Photoshop CS Bible [Electronic resources] نسخه متنی

Working in Different Color Modes

The four sets of option boxes in the Color Picker dialog box represent color models — or, if you prefer, color modes (one less letter, no less meaning, perfect for you folks who are trying to cut down in life). Color models are different ways to define colors on both the screen and the printed page.

Outside the Color Picker dialog box, you can work in any one of these color models by choosing a command from the Image Mode submenu. In doing so, you generally change the colors in your image by dumping a few hundred, or even thousand, colors with no equivalents in the new color model. The only exception is Lab, which in theory encompasses every unique color your eyes can detect.

Rather than discuss the color models in the order in which they occur in the Mode submenu, I cover them in logical order, starting with the most common and widely accepted color model, RGB. Also, note that I don't discuss the duotone or multichannel modes now. Image Mode Duotone represents an alternative method for printing grayscale images, so it is discussed in Chapter 18. The multichannel mode, meanwhile, is not even a color model. Rather, Image Mode Multichannel enables you to separate an image into independent channels, which you then can swap around and splice back together to create special effects. For more information, see the "Using multichannel techniques" section later in this chapter.

RGB

RGB is the color model of light. RGB comprises three primary colors — red, green, and blue — each of which can vary between 256 levels of intensity (called brightness values, as discussed in previous chapters). The RGB model is also called the additive primary model because a color becomes lighter as you add higher levels of red, green, and blue light. All monitors, projection devices, and other items that transmit or filter light — including televisions, movie projectors, colored stage lights, and even stained glass — rely on the additive primary model.

Red, green, and blue light mix as follows:

Red and green: Full-intensity red and green mix to form yellow. Subtract some red to make chartreuse; subtract some green to make orange. All these colors assume a complete lack of blue.

Green and blue: Full-intensity green and blue with no red mix to form cyan. If you try hard enough, you can come up with 65,000 colors in the turquoise/ jade/sky-blue/sea-green range.

Blue and red: Full-intensity blue and red mix to form magenta. Subtract some blue to make rose; subtract some red to make purple. All these colors assume a complete lack of green.

Red, green, and blue: Full-intensity red, green, and blue mix to form white, the absolute brightest color in the visible spectrum.

No light: Low intensities of red, green, and blue plunge a color into blackness.

As far as image editing is concerned, the RGB color model is ideal for editing images on screen because it provides access to the entire range of 24-bit screen colors. Furthermore, you can save an RGB image in every file format supported by Photoshop except GIF and the two DCS formats. As shown in Table 4-1, grayscale is the only other color mode compatible with a wider range of file formats.

On the negative side, the RGB color model provides access to a wider range of colors than you can print. If you are designing an image for full-color printing, therefore, you can expect to lose many of the brightest and most vivid colors in your image. The only way to avoid any color loss whatsoever is to have a professional scan your image to CMYK and then edit it in the CMYK mode, but then you're working within a limited color range. Colors can get clipped when you apply special effects, and the editing process can be exceptionally slow. The better solution is to scan your images to RGB and edit them in the Lab mode, as explained in the upcoming "CIE's Lab" section.

HSB

Back in Photoshop 2, the Modes submenu provided access to the HSB — hue, saturation, brightness — color model, now relegated to the Color Picker dialog box and the Color palette (discussed later in this chapter). Hue is pure color, the stuff rainbows are made of, measured on a 360-degree circle. Red is located at 0 degrees, yellow at 60 degrees, green at 120 degrees, cyan at 180 degrees (midway around the circle), blue at 240 degrees, and magenta at 300 degrees. This is basically a pie-shaped version of the RGB model at full intensity.

Saturation represents the purity of the color. A zero saturation value equals gray. White, black, and any other colors you can express in a grayscale image have no saturation. Full saturation produces the purest version of a hue.

Brightness is the lightness or darkness of a color. A zero brightness value equals black. Full brightness combined with full saturation results in the most vivid version of any hue.

