Using Adobe Photoshop CS, Part 1 - Channels
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No, I'm not referring to the channels on your TV (there are so many of them, yet nothing is ever on). Rather, these channels, also called alpha channels, can be thought of as selections, because in reality that's what they are. You can find them in the Channels palette.
Every file has at least one channel by default, and three or more if it is a color file. For example, an RGB file starts with three channels: one each for red, green, and blue. Photoshop also displays a composite, one for all of the channels combined, although this composite isn't actually a channel itself (see Figure 5.39). You can view and edit each channel individually, giving you total control over your image.
But the real strength here is that you can create your own channels. When you save a selection (as mentioned earlier), Photoshop is creating a channel, and that is how the selection is stored (see Figure 5.40). Channels that you create can contain 256 levels of gray. There are certain file formats that can use the information in channels as well. For example, you can specify that a channel should be a transparency mask when you export a file as a PNG from Photoshop. In contrast to clipping masks (which we'll discuss later in the chapter) that you might save in EPS format, an alpha channel transparency mask can utilize 256 levels of gray.
Figure 5.39 The channels of an RGB document.
Figure 5.40 A file with several channels saved.
Rather than having to load selections via the Select menu, you can simply (Command-click) [Ctrl-click] on a channel (in the Channels palette) to load that selection. This is true with any layer, actually-you can select all of a layer's contents this way.
Paths
If you've used Illustrator before, you know what a Bézier path is. We'll learn more about it in Chapter 7, "Using Adobe Illustrator CS," but at a basic level, it's an object-based path that you can draw using Photoshop's Pen tool (see Figure 5.41). It just so happens that Illustrator, InDesign, and Photoshop all have Bézier Pen tools-mainly because these tools are basic drawing tools.
Figure 5.41 Drawing a Bézier path with the Pen tool.
The selection tools we've discussed until now aren't really precise drawing tools at all. When you want to draw a high-quality selection, the Pen tool is the way to go. As you draw a path, it automatically creates a vector mask layer (see Figure 5.42). We'll talk about masks later in the chapter, but you're basically creating a vector shape that is filled with colored pixels. A listing also appears in the Paths palette. From the Paths palette, you can choose Save Path from the palette flyout menu to save the path for future use. At any time you can also (Command-click) [Ctrl-click] on the path in the Paths palette to turn it into an active selection.
Figure 5.42 A vector mask layer in the Layers palette.
You can also use paths in the reverse context. You start by creating a selection using any of the methods we've discussed. Then, with the selection active, choose Make Work Path from the Paths palette flyout menu (see Figure 5.43) to turn that selection into a vector path, at which point you can use the Pen tools to further edit that path if necessary.
Figure 5.43 Creating a work path from a selection.
 This chapter is from Adobe Creative Suite, by Mordy Golding (Sams, 2003, ISBN: 0672325918). Check it out at your favorite bookstore today. Buy this book now.
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