Back in "Masking-Tape Selections," we told you that selections, masks, and channels are all the same thing down deep: grayscale images. This is not intuitive, nor is it easy to grasp at first. But once you really understand this point, you've taken the first step toward really surfing the Photoshop big waves.
A channel is a solitary grayscale imageeach pixel described using either 8 bits or 16 bits of data, depending on whether or not it's a high-bit image. You can have up to 56 channels in a documentthat includes the three in an RGB or four in a CMYK image. (Actually, there are two exceptions: First, images in Bitmap mode can only contain a single 1-bit channel; second, Photoshop allows one additional channel per layer to accommodate layer masks, which we'll talk about later in this chapter.)
But in the eyes of the program, not all channels are created equal. There are three types of channels: alpha, color, and spot-color channels (see Figure 8-13). We discuss the first two here, and the third in Chapter 10, Spot Colors and Duotones.
Figure 8-13. The Channels palette
People get very nervous when they hear the term "alpha channel," because they figure that with such an exotic name, it has to be a complex feature. Not so. An alpha channel is simply a grayscale picture. Alpha channels let you save selections, but a solid understanding of these beasts is also crucial to tackling layer masks. Note that since Photsoshop CS, 16-bit images are first-class citizens: You can make, save, and load any sort of selection or channel in high-bit images.
Selections and channels are really the same thing down deep (even though they have different outward appearances), so you can turn one into the other very quickly. Earlier in this chapter we discussed how you can see and edit a selection by switching to Quick Mask mode. But quick masks are ephemeral things, and aren't much use if you want to hold on to that selection and use it later.
When you turn a selection into an alpha channel, you're saving that selection in the document. Then you can go back later and edit the channel, or turn it back into a selection.
As we pointed out in the last chapter, the slow way to save a selection is to choose Save Selection from the Select menu. It's a nice place for beginners because Photoshop provides you with a dialog box (see Figure 8-14). But pros don't bother with menu selections when they can avoid them. Instead, click the Save Selection icon in the Channels palette. Or, if you want to see the Channel Options dialog box first (for instance, if you want to name the channel), hold down the Option key while clicking the icon (see Figure 8-15). Of course, you can also assign a keyboard shortcut to the New Channel feature if you use this a lot.
Figure 8-14. Save Selection dialog box
Figure 8-15. Saving a selection
Tip: Loading Selections
Saving a selection as an alpha channel doesn't do you much good unless you can retrieve it. Again, the slowest method is to use the Load Selection item from the Select menu (though there are benefits to this method; see "Tip: Saving Channels in Other Documents," later in this chapter).
One step better is to Command-click on the channel that you want to turn into a selection. Even better, press Command-Option-#, where the number is the channel you want. For instance, if you want to load channel six as a selection, press Command-Option-6. Note that if you press Command-Option-~(tilde), you load the luminosity mask. This isn't really the "lightness" of the image; rather, it's like getting a grayscale version of your image.
Tip: Be a Packrat!
Every time it takes you more than 10 seconds to make a selection in your image, you should be thinking: Save This Selection. We try to save every complex selection as a channel or a path until the end of the project (and sometimes we even archive them, just in case). The reason? You never know when you'll need them again.
Tip: Channels in TIFFs
If you're saving a mess of channels along with the image you're working on, and you want to save the file as a TIFF, you should probably turn on LZW compression in the Save as TIFF dialog box. Zip compression is even better, though QuarkXPress and most other programs can't read Zip-compressed TIFFs yet. However, Adobe InDesign can, and of course you can always reopen Zip-compressed TIFFs in Adobe Photoshop. Whatever the case, use some kind of compressionotherwise, the TIFF will be enormous. Of course, you could save in the native Photoshop format, but we find that a Zip-compressed TIFF file is almost always smaller on disk (see Chapter 13, Image Storage and Output).
Tip: Adding, Subtracting, and Intersecting Selections
Let's say you have an image with three distinct elements in it. You've spent an hour carefully selecting each of the elements, and you've saved each one in its own channel (see Figure 8-16). Now you want to select all three objects at the same time.
Figure 8-16. Adding, subtracting, and intersecting selections
In the good old days, you would have sat around trying to figure out the appropriate channel operations (using Calculations) to get exactly what you wanted. But it's a kinder, gentler Photoshop now. After you load one channel as a selection, you can use Load Selection from the Select menu to add another channel to the current selection, subtract another channel, or find the intersection between the two selections.
