SAMS Teach Yourself Adobe® Photoshop® CS2 in 24 Hours [Electronic resources] نسخه متنی

The Modes and Models of Color

First, forget about CIE Lab color unless you already have a background in color theory. It's there. Photoshop uses it in the background, but you needn't concern yourself with it. The other three modelsHSB, RGB, and CMYKwill have much greater impact on your work in Photoshop. The difference between the modes and the models is simple. The

models are methods of defining color.

Modes are methods of working with color based on the models. HSB is the only model without a directly corresponding mode. CMYK and RGB have corresponding modes in Photoshop. There are also modes for black-and-white, grayscale, and limited color work.

The Photoshop modes available under ImageMode are as follows:

Bitmap
Grayscale
Duotone
Indexed Color
RGB Color
CMYK Color
Lab Color
Multichannel

There are only four of these color modes that you'll use often: Grayscale, RGB, CMYK, and Indexed Color. Let's take a closer look at them.

Bitmap and Grayscale

Let's start with the most basic of the color modes available within PhotoshopBitmap and Grayscale.

The Grayscale mode offers 256 shades of gray that range from white to black, whereas the Bitmap mode uses only two color values to display imagesblack and white (see Figures 5.4 and 5.5 for examples).

Figure 5.4. A photo rendered in the Grayscale mode.

Figure 5.5. The same image in the Bitmap mode.

Notice the vast difference in quality. The Grayscale image has a smooth transition between values, whereas the Bitmap image does not. There are, however, a number of ways to convert to Bitmap mode, discussed later in this hour.

Whenever a picture is printed in black-and-white or grayscalefor instance, as part of a newsletter or brochureit makes sense for you to work on it in Grayscale mode. Doing the conversion yourself, rather than sending a color photo to the printer, gives you the opportunity to make sure that the picture will print properly. You can tell by looking at it whether the darks need to be lightened or the light grays intensified to bring out more detail. You can adjust the overall level of contrast as well as work on individual trouble spots.

To convert a color photo to Grayscale, simply choose ImageModeGrayscale. You'll be asked for permission to discard the color information. Click OK to confirm, and the picture is converted to grays. To convert the picture to Bitmap mode, as you might want to for certain effects, you must first convert to grayscale and then to bitmap.

RGB

RGB is the color mode for working on pictures that will be viewed on a computer screen. If you are preparing pictures in Photoshop that will eventually become part of a desktop presentation, a video, or a web page, stick with RGB for the best color rendition. If your work is only going on the Web, I still recommend doing the color adjustments in RGB and then converting the picture to Indexed Color if you choose to save it as a GIF in its final form. Also, if you work in Indexed Color, you can't use Photoshop's filters or layers. That's too much of a limitation!

Indexed Color

Indexed Color, when it can work for you, is a wonderful thing. Because of cross-platform compatibility issues, web designers are theoretically limited to the 216 colors shared by Macintoshes and PCs. Indexed Color is a palette or, rather, a collection of colors256 to be exact. With this mode, you know exactly what you are getting, and if you don't like any of the palettes Photoshop supplies, you can build your own. Many web designers stick to indexed palettes to ensure consistent color. Others use any colors they want, knowing that most users don't calibrate their monitors anyway.

Indexed Color is perfect for the World Wide Web. The Indexed Color mode includes a specific Web palette. Indexed Color doesn't really limit you to 216 colors, however. Dithering takes place in Indexed Color images. From RGB mode, choose ImageModeIndexed Color to take a look at the Indexed Color dialog box (see Figure 5.6).

Figure 5.6. The Indexed Color dialog box.

Dithering means that certain colors are combined, that is, adjacent pixels are interspersed, visually blending onscreen to create a new color although they retain their original coloror the closest index equivalentwhen viewed at a large magnification.

You are given a number of palette choices when you work with Indexed Color. They are as follows:

ExactThis option takes the colors that are in the RGB version of the image for its palette. This works only if there are fewer than 256 colors in the original image.
System (Mac OS)This option uses the Macintosh System palette.
System (Windows)This option uses the Windows System palette.
WebThis palette uses the 216 colors discussed previously. If you are planning to publish your work on the World Wide Web, this is the "safe" palette. Otherwise, you might have problems with incompatible colors dropping out when an image is viewed with a web browser.
UniformThe Uniform option bases the colors in the palette on a strict sampling of colors across the color spectrum.
PerceptualThis option creates a custom palette by giving priority to colors for which the human eye has greater sensitivity. You can use a local palette (based on the current image) or, if you switch to ImageReady, a master palette that draws colors from a group of images you plan to display on a website or CD-ROM.
SelectiveThe Selective option creates a color table similar to the Perceptual color table, but favoring broad areas of color and the preservation of Web colors. Again, you can choose a local palette or a master palette.
AdaptiveThis is your best bet for most work in Indexed Color. During conversion, this option samples the most frequently used colors from the original. Adaptive usually provides you with the closest match to the original image. This option also comes in both local and master flavors.
CustomIf none of the other options suits you, you can always build your own palette. See the Photoshop manual for instructions.
PreviousThis option simply remembers and reverts to whichever option you chose last time you converted to Indexed Color.