CMYK

In nature, our eyes perceive pigments according to the subtractive color model. Sunlight contains every visible color found on Earth. When sunlight is projected on an object, the object absorbs (subtracts) some of the light and reflects the rest. The reflected light is the color you see. For example, a fire engine is bright red because it absorbs all non-red — meaning all blue and green — from the white-light spectrum.

Pigments on a sheet of paper work the same way. You can even mix pigments to create other colors. Suppose you paint a red brushstroke, which absorbs green and blue light, over a blue brushstroke, which absorbs green and red light. You get a blackish mess with only a modicum of blue and red light left, along with a smidgen of green because the colors weren't absolutely pure.

But wait — every child knows red and blue mix to form purple. So what gives? What gives is that what you learned in elementary school is only a rude approximation of the truth. Did you ever try mixing a vivid red with a canary yellow only to produce an ugly orange-brown glop? The reason you didn't achieve the bright orange you wanted is because red starts out darker than bright orange, which means you must add a great deal of yellow before you arrive at orange. And even then, the yellow had better be an incredibly bright lemon yellow, not some deep canary yellow with a lot of red in it.

Commercial subtractive primaries

The subtractive primary colors used by commercial printers — cyan, magenta, and yellow — are for the most part very light. Cyan absorbs only red light, magenta absorbs only green light, and yellow absorbs only blue light. On their own, these colors unfortunately don't do a good job of producing dark colors. In fact, at full intensities, cyan, magenta, and yellow all mixed together don't get much beyond a muddy brown. That's where black comes in. Black helps to accentuate shadows, deepen dark colors, and, of course, print real blacks.

In case you're wondering how colors mix in the CMYK model, it's basically the opposite of the RGB model. Because pigments are not as pure as primary colors in the additive model, though, some differences exist:

Cyan and magenta: Full-intensity cyan and magenta mix to form a deep blue with a little violet. Subtract some cyan to make purple; subtract some magenta to make a dull medium blue. All these colors assume a complete lack of yellow.

Magenta and yellow: Full-intensity magenta and yellow mix to form a brilliant red. Subtract some magenta to make vivid orange; subtract some yellow to make rose. All these colors assume a complete lack of cyan.

Yellow and cyan: Full-intensity yellow and cyan mix to form a bright green with a hint of blue. Subtract some yellow to make a deep teal; subtract some cyan to make chartreuse. All these colors assume a complete lack of magenta.

Cyan, magenta, and yellow: Full-intensity cyan, magenta, and yellow mix to form a muddy brown.

Black: Black pigmentation added to any other pigment darkens the color.

No pigment: No pigmentation results in white (assuming white is the color of the paper).

Editing in CMYK

If you're used to editing RGB images, editing in the CMYK mode can require some new approaches, especially when editing individual color channels. When you view a single color channel in the RGB mode (as discussed later in this chapter), white indicates high-intensity color, and black indicates low-intensity color. It's the opposite in CMYK. When you view an individual color channel, black means high-intensity color, and white means low-intensity color.

This doesn't mean RGB and CMYK color channels look like inverted versions of each other. In fact, because the color theory is inverted, they look much the same. But if you're trying to achieve the full-intensity colors mentioned in the preceding section, you should apply black to the individual color channels, not white as you would in the RGB mode.

Should I edit in CMYK?

RGB doesn't accurately represent the colors you get when you print an image because the RGB color space contains many colors — particularly very bright colors — that CMYK can't touch. This is why when you switch from RGB to CMYK, the colors appear duller. (For those familiar with painting, RGB is like oils and CMYK is like acrylics. The latter lacks the depth of color provided by the former.)

For this reason, many folks advocate working exclusively in the CMYK mode, but I do not. Although working in CMYK eliminates color disappointments, it is also much slower because Photoshop has to convert CMYK values to your RGB screen on-the-fly.

