Chapter 2. Channels

BEFORE THERE WERE LAYERS, before layer masks and adjustment layers, before clipping masks, Smart Objects, and all the other selection-related features that Photoshop offers today, there were channels. And as impressive as those features are, they can never supplant channels in terms of their fundamental importance. Channels underpin every Photoshop image you work with, and working in Photoshop you're working with channels whether you know it or not.

Every image is made up of channelsor a single channel if it's a grayscale, bitmap, or indexed color imageand utilizing those channels will vastly expand your range of image-editing possibilities and massively speed up your workflow. Channels come in three flavors: color channels, which store the color information in our images; alpha channels, which are saved selections; and spot color channels, which expand our range of printing options.

Color channels store information about the color of each pixel in your image. The most common way to describe a pixel's color is in RGBred, green, bluealthough as we'll see there are other ways. Much as you might mix paints on a palette to create a range of colors, Photoshop mixes red, green, and blue in different proportions to represent the colors in an image. Every pixel in an RGB image has three numbers that describe its red, green, and blue components. Collectively, the red values for all the pixels in an image make up the Red channel. The same is true of the green and blue values and their channels.

How many color channels an image starts out with is determined by its color modethree for RGB, four for CMYK. For example, when you open an RGB image, it's made up of three channels: Red, Green, andyou guessed itBlue, superimposed on each other. A fourth, composite channel (RGB) allows you to view all three color channels together. You can also view the channels individually or in combination. Let's take a look at this deconstructed image.

Figure 2.1. The composite channel (example A) is made up of three channels in an RGB image (examples B and C) and four in a CMYK image (example D). The number of color channels depends on the color mode of an image.

If you're familiar with how offset printing works, you can think of channels as printing plates, with a separate plate applying each channel of color. In addition to color channels Photoshop has two other types of channels: the absolutely essential alpha channels, for saving selections, and the occasionally useful spot color channels, for expanding the range of printing options.

An understanding of channels is the foundation for working with all selection tools in Photoshop. Here's what channels can do for you:

• You can save selections as alpha channels. Don't be put off by the name, alpha channels are nothing more than potential selections, which you can load on an as-needed basis. Like stencils, alpha channels are black-and-white representations of what is selected in the image (white) and what is unselected or masked (black).

• You can use channels to create more accurate selections than is possible with Selection tools. By converting a selection to a Quick Mask (see "Using Quick Masks to Paint Selections" later in this chapter), you can refine it with your painting tools and then convert it to a (permanent) alpha channel.

• You can use channels for more than just storing selections. In this chapter, we'll see how you can optimize Web images by compressing specific parts of an image and so reduce their download time, as well as how to mix channels to create high-quality grayscale images.

  Note

  With the evolution of layers, layer masks, and adjustment layers (discussed in Chapters 3, 4, and 5 respectively) some of the things that you could previously only achieve with channelssuch as gradient masks or combining channel blend modes with Calculationsare now more easily, and more safely, achieved with layers. For example, layer masks are channels that are applied to specific layers and can be used to make nondestructive, that is, reversible changes. An understanding of channels is the foundation for working with all masking tools in Photoshop.

  
  

• You can create a duotone to allow you to print with a limited number of inks as well as to extend the tonal range of a printed image.

• You can create spot color channels to specify additional plates for printing with spot color inks. For example, you can create specific print effects using a channel to apply a spot (custom color) ink or varnishsuch as for a logo or to make a particular color "pop" on the pagethat will print to a separate plate.

• You can use channels as the starting point for making complex selections. Often the selection you want to make is right there, in the channel, waiting to be teased out. The best approach to selecting images with fine-edge detail, like hair, fur, tree branches, and so forth, is to make a duplicate of one of your existing color channels and then refine this channel to make your mask. This technique is the subject of Chapter 6, "Channel Masks."

Note

Not all color modes use multiple channelsbitmap, grayscale, and indexed color modes use a single channel.