Even easier, use the key-click combinations in Table 8-1. Confused? Don't forget to watch Photoshop's cursor icons; as you hold down the various key combinations, Photoshop indicates what will happen when you click.
As we said earlier, your alpha channels don't all have to be in the same document. In fact, if you've got more than 56 channels, you have to have them in multiple documents. But even if you have fewer than 56, you may want to save off channels in order to reduce the current file's size. Here are a bunch of tips we've found helpful in moving channels back and forth between documents.
Tip: Saving Channels in Other Documents
As long as another file is currently open, you can save a selection into it using Save Selection, or load a selection (from a channel) from the file using the menu items. You can even save selections into a new document by selecting New from the Document popup menu in the Save Selection dialog box.
If you have two similar documents open and you've carefully made and saved a selection in one image, you might want to use it in the other image. Instead of copying and pasting the selection channel, take a shortcut route and use the Load Selection dialog box. You can load the selection channel directly by choosing it from the Document and Channel popup menus.
The catch here is that both documents have to have exactly the same pixel dimensions (otherwise, Photoshop wouldn't know how to place the selection properly).
Tip: More Saving Off Channels
If you've already saved your selection into a channel in document A, how can you then get that channel into document B? One method is to select the channel and choose Duplicate Channel from the Channel palette's popout menu (see Figure 8-17). Here you can choose to duplicate the channel into a new document or any other open document (as long as the documents have the same pixel dimensions).
Figure 8-17. Duplicating a channel to another document
Tip: Copying Channels the Fast Way
When it comes right down to it, the fastest way to copy a channel from document A to document B is simply by dragging the channel's tile (in the Channels palette) from document A onto document B. It's nice, quick, simple, and elegant. The problem is that the two documents have to be the same size, or else Photoshop won't align the channels properly.
We hear you asking, "Why not just copy and paste the channel from one document into another?" The answer is that copy and paste can really bog down in large files. ("Large" means different things to different people; Bruce counts in gigabytes!) In smaller files, however, copy and paste works just as well.
Tip: Dragging Selections
Photoshop is full of little, subtle features that make life so much nicer. For instance, you can drag any selection from one document into another document using one of the selection tools. (The Move tool actually moves the pixels inside the selection; the selection tools move the selection itself.)
Normally, the selection "drops" wherever you let go of the mouse button. However, if the two documents have the same pixel dimensions, you can hold down the Shift key to pin-register the selection (it lands in the same location as it was in the first document). If the images aren't the same pixel dimension, the Shift key centers the selection.
When a color image is in RGB mode (under the Mode menu), the image is made up of three channels: red, green, and blue. Each of these channels is exactly the same as an alpha channel, except that they're designated as color channels. You can edit each color channel separately from the others. You can independently make a single color channel visible or invisible. But you can't delete or add a color channel without changing the image mode.
The first tile in the Channels palette (above the color channels) is the composite channel. Actually, this isn't really a channel at all. Rather, the composite channel is the full-color representation of all the individual color channels mixed together. It gives you a convenient way to select or deselect all the color channels at once, and also lets you view the composite color image even while you're editing a single channel.
Selecting and Seeing Channels
The tricky thing about working with channels is figuring out which channel(s) you're editing and which channel(s) you're seeing on the screen. They're not always the same!
The Channels palette has two columns. The left column contains little eyeball checkboxes that you can turn on and off to show or hide individual channels. Clicking on one of the tiles in the right column not only displays that channel, but lets you edit it, too. The channels that are selected for editing are highlighted. The two columns are independent of each other because editing and seeing the channels are not the same thing.
Tip: Keystroke Channels
When you're jumping from one channel to another, skip the clicking altogether and use a keystroke instead. Command-# displays the channel number you press; for instance, Command-1 shows the red channel (or whatever the first channel is), and Command-4 shows the fourth channel (the first alpha channel in an RGB image or Black in a CMYK image). Sorry, there's no way (that we know of) to select channels above number nine with keystrokes.
In ancient versions of Photoshop, Command-0 would select the color composite channel (deselecting all other channels in the process). Now, the keystroke is Command-~ (tilde).
You can see as many channels at once as you want by clicking in the channel's eyeball checkboxes. To edit more than one channel at a time, Shift-click on the channel tiles.
Note that when you display more than one channel at a time, the alpha channels automatically switch from their standard black and white to their channel color (you can specify what color each channel uses in Channel Optionsdouble-click on the channel tile).