Color Channels In Photoshop

After I’ve droned on for pages about color in Photoshop, it might surprise you when I say that Photoshop is at its heart a grayscale editor. Oh sure, it offers an array of color conversion features and it displays and prints spectacular full-color images. But when it comes to editing the image, everything happens in grayscale.

This is because Photoshop approaches every full-color image not as a single collection of 24-bit pixels, but as three or four bands of 8-bit (grayscale) pixels. An RGB file contains a band of red, a band of green, and a band of blue, each of which functions as a separate grayscale image. A Lab image likewise contains three bands, one corresponding to luminosity and the others to the variables a and b.

A CMYK file contains four bands, one for each of the process-color inks. These bands are known as channels. Channels frequently correspond to the structure of an input or output device. Each channel in a CMYK image, for example, corresponds to a different printer’s plate when the document goes to press.

The cyan plate is inked with cyan, the magenta plate is inked with magenta, and so on. Each channel in an RGB image corresponds to a pass of the red, green, or blue scanner sensor over the original photograph or artwork. Only the Lab mode is device independent, so its channels don’t correspond to any piece of hardware.

Why You Should Care

But so what, right? Who cares how many planes of color an image comprises? You want to edit the photograph, not dissect it. Well, even if you don’t like to rebuild car engines or poke preserved frog entrails with sharp knives, you’ll get a charge out of editing channels.

The fact is, channels provide you with yet another degree of selective control over an image. Consider this example: Your client scanned a photograph of his gap-toothed daughter that he wants you to integrate into some goofy ad campaign for his car dealership.

Unfortunately, the scan is downright rotten. You don’t want to offend the guy, so you praise him on his fine offspring and say something to the effect of, “No problem, boss.” But after you take it back to your office and load it into Photoshop, you break out in a cold sweat.

You try swabbing at it with the edit tools, applying a few filters, and even attempting some scary-looking color correction commands, but the image continues to look like the inside of a garbage disposal. (Not that I’ve ever seen the inside of a garbage disposal, but it can’t be attractive.)

Suddenly, it occurs to you to look at the channels. What the heck, it can’t hurt. With very little effort, you discover that the red and green channels look okay, but the blue channel looks like it’s melting.

Her mouth is sort of mixed in with her teeth, her eyes look like an experiment in expressionism, and her hair has taken on a slightly geometric appearance. (If you think that this is a big exaggeration, take a look at a few blue channels from a low-end scanner or digital camera. They’re frequently rife with tattered edges, random blocks of color, stray pixels, and other so-called digital artifacts.)

The point is, you’ve located the cancer. You don’t have to waste your time trying to perform surgery on the entire image; in fact, doing so may very well harm the channels that are in good shape. You merely have to fix this one channel.

A wave of the Gaussian Blur filter here, an application of the Levels command there, and some selective rebuilding of missing detail borrowed from the other channels all of which I’ll get to in future sections and chapters result in an image that resembles a living, breathing human being. Granted, she still needs braces, but you’re an artist, not an orthodontist.

How Photoshop Channels Work

Photoshop devotes 8 bits of data to each pixel in each channel, thus permitting 256 brightness values, from 0 (black) to 255 (white). Therefore, each channel is actually an independent grayscale image. At first, this may throw you off. If an RGB image is made up of red, green, and blue channels, why do all the channels look gray?

Photoshop provides an option in the Display & Cursors panel of the Preferences dialog box (Ctrl+K, Ctrl+3) called Color Channels in Color. When selected, this function displays each channel in its corresponding primary color. But although this feature can be reassuring particularly to novices it’s equally counterproductive.

When you view an 8-bit image composed exclusively of shades of red, for example, it’s easy to miss subtle variations in detail that may appear obvious when you print the image. You may have problems accurately gauging the impact of filters and tonal adjustments.

I mean, face it, red isn’t a friendly shade to stare at for a half hour of intense editing. So leave the Color Channels in Color option off and temporarily suspend your biological urge for on-screen color. With a little experience, you’ll be able to better monitor your adjustments and predict the outcome of your edits in plain old grayscale.

Images that include 256 or fewer colors can be expressed in a single channel and therefore do not include multiple channels that you can edit independently. A grayscale image, for example, is just one channel. A black-and-white bitmap permits only one bit of data per pixel, so a single channel is more than enough to express it.

You can add channels above and beyond those required to represent a color or grayscale image for the purpose of storing masks. But even then, each channel is typically limited to 8 bits of data per pixel meaning that it’s just another grayscale image. Mask channels do not affect the appearance of the image on screen or when it is printed. Rather, they serve to save selection outlines.

How To Switch and View Channels On Photoshop

To access channels in Photoshop, display the Channels palette by choosing Window>Show Channels. Every channel in the image appears in the palette including any mask channels as shown in Figure-6.

Figure-6: Photoshop displays tiny thumbnails of each color channel in the Channels palette.

Photoshop even shows little thumbnail views of each channel so that you can see what it looks like. To switch to a different channel, click a channel name in the Channels palette. The channel name becomes gray like the Blue channel in Figure-6 showing that you can now edit it independently of other channels in the image.

To edit more than one channel at a time, click one channel name and then Shiftclick another. You can also Shift-click an active channel to deactivate it independently of any others. When you select a single channel, Photoshop displays just that one channel on screen. However, you can view additional channels beyond those that you want to edit.

To specify which channels appear and which remain invisible, click in the farleft column of the Channels palette. Click an eyeball icon to make it disappear and hence hide that channel. Click where there is no eyeball to create one and thus display the channel.

When only one channel is visible, that channel appears as a grayscale picture in the image window (possibly colorized in accordance with the Color Channels in Color check box in the Preferences dialog box).

However, when more than one channel is visible, you always see color. If both the blue and green channels are visible, for example, the image appears blue-green. If the red and green channels are visible, the image has a yellow cast, and so on.

In addition to the individual channels, Photoshop provides access to a composite view that displays all colors in an RGB, CMYK, or Lab image at once. (The composite view does not show mask channels; you have to specify their display separately.)

The composite view is listed first in the Channel palette and is displayed by default. Notice that when you select the composite view, all the names of the individual color channels in the Channels palette turn gray along with the composite channel. This shows that all the channels are active.

The composite view is the one in which you will perform the majority of your image editing. Press Ctrl plus a number key to switch between color channels. Depending on thecolor mode you’re working in, Ctrl+1 takes you to the red (RGB), cyan (CMYK), or luminosity (Lab) channel; Ctrl+2 takes you to the green, magenta, or a channel; and Ctrl+3 takes you to the blue, yellow, or b channel.

In the CMYK mode, Ctrl+4 displays the black channel. Other Ctrl+key equivalents up to Ctrl+9 take you to mask or spot-color channels (if there are any). To go to the composite view, press Ctrl+tilde (~). Tilde is typically the key to the left of 1, or on some keyboards, to the right of the spacebar.

When editing a single channel, you may find it helpful to monitor the results in both grayscale and full-color views. Choose View>New View to create a new window for the image, automatically set to the color composite view. Then return to the first window and edit away on the individual channel.

One of the amazing benefits to creating multiple views in Photoshop is that the views may show entirely different channels, layers, and other image elements.

The shortcuts are slightly different when you’re working on a grayscale image. You access the image itself by pressing Ctrl+1. Ctrl+2 and higher take you to extra spotcolor and mask channels.