically creates a new channel for the duplicate.

* Delete Channel: To delete a channel from an image, click the channel name in the palette and choose this command. You can delete only one channel at a time. The Delete Channel command is dimmed when any essential color channel is active, or when more than one channel is selected.

Tip If choosing a command is too much effort, just drag the channel onto the delete channel icon (which is the little trash icon in the lower right corner of the Channels palette). Or you can just click the trash icon, in which case Photoshop asks you if you really want to delete the channel. To bypass this warning, Alt-click the trash icon.

♦ New Spot Channel: Photoshop lets you add spot color channels to an image. Each spot color channel prints to a separate plate, just like spot colors in Illustrator or QuarkXPress. When you choose the New Spot Color command, Photoshop asks you to specify a color and a Solidity. Click the color square to bring up the Custom Colors dialog box, from which you can select a Pantone or other spot color (see Figure 4-20). The Solidity option lets you increase the opacity of the ink, perfect for Day Glo fluorescents and metallic inks.

Tip To create a spot color channel without choosing a command, Ctrl click the page icon at the bottom of the Channels palette. For more information on spot color channels, read the "Spot Color Separations" section at the end of Chapter 18.

Figure 4-20: When creating a spot-color channel, Photoshop asks you to select a color and specify the degree to which the spot color will cover up other inks in the printed image.

♦ Merge Spot Channel: Select a spot-color channel and choose this command to merge the spot color in with the RGB, Lab, or CMYK colors in the image. Most spot colors don't have precise RGB or CMYK equivalents, so you will lose some color fidelity in the merge. Adobe includes this command to enable you to proof an image to a typical midrange color printer.

♦ Channel Options: Choose this command or double-click the channel name in the palette's scrolling list to change the settings assigned to a spot-color or mask channel. The Channel Options command is dimmed when a regular, everyday color channel is active.

♦ Split Channels: When you choose this command, Photoshop splits off each channel in an image to its own independent grayscale image window. As demonstrated in Figure 4-21, Photoshop automatically appends the channel color to the end of the window name. The Split Channels command is useful as a first step in redistributing channels in an image prior to choosing Merge Channels, as I will demonstrate later in this same chapter.

Figure 4-21: When you choose the Split Channels command, Photoshop relocates each channel to an independent image window.

♦ Merge Channels: Choose this command to merge several images into a single multichannel image. The images you want to merge must be open, they must be grayscale, and they must be absolutely equal in size — the same number of pixels horizontally and vertically. When you choose Merge Channels, Photoshop displays the Merge Channels dialog box, shown in Figure 4-22. It then assigns a color mode for the new image based on the number of open grayscale images that contain the same number of pixels as the foreground image.

Figure 4-22: The two dialog boxes that appear after you choose Merge Channels enable you to select a color mode for the merged image (top) and to associate images with color channels (bottom).

You can override Photoshop's choice by selecting a different option from the Mode pop-up menu. (Generally, you won't want to change the value in the Channels option box because doing so causes Photoshop to automatically select Multichannel from the Mode pop-up menu. I explain multichannel images in the upcoming "Using multichannel techniques" section.)

After you press Enter, Photoshop displays a second dialog box, which also appears in Figure 4-22. In this dialog box, you can specify which grayscale image goes with which channel by choosing options from pop-up menus. When working from an image split with the Split Channels command, Photoshop automatically organizes each window into a pop-up menu according to the color appended to the window's name. For example, Photoshop associates the window Sunbat_C.jpg with the Cyan pop-up menu.

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