Photoshop.CS.Bible [Electronic resources] نسخه متنی

Quick Corrections

Photoshop offers four quick-correctors under the Image Adjustments submenu. Introduced in Photoshop 7, the Auto Color command is the most useful of the quick-correctors. Desaturate sucks the saturation out of a selection and leaves it looking like a grayscale image. Auto Levels and Auto Contrast automatically increase the contrast of an image according to what Photoshop deems to be ideal brightness values. Auto Levels examines each color channel independently; Auto Contrast examines the image as a whole. And Auto Color, which works only with RGB images, looks at the entire image as it attempts to automatically correct the image’s midtones and remove color casts.

Sucking saturation

The Desaturate command is mostly useful for robbing color from selected areas or from independent layers. There’s really very little reason to apply Desaturate to an entire image; you can just as easily choose Image Mode Grayscale to accomplish the same thing and dispose of the extra channels that would otherwise consume room in memory and on disk. I know of only two reasons to sacrifice all colors in the RGB mode:

You want to retain the option of applying RGB-only filters, such as Lens Flare and Lighting Effects.

You intend to downsize the colors using Image Mode Indexed Colors and save the final image in the GIF format for use on the Web.

Desaturate isn’t the only way to suck colors out of an image. You can also invert the colors and mix them with their original counterparts to achieve a slightly different effect. Just press Ctrl+I (z -I on the Mac) to invert the area you want to desaturate. Then press Ctrl+Shift+F (z -Shift-F on the Mac), change the Opacity setting to 50 percent, and — here’s the important part — select Color from the Mode pop-up menu. You can get a close approximation of converting an image to the Grayscale mode simply by creating a new layer, filling it with black, and switching the blend mode to Color. You can also use the Channel Mixer, which gives you the most exacting control over converting to grayscale. For details, see Chapter 4.

The Auto Levels command

Image Adjustments Auto Levels (Ctrl+Shift+L or z -Shift-L) goes through each color channel and changes the lightest pixel to white, changes the darkest pixel to black, and stretches all the shades of gray to fill out the spectrum. In Figure 17-8, I started with a drab and murky image. But when I applied Auto Levels, Photoshop pumped up the lights and darks, bolstering the contrast. You could probably get better results by applying Levels and tweaking by hand, but it’s not half bad for an automated, no-brainer command that you just choose and let rip.

Figure 17-8: A grayscale image before (left) and after (right) applying the Auto Levels command.

Unlike the Equalize command, which considers all color channels as a whole, Auto Levels looks at each channel independently. Therefore, the active color mode has a profound effect on Auto Levels. Like Invert, Equalize, and other automatic color mappers, Auto Levels is designed for use in the RGB mode. If you use it in CMYK, you’re more likely to achieve special effects than color correction.

The Auto Contrast command

The problem with Image Adjustments Auto Levels is that it modifies values on a channel-by-channel basis, which means it has a habit of upsetting the balance of colors in an image. Consider Color Plate 17-2, for example. The first image is severely washed out. As shown in the second image, choosing the Auto Levels command results in a bolder and more vibrant image, but it also changes the background from green to blue.

The solution is Image Adjustments Auto Contrast (Ctrl+Shift+Alt+L or z -Shift- Option-L). The Auto Contrast command adjusts the composite levels, thus preserving the colors, as in the case of the third image in Color Plate 17-2.

Which should you use when? If a low-contrast image suffers from a color cast that you want to correct, try Auto Levels. If the image is washed out but the colors are okay, try Auto Contrast. (When working on a grayscale image, the two commands work the same, so choose whichever is more convenient.) Bear in mind that neither command is perfect, so you’ll likely want to make additional Levels and Variations adjustments — or try the next command on our roster.

The Auto Color command

The Auto Color command is such a surprisingly smart and useful tool that it even manages to erode my faint disdain for any command that starts with the word “Auto.” It’s true that, depending on the image you’re attempting to correct, you might not see any difference between an application of Auto Levels and Auto Color. But on some images, like the one pictured in Color Plate 17-2, Auto Color is definitely worth a shot. Auto Levels helps the skin tones and adds a pleasing blue tint to the background, while Auto Contrast fixes the brightness but leaves the skin jaundiced. Auto Color does the best job on the skin tones, which after all represent the most important colors in a portrait of this sort.

Choose Image Adjustments Auto Color or press Ctrl+Shift+B (z -Shift-B on the Mac) to apply the Auto Color command. To get a sense of how the command works, and how it compares with Auto Levels in particular, take a look at Figure 17-9, which shows how each command affects an image on a channel-by-channel basis. (Note that Auto Color works only when applied to images in the RGB mode.)

Figure 17-9: The effects of Auto Levels (top row) and Auto Color (bottom row) on the individual channels of an RGB image. Notice that Auto Color turned the red and green channels slightly darker than did Auto Levels, but the blue channel is slightly lighter.

So what’s going on? Like Auto Levels, Auto Color corrects an image one channel at a time. The difference is, rather than set the darks to black and the lights to white, Auto Color neutralizes an image’s highlights, midtones, and shadows, thereby restoring balance to other colors in the image. Or, at least, that’s the idea. Like any automated command, it fails to achieve the desired results as often as it succeeds. But that’s okay. Because although Auto Color might occasionally fix colors when left to its default settings, the real power comes when you customize it using the Levels and Curves commands — a process I discuss in “The Levels command” section later in this chapter.