Furthermore, your scanner and monitor are RGB devices. No matter how you work, a translation from RGB to CMYK color space must occur at some time. If you pay the extra bucks to purchase a commercial drum scan, for example, you simply make the translation at the beginning of the process — Scitex has no option but to use RGB sensors internally — rather than at the end. In fact, nearly every color device on Earth is RGB except the printer.

You should wait to convert to the CMYK mode until right before you print. After your artwork is finalized, choose Image Mode CMYK Color and make whatever edits you deem necessary. For example, you might want to introduce a few color corrections, apply some sharpening, and even retouch a few details by hand. Photoshop applies your changes more slowly in the CMYK mode, but at least you're slowed down only at the end of the job, not throughout the entire process.

Previewing the CMYK color space

While you're editing in RGB mode, you can soft proof your image — display a rough approximation of what the image will look like when converted to CMYK and printed. To display colors in the CMYK color space, choose View Proof Colors. You also can press Ctrl+Y (z -Y on the Mac).

But before you do either, select the output you want to preview from the View Proof Setup submenu. Photoshop creates the proof display based on your selection. You can preview the image using the current CMYK working space, choose Custom to specify a particular output device, or preview the individual cyan, magenta, yellow, and black plates. The plates appear as grayscale images unless you colorize them by selecting the Color Channels in Color option in the Display & Cursors panel of the Preferences dialog box (that's Ctrl+K, Ctrl+3 on the PC and z -K, z -3 on the Mac). If you work with an older model color inkjet printer that prints using just cyan, magenta, and yellow, you can choose the Working CMY Plates option to see what your image will look like when printed without black ink.

View Gamut Warning (Ctrl+Shift+Y on the PC or z -Shift-Y on the Mac) is a companion to Photoshop's CMYK preview commands that covers so-called out-of-gamut colors — RGB colors with no CMYK equivalents — with gray. I find this command less useful because it demonstrates a problem without suggesting a solution. You can desaturate the grayed colors with the sponge tool (which I explain in Chapter 5), but this accomplishes little that Photoshop won't do automatically. A CMYK preview is much more serviceable and representative of the final CMYK image.

CIE's Lab

RGB isn't the only mode that responds quickly and provides a bountiful range of colors. Photoshop's Lab color space comprises all the colors from RGB and CMYK and is every bit as fast as RGB. Many high-end users prefer to work in this mode, and I certainly advocate this if you're brave enough.

Whereas the RGB mode is the color model of your luminescent computer screen and the CMYK mode is the color model of the reflective page, Lab is independent of light or pigment. Perhaps you've already heard the bit about how, in 1931, an international color organization called the Commission Internationale d'Eclairage (CIE) developed a color model that, in theory, contains every single color the human eye can see. (Gnats, iguanas, fruit bats, go find your own color models; humans, you have CIE. Mutants and aliens — maybe CIE, maybe not, too early to tell.) Then, in 1976, the CIE came up with two additional color systems. One of those systems was Lab, and the other was shrouded in secrecy. Well, at least I don't know what the other one was. Probably something that measures how, when using flash photography, the entire visible spectrum of color can bounce off your retina and come out looking the exact shade of red one normally associates with lab (not Lab) rabbits. But this is just a guess.

The beauty of the Lab color model is it fills in gaps in both the RGB and CMYK models. RGB, for example, provides an overabundance of colors in the blue-to-green range but is stingy on yellows, oranges, and other colors in the green-to-red range. Meanwhile, the colors missing from CMYK are as numerous as the holes in the Albert Hall. Lab gets everything right.

Understanding Lab anatomy

The Lab mode features three color channels, one for luminosity and two others for color ranges, known simply by the initials a and b. (The Greeks would have called them alpha and beta, if that's any help.) Upon hearing luminosity, you might think, "Ah, just like HSL." Well, to make things confusing, Lab's luminosity is like HSB's brightness. White indicates full-intensity color.

