From the book
Image Mode
As we said earlier, a pixel can have a value of 165, but that doesn’t mean anything until you know the image’s mode. That 165 could represent a level of gray or a particular color, or it might be only one member of a set of three or four other 8-bit values. Fortunately, Photoshop makes it easy to see the image mode of a file, and you can convert it to a different mode if you want.
Ultimately, an image mode is simply a method of organizing the bits to describe a color. In a perfect world, you could say to a printer, “I’d like this box to be navy blue,” and they’d know exactly what you were talking about. However, even we can’t agree on what navy blue looks like, much less you and your printer. So color scientists created a whole mess of ways for us to describe colors—to each other and to a computer—with some precision.
Photoshop reads and writes only a handful of the many different color modes these scientists came up with. Fortunately, these modes are the most important of the bunch, at least for those in the world of graphic arts. Each of the following image modes appears on the Mode menu in Photoshop.
Bitmap
As far as Photoshop is concerned, Bitmap means one thing: an image that contains only black or white (no shades of gray)—technically, a 1-bit image. Compared to other image modes, the kinds of image editing you can do are severely limited in Bitmap mode. For instance, you can’t use any filters, and you can’t use tools such as the Smudge tool, the Blur tool, or the Dodge/Burn tool. Those features are designed to finesse shades of gray and color, which simply don’t exist in Bitmap mode.
Grayscale
Grayscale files in Photoshop can use 8, 16, or 32 bits to represent gray shades. If a file uses a different number of bits, it’s rounded up. For example, if you open a 10-bit grayscale file in Photoshop, it’s opened and saved as a 16-bit file.
In an 8-bit grayscale image, each pixel has a value from 0 (black) to 255 (white), so there are a maximum of 256 levels of gray possible. In a 16-bit grayscale image, each pixel has a value from 0 (black) to 32,768 (white), for a theoretical maximum of 32,769 possible gray shades.
Few capture devices can actually deliver all those gray shades, so 16-bit files usually have a lot of redundancy. But that redundancy translates into editing headroom, so if your camera or scanner can capture 12 or more bits per pixel, it’s often worthwhile to bring the high-bit data into Photoshop.
Duotone
When you print a grayscale image on a printing press, those 256 levels of gray often get reduced to 100 or so because of the limitations of the press. People get around this by printing the image with more than one color of ink, increasing the tonal range of the printed image. This is called printing a duotone (for two inks), a tritone (for three inks), or a quadtone (for four inks).
The extra colors aren’t typically used to simulate colors in the image; rather, they’re used to extend the dynamic range of the underlying grayscale image. Those expensive Ansel Adams books on your coffee table were very likely printed using three or four (or even five or six) different black and gray inks.
Photoshop has a special image mode for duotones, tritones, and quadtones, and even though the file may appear to be in color, each pixel is still saved using only 8 bits of information. The trick is that Photoshop saves a set of contrast curves for each ink along with the 8-bit grayscale image. Creating a good duotone is as much art as science. For more information, see our online chapter, “Spot Colors and Duotones.”
Indexed Color
Indexed Color mode is like a paint-by-numbers approach. It uses 8 bits to store a table of 256 colors, and a pixel in the image can use any of the 256 colors in the table. For example, instead of defining a pixel’s color as Red 32, Green 140, Blue 96, as it might be in RGB mode, in Indexed Color mode a pixel simply says “I’m using the 35th color in the table.” Photoshop then finds out which color is 35th and uses it for that pixel. You can completely change the look of the image by loading a different table.
While indexed color can save disk space (it requires only 8 bits per sample point), it gives your image only 256 different colors. That’s not a lot when you compare it to the millions of colors you can get in RGB. But back when video cards and network speeds were very limited, the small size of indexed-color images outweighed the lack of subtle color variations, so indexed color was popular as part of the GIF standard for Web graphics.
Most Photoshop editing tools don’t work in Indexed Color mode because it can’t store enough colors to create gradual variations in color and tone. Do your major image edits in RGB mode first.
RGB
Every color computer monitor and television in the world displays color using the RGB image mode, in which every color is produced with varying amounts of red, green, and blue light. (These colors are called additive primaries because the more red, green, or blue light you add, the closer to white you get.) In Photoshop, files saved in RGB mode typically use a set of three 8-bit grayscale files, so we say that RGB files are 24-bit files. Digital cameras and scanners capture images in RGB format, and we prefer to work in RGB mode when editing color images.
CMYK
Traditional full-color printing presses reproduce colors using just four inks: cyan, magenta, yellow, and black, simulating all other colors using various combinations of those inks. When you see the image on your RGB monitor, Photoshop converts the CMYK values to RGB values on the fly for display.
If you reuse images from old press runs, they may be CMYK files, because drum scanning to a CMYK file used to be the most common way to obtain a color image for use in prepress. These days, you’re more likely to start out with an RGB file from a digital camera or desktop scanner, so for many prepress jobs you’ll have to convert those RGB images to CMYK. We discuss Photoshop’s tools for doing so in Chapter 4, “Color Settings.”
Lab
The problem with RGB and CMYK modes is that a given RGB or CMYK specification doesn’t really describe a color. Rather, it’s a set of instructions that a specific output device uses to produce a color. The problem is that different devices produce different colors from the same RGB or CMYK specifications. If you’ve ever seen a wall full of television screens at a department store, you’ve seen what we’re talking about: The same image—with the same RGB values—looks different on each screen. Similarly, if you’ve ever sat through a printing-press run, you know that the 50th impression probably isn’t exactly the same color as the 5000th or the 50,000th. So while a pixel in a scanned image may have a particular RGB or CMYK value, you can’t tell what that color really looks like. RGB and CMYK are both device-specific color modes.
However, a class of device-independent or perceptually based modes has been developed over the years. All of them are based, more or less, on a color space defined by the Commission Internationale de l’Éclairage (CIE) in 1931. The Lab mode in Photoshop is one such derivative.
A file saved in Lab mode describes what a color looks like under rigidly specified conditions; it’s up to you (or Photoshop, or your color management software) to decide what RGB or CMYK values are needed to create that color on your chosen output device.
Lab doesn’t describe a color by the components that make it up (RGB or CMYK, for instance). Instead, it describes what a color looks like. Device-independent color spaces are at the heart of the various color management systems now available that improve color correspondence between your screen, color printouts, and final printed output. Photoshop uses Lab mode as a reference when switching between CMYK and RGB modes, taking the values in your RGB Setup and CMYK Setup dialogs into account (see Chapter 4, “Color Settings,” for more information on this conversion).
Lab is considerably less intuitive than the other color modes. The Lightness channel is relatively easy to understand, but the a* and b* channels (pronounced “A-star” and “B-star”) are less so. The a* channel represents how red or green a color is—negative values represent greens, positive ones represent magentas—and the b* channel represents how blue or yellow the color is—negative values represent blue, positive ones represent yellow. Neutrals and near-neutrals always have values close to zero in both channels. Most hard-core Photoshop geeks have a few tricks that rely on Lab mode, but many of them can be accomplished more easily by using blending modes instead. Luminosity blending, for example, produces results that are extremely similar to those obtained when working in the Lightness channel in Lab mode.
Multichannel
The last image mode that Photoshop offers is Multichannel mode. This is a generic mode: Like RGB or CMYK, Multichannel mode has more than one 8-bit channel; however, you can set the color and name of each channel to anything you like.
Today, many scientific and astronomical images are made in “false color”—the channels may be a combination of radar, infrared, and ultraviolet, in addition to various colors of visible light. Some adventurous digital photographers use Multichannel mode to combine infrared and visible-spectrum photographs into composite images of surreal beauty.