CMYK

As you saw earlier, CMYK mode should be used only when your image is printed commercially. By converting to CMYK before you start to print (and being aware of gamut warnings), you can make sure that your nice yellow banana or flower doesn't end up a muddy brown, or your bright blue sky doesn't print as purple.

Gamut refers to the range of colors that the combination of CMYK inks can print. Some colors are

out of gamut , and can't be printed accurately. Very bright colors, particularly oranges and greens, are often out of gamut, and would trigger the gamut warning. The

gamut warning shows up on the Color palette and looks like a small triangular traffic warning sign with an exclamation point in the middle.

Converting Between Modes

All you have to do to convert between modes, at least mechanically (this is not taking image degradation or changes into account), is choose ImageMode and then choose your poison.

Although Photoshop uses the model (Lab) with the broadest gamut of color to change color modes (as if all the other modes are circles that will fit within Lab color), this is no guarantee that your colors will turn out the same in another mode as they did in the original mode.

The rule of thumb is this, and I can't stress it enough

Do your work in

RGB , even if you are going to output your images to print. Convert a copy of your image to CMYK immediately before you send it to the commercial print shop. To see whether all your colors are within the CMYK gamut, use the menu command ViewGamut Warning. If you are going to publish your images on the Web, stick with RGB or use Indexed Color, if file size is an issue. Knowing this will save you many hours of wondering why the Web page that looks great on the office Macintosh looks funky on the Windows machine you use at home, or why the yellow in your printed piece looks brownish.

Try it Yourself

Getting Started with Color

Just for fun, why don't we dive in with some hands-on before we go any further? Because the pictures in the book are in black-and-white, working through this exercise will give you a better idea of the concepts and ideas that we have been talking about. Let's look at a colorful image and examine how the modes affect the way the color appears.

First, find a colorful picture and open it. You can download the photo in Figure 5.7 from the Sams website. It's called YellowLily. To get to the website, point your Web browser to http://www.samspublishing.com/. In the Search box, type

Photoshop in 24 . Find this book in the list that appears, and click the link. On the book's main page, find and click the Related Materials link to get to the files. If your picture doesn't have the letters RGB in parentheses after the filename shown in the image window's title bar, choose ImageModeRGB Color. This is your starting point. If your monitor is correctly adjusted, you should see very good color.

Figure 5.7. Yellow lily.

Choose ImageModeGrayscale. A dialog box appears, asking whether you want to discard the image's color information. Click OK. Photoshop then proceeds to examine your image and assigns all the colors to 256 shades of gray that range from white to black.

You can monitor, using the status bar at the bottom of the picture, how the size of your file diminishes. (If you don't see the file size at the bottom of the document window, choose ShowDocument Sizes from the pop-up menu.) This is because the amount of information or data in a color image is much greater than that required to display a grayscale image. In this case, the file size decreased by more than 1MB.

Before moving on, you need to return the image to its original RGB state. Choose FileRevert.

This time, you're going to change the RGB image to CMYK. This process becomes enormously important if you'll be taking your images to a commercial printer. RGB can display a number of colors that CMYK, by the nature of its four inks, cannot reproduce. The inks, for instance, can only approximate neon colors.

Before making the mode change, let's take a closer look at some of the colors in this RGB image to see whether they can be reproduced in CMYK (see Figure 5.8).

Figure 5.8. The triangle symbol means that the color is out of the CMYK gamut.

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A. Click the Eyedropper tool in the toolbox.

B. Next, open the Color palette by choosing Window

Color.

C. Use the Eyedropper to select (click) a color in the image. Try clicking the orange stamens in the center of the lily.

D. Look in the Color palette. Is there an out-of-gamut warning there? This little triangle indicates that the selected color cannot be reproduced precisely by the process colors of CMYK.

E. To get an idea how far out of gamut your colors are, choose View

Gamut Warning. This gives you an indication of the colors that will be lost or modified during the translation of RGB mode to CMYK. Figure 5.9 shows what the gamut warning looks like for this picture. Out-of-gamut areas are shown as gray patches.

Figure 5.9. The dark patches are out of gamut.

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F. Click the warning triangle to select the nearest color that can be achieved with CMYK colors. You can quickly adjust a picture like this one that has many out-of-gamut colors by activating the gamut warning. You can then use the color adjustment tools you'll learn about in the next hour to bring the picture into a printable range.

To change the mode from RGB to CMYK, choose ImageModeCMYK.

After you've seen and perhaps printed the picture in CMYK mode, feel free to experiment with the other modes, too.

Hard to See the Gamut Warning?

To change the color used in the display of the Gamut Warning, open the preferences (choose PhotoshopPreferences on Mac and EditPreferences on Windows) and turn to the Transparency & Gamut prefs. Click the color swatch at the bottom of the dialog box and choose a color that contrasts with the colors in the picture.Hour 23, "Printing and Publishing." If your monitor seems to need calibration, you can jump ahead to "What's Color Management" on page 441.

Color Is Critical

The human eye is extremely sensitive to even the slightest variation in color. Think for a moment about something familiara can of Coca-Cola. I'll bet that if you were shown two swatches of red you could, without much hesitation, select the Coke can's red and differentiate it from, say, the red used on the cover of Time magazine. If you saw cans of Coke displayed with a slightly off-color red, you'd probably think they were either outdated or perhaps counterfeit. Most people are very much aware of even slight color changes. That is why color becomes so important in product branding through advertising.