Meanwhile, the a channel contains colors ranging from deep green (low-brightness values) to gray (medium-brightness values) to vivid pink (high-brightness values). The b channel ranges from bright blue (low-brightness values) to gray to burnt yellow (high-brightness values). As in the RGB model, these colors mix together to produce lighter colors. Only the brightness values in the luminosity channel darken the colors. So you can think of Lab as a two-channel RGB with brightness thrown on top.

To get a glimpse of how it works, try the following simple experiment.

STEPS: Testing Out the Lab Mode

Create a new image in the Lab mode — say, 300300 pixels, setting the Background Contents option to White.

Press D to return the default colors to the toolbox. The foreground color is now black and the background color is white.

Press Ctrl+2 (z -2 on the Mac). This takes you to the a channel.

Click the gradient tool in the toolbox. In the Options bar, select the Foreground to Background option from the gradient pop-up menu, select the Linear gradient style, and select Normal from the Mode pop-up menu. (See Chapter 6 if you need help using these controls in the Options bar.)

Shift-drag with the gradient tool from the top to the bottom of the window. This creates a vertical black-to-white gradation.

Press Ctrl+3 (z -3 on the Mac). This takes you to the b channel.

Shift-drag from left to right with the gradient tool. Photoshop paints a horizontal gradation.

Press Ctrl+tilde (~) (z -tilde on the Mac) to return to the composite display. Now you can see all channels at once. If you're using a 24-bit monitor, you should be looking at a window filled with an incredible array of super bright colors. In theory, these are the brightest shades of all the colors you can see. In practice, however, the colors are limited by the display capabilities of your RGB monitor.

Using Lab

Because the Lab mode is device independent, you can use it to edit any image. Editing in the Lab mode is as fast as editing in the RGB mode and several times faster than editing in the CMYK mode. If you plan on printing your image to color separations, you may want to experiment with using the Lab mode instead of RGB, because Lab ensures no colors are altered when you convert the image to CMYK, except to change colors that fall outside the CMYK range. In fact, any time you convert an image from RGB to CMYK, Photoshop automatically converts the image to the Lab mode as an intermediate step.

Indexed Color

Choose Image Mode Indexed Color to display the dialog box shown in Figure 4-5. This command permits you to strip an image of all but its most essential colors, a necessary step when saving GIF images and other graphics for display on the Web. Photoshop then generates a color look-up table (LUT), which describes the few remaining colors in the image. The LUT serves as an index, which is why the process is called indexing.

Figure 4-5: Use the Palette option to select the kinds of colors that remain in the image. Use the Colors option to specify how many colors remain.

For some reason, Photoshop doesn't let you apply the Indexed Color command to Lab or CMYK images. And although you can apply Indexed Color to a grayscale image, you don't get any control over the indexing process; Photoshop doesn't let you reduce the image to fewer than 256 colors, for example. So if you want to index a Lab or CMYK image or custom-prepare a grayscale image, choose Image Mode RGB to convert the image to the RGB mode and then choose Image Mode Indexed Color.

Now that I have all the warnings and special advice out of the way, the following list provides a brief rundown of the options inside the Indexed Color dialog box, along with some recommended settings for Web graphics:

Palette: This pop-up menu tells Photoshop how to compute the colors in the look-up table. You have lots of options here, but only a handful are really useful. If your image already contains fewer than 256 colors, the Exact option appears by default, in which case you should just press Enter or Return and let the command do its stuff. The Web option converts your image to the 216 so-called "Web-safe" colors. The Adaptive option selects the most frequently used colors in your image, which typically delivers the best possible results. The Perceptual and Selective options are variations on Adaptive. But where Adaptive maintains the most popular colors, Perceptual is more intelligent, sampling the colors that produce the best transitions. The Selective option tries to maintain key colors, including those in the Web-safe palette. The Adaptive, Perceptual, and Selective options each come in two flavors, Local and Master. Choose Local if you want Photoshop to consider the colors in only the current image. If you have several images open and want to create a palette based on all the images, choose Master.

