Real World Adobe® Photoshop® CS2 [Electronic resources] : Industrial-Strength Production Techniques نسخه متنی

Color Concerns

Editing grayscale is relatively straightforwardit's basically concerned with making pixels lighter or darker to improve the image appearance. Color, however, is considerably more complex. As with grayscale, the basic mechnisms let you make pixels lighter or darker, but depending on whether you do so in the composite channel or in individual color channels, the primary effect may be on tone or on color appearance.

As a result, even though the tools are the same whether you're editing grayscale or color, we use them somewhat differently depending on the image's mode. We'll point out the special concerns in context in the next section, "Hands-On Levels and Curves." But before we look at typical uses of the tools, there's one more topic we should address.

RGB or CMYK?

The debate over whether to work in RGB or CMYK has been the subject of countless magazine articles, several online flame wars, and even a book or two. Of course, if your work is destined for a film recorder, the computer screen, or videotape, then CMYK is quite irrelevant; but if you're working in the print medium, it's very important indeed.

There are still people who continue to maintain that if your work is destined for print, you should work exclusively in CMYK. When confronted, they usually give four reasons for this.

It's the only color space that matters.

RGB is meaningless.

Monitor calibration is inherently impossible.

All that matters are the CMYK dot percentages.

While all these points have some validity, we beg to differ with the philosophy as a whole. As we've noted before, when all you have is a hammer, everything starts to look like a nail. People who tell you to do everything in CMYK may have excellent traditional prepress skills and a deep understanding of process-color printing, but they just don't realize how much image information Photoshop loses during the conversion, and they probably are not comfortable working in RGB.

As a result, they convert unedited scans (or even worse, JPEGs from consumer digital cameras) to CMYK immediately, damaging the image irretrievably. Then they make huge corrections in CMYK, trying to salvage a printable image from what's left. Once they're done, they congratulate themselves on their exquisite skills while pointing out the limited quality you can obtain from tools like Photoshop.

These limits are self-imposed and largely illusory. If you follow these people's recommendations, you too will throw away about a third of your image right off the bat; then you'll be able to labor mightily, use all sorts of nifty tricks, and be rewarded with mediocre results for your efforts.

We have a very simple rule: Don't change color modes unless or until you have to, and do as much of your correction as possible in the image's original color mode. The ideal number of color-mode conversions is one or none.

All images ultimately come from an RGB source, so even if the desired end result is a set of CMYK separations, you should do as much of your image correction as possible in the RGB file. This is not a universally accepted view, but we believe that the arguments in its favor are compelling. When you prematurely convert to CMYK, you restrict yourself in at least four ways.

You lose a great deal of image information, making quality tonal and color correction much more difficult.

You optimize the image for a particular set of press conditions (paper, press, inks, and so on). If press conditions change, or if you want to print the image under various press conditions, you're in a hole that's difficult to climb out of.

You increase your file size by a third, slowing most operations by that same amount.

Levels Command Goodies," earlier in this chapter).

More important, correcting images in RGB prior to CMYK conversion just plain works.

For the vast majority of Photoshop users, that means working in RGB for as long as possible, and converting your image to CMYK only after you're finished with your other corrections and you know the printing conditions. It's useful to keep an eye on the CMYK dot percentages while you work, but you don't need to work in CMYK to do thatthey're always available in the Info palette, even when you're working in RGB.

We're not saying that you should never make corrections in CMYKfar from it. Your CMYK separations will often benefit from fine-tuning. Editing the black plate is a particularly powerful technique, but it's most effective as a fine-tuner, making small moves. Similarly, Hue/Saturation changes in CMYK are generally more delicate than in RGB. If you work in print, you must learn to edit in CMYK. But it isn't the only game in town.

When to Use CMYK

If your images come from a traditional drum scanner in CMYK form, it makes no sense to convert them to RGB for correction. You should stay in CMYK. If you find that you have to make major corrections, though, you'll almost certainly get better results by rescanning the image instead of editing it in Photoshop. In high-end prepress shops, it's not unusual to scan an image three times before the client signs off on it.

Chapter 5, Color Settings). In that case, it makes more sense to go back to the RGB original and reseparate it using new settings that get you closer to the desired result.

If all you have is a CMYK file, work in CMYK if at all possible. In dire emergencies, such as when you have a CMYK file separated for newsprint and you need to reproduce it on glossy stock, you may want to take the desperate step of converting it back to RGB, correcting, and reseparating to CMYK; but in general, treat RGB to CMYK as strictly a one-way trip.

Tip: CMYK to RGB

We can think of two reasons to convert a CMYK image to RGB: you need an image for the Web or multimedia, and a CMYK scan is all you have, or you need to repurpose the CMYK image for a larger-gamut output process. In either case, you need to expand the tonal range and color gamutif you just do a mode change from CMYK to RGB, you'll get a flat, lifeless image with washed-out color, because the tonal range and color gamut of the original were compressed in the initial RGB-to-CMYK conversion, and the mode change reproduces the compressed gamut and squashed tonal range faithfully in RGB.

Instead, use the Convert to Profile command to do the transformation, using perceptual intent, with black point compensation and dither turned on. Then, open Levels, and click Auto. If this makes the image too saturated, back the black and white point sliders off a few levels. You'll find it works surprisingly well (see Figure 6-31). That said, this is a last-resort technique. Work on a copy of the file, and watch for color shifts and posterization.

When to Use RGB

If your image comes from an RGB source, such as a desktop scanner or a digital camera, you should do as much of your work as possible in RGB. The files are smaller, so your work goes faster, and you have the entire tonal range and color gamut of the original at your disposal, allowing you to take full advantage of the small differences between pixels that you want to emphasize in the image. It's also much easier to repurpose RGB images for different kinds of output than it is to do so with CMYK.

RGB has less-obvious advantages, as well. Some features (such as the clipping display in Levels) are available only in RGB, not in CMYK. And when you work in RGB, you have a built-in safeguard: It's impossible to violate the ink limits specified by your CMYK profile or Custom CMYK settings (because they'll always be imposed when you convert to CMYK).

When you edit CMYK files directly, you have no such constraintyou can build up so much density in the shadows using Levels or Curves that you're calling for 400-percent ink coverage. On a sheetfed press, this will create a mess. On a web press, it will create a potentially life-threatening situation! In any case, it's something to avoid unless you're printing to the rare desktop color printer that can handle 400-percent total ink.