Colors: You can specify the number of colors in the palette by entering a number in this option box. As you can guess, fewer colors result in smaller files. For GIF images, I generally start with 64 colors. If the image looks okay, I try going even lower.

Forced: This option enables you to lock in important colors so that they don't change. Black and White locks in black and white. Primaries protects eight colors — white, red, green, blue, cyan, magenta, yellow, and black. And Web protects the 216 colors in the Web-safe palette. If you choose Custom, you can select the colors that you want to lock in.

Transparency: If an image is set on a layer against a transparent background, selecting this check box maintains that transparency. Bear in mind, however, that transparency in a GIF file is either on or off; there are no soft transitions as in a Photoshop layer.

Matte: The Matte option works in collaboration with the Transparency check box. (If an image has no transparency — that is, all layers cover one another to create a seamless opacity — the Matte option is dimmed.) When you select Transparency, the specified Matte color fills the translucent pixels in the image. When Transparency is turned off, the Matte color fills all translucent and transparent areas.

Dither: This option controls how Photoshop mimics the colors that you asked it to remove from an image. The None setting maps each color in the image to its closest equivalent in the look-up table, pixel for pixel. This results in the harshest color transitions, but it is frequently the preferable option. Diffusion dithers colors randomly to create a naturalistic effect. Pattern dithers colors in a geometric pattern, which is altogether ugly. Noise mixes pixels throughout the image, not merely in areas of transition.

Amount: When you choose Diffusion as the dithering mode, you can modify the amount of dithering by raising or lowering this value. Lower values produce harsher color transitions but decrease the file size. It's a trade-off. Keep an eye on the image window to see how low you can go.

Preserve Exact Colors: This check box is available only when the Diffusion option is selected from the Dither pop-up menu. When turned on, this option turns off dithering inside areas of flat color that exactly match a color in the active palette. As I mentioned before, you may often get better looking images if you apply no dithering. But if you decide to dither, turn Preserve Exact Colors on. Even if you can't see a difference on your screen, it may show up on another screen.

Grayscale

Grayscale is possibly my favorite color mode. Grayscale frees you from all the hassles and possible expense of working with color and provides access to every bit of Photoshop's power and functionality. Anyone who says you can't do as much with grayscale as you can with color missed out on Citizen Kane, Grapes of Wrath, Manhattan, and Raging Bull. You can print grayscale images to any laser printer, reproduce them in any publication, and edit them on nearly any machine. Besides, they look great, they remind you of old movies, and they make a hefty book such as this one affordable. What could be better?

Other than extolling its virtues, however, there isn't a whole lot to say about grayscale. You can convert an image to the grayscale mode regardless of its current mode, and you can convert from grayscale to any other mode just as easily. In fact, choosing Image

Search your channels before converting

When you convert an image from one of the color modes to the grayscale mode, Photoshop normally weights the values of each color channel in a way that retains the apparent brightness of the overall image. For example, when you convert an image from RGB, Photoshop weights red more heavily than blue when computing dark values. This is because red is a darker-looking color than blue (much as that might seem contrary to popular belief).

So before switching to the grayscale mode, be sure to look at the individual color channels — particularly the red and green channels (the blue channel frequently contains substandard detail) — to see how each channel might look on its own. To browse the channels, use the following shortcuts (on the Mac, substitute the z key for Ctrl): Press Ctrl+1 for red, Ctrl+2 for green, and Ctrl+3 for blue. Or Ctrl+1 for cyan, Ctrl+2 for magenta, Ctrl+3 for yellow, and Ctrl+4 for black. Or even Ctrl+1 for luminosity, Ctrl+2 for a, and Ctrl+3 for b. I describe color channels in more detail later in this chapter.

16 bits per channel

The potential number of colors an image can contain depends on the image's bit depth. A pixel 2 bits long can be one of four colors, and each additional bit doubles the number of colors available to the pixel. A typical RGB image contains 8 bits per channel, or a total of 24 bits (3 channels 8 bits = 24 bits), which translates to 2²⁴ = 16.8 million colors. You can increase the bit depth of an image by choosing Image Mode 16 Bits/Channel to convert the image to 16-bit mode, which gives you 2⁴⁸ = 281.5 trillion colors.

But Photoshop's 16-bit mode has its drawbacks. Most significantly, when you double the bit depth of an image, you double the size of the file in memory, which can make for some very large and unwieldy files. You have a limited choice of file formats, including TIFF and PSD but not JPEG, and most of the commands on the Filter menu are unavailable to images in 16-bit mode. Also note that the 16 Bits/Channel command is applicable to RGB, CMYK, Lab, and grayscale images but not indexed or black-and-white files.

Black-and-white (bitmap)

Choose Image Mode Bitmap to convert a flattened grayscale image to exclusively black-and-white pixels. This may sound like a boring option, but it can prove useful for gaining complete control over the printing of grayscale images. After all, output devices, such as laser printers and imagesetters, render grayscale images as a series of tiny dots. Using the Bitmap command, you can specify the size, shape, and angle of those dots.

When you choose Image Mode Bitmap, Photoshop displays the Bitmap dialog box, shown in Figure 4-6. Here you specify the resolution of the black-and-white image and select a conversion process. The options work as follows:

Figure 4-6: The Bitmap dialog box converts images from grayscale to black and white.

Output: Specify the resolution of the black-and-white file. If you want control over every single pixel available to your printer, raise this value to match your printer's resolution. As a general rule, try setting the Output value somewhere between 200 to 250 percent of the Input value.

50% Threshold: Select this option from the Use pop-up menu to change every pixel that is darker than 50 percent gray to black and every pixel that is 50 percent gray or lighter to white. Unless you are working toward some special effect — for example, overlaying a black-and-white version of an image over the original grayscale image — this option most likely isn't for you. (And if you're working toward a special effect, Image Adjustments Threshold is the better alternative.)

Pattern Dither: To dither pixels is to mix them up to emulate different colors. In this case, Photoshop mixes up black and white pixels to produce shades of gray. The Pattern Dither option (in the Use pop-up menu) dithers an image using a geometric pattern. Unfortunately, the results are pretty ugly, as demonstrated in the top example in Figure 4-7. And the space between dots has a tendency to fill in, especially when you output to a laser printer.

Figure 4-7: The results of selecting the Pattern Dither option (top) and the much more acceptable Diffusion Dither option (bottom).

Diffusion Dither: Select this option from the Use pop-up menu to create a mezzotint-like effect, as demonstrated in the bottom example in Chapter 17) before selecting this option.

Halftone Screen: When you select this option from the Use pop-up menu and press Enter or Return, Photoshop displays the dialog box shown in Figure 4-8. These options enable you to apply a dot pattern to the image, as demonstrated in Figure 4-9. Enter the number of dots per inch in the Frequency option box and the angle of the dots in the Angle option box. Then select a dot shape from the Shape pop-up menu. Figure 4-9 shows examples of four shapes, each with a frequency of 24 lines per inch.

Figure 4-8: This dialog box appears when you select the Halftone Screen option in the Bitmap dialog box.

Figure 4-9: Four random examples of halftone cell shapes. In all cases, the Frequency value was set to 24.

Custom Pattern: To use a custom dither pattern, choose this option from the Use pop-up menu and open the Custom Pattern palette, as shown in Figure 4-6. The palette includes a number of predefined patterns that ship with Photoshop as well as any custom preset patterns that you may have defined using Edit Define Pattern. Simply click the icon for the pattern you want to use. Figure 4-10 shows two examples of predefined patterns (Metal Landscape and Herringbone 2) used as custom halftoning patterns.

Figure 4-10: Two examples of employing repeating patterns as custom halftoning patterns.