When are 32 Bits Actually 24 Bits? and When is 32 more than 32?

isdocumentiscreatedwithtrial ^retonof^l^C^Not2.16.100.,^ a 48-bit (16 bits x 3 channels)representation. Photoshop 2.0 added an extra dimensionthat of high-dynamic range color. HDR color also uses 16-bits of information to store information about each channel. However, there is a difference, one that can be confusing until you wrap your mind around it. Those 16 bits of information are stored as floating decimal point numbers, which are inherently a lot more accurate. If it's been as long for you as it has for me since math class, the easiest way to understand the difference is to think of expressing the idea of one-third only with integers (33 percent) or with a floating point number like 33.3333333333333333 percent. Less information is lost to rounding errors, etc. HDR uses 32-point floating point numbers to help preserve the dynamic range of your color images. You'll learn more about this later in the chapter.

For most applications, 16-bit color is as good as 24-bit color. Image editing with Photoshop CS2 is not one of them. It actually has robust high dynamic range (HDR) capabilities that extend even beyond 24 bit color. So, 24bit color is, at best, the minimum you should work with. Happily, the standard today is that video cards generally have 64MB or more of memory, and are fully capable of displaying 24-bit full color at any supported resolution, and 16.8 million different hues. Scanners and some high-end digital cameras can even capture 36 bits or 48 bits of color, for a staggering billions and billions of hues. The extra colors are useful to provide detail in the darkest areas of an image, especially when you consider that many bits of information are lost during the conversion from an analog signal (the captured light) to digital (the image file stored on your computer).

Subtractive Color

There is a second way of producing color that is familiar to computer usersone that is put to work whenever we output our Photoshop images as hard copies using a color printer. This kind of color also has a color model that represents a particular color gamut. The reason a different kind of color model is necessary is simple: When we represent colors in hardcopy form, the light source we view by comes not from the image itself, as it does with a computer display. Instead, hard copies are viewed by light that strikes the paper or other substrate and then is filtered by the image on the paper, then reflected back to our eyes.

This light starts out with (more or less) equal quantities of red, green, and blue light and looks white to our eyes. The pigments the light passes through before bouncing off the substrate absorb part of this light, subtracting it from the spectrum. The components of light that remain reach our eyes and are interpreted as color. Because various parts of the illumination are subtracted from white to produce color, this color model is known as the subtractive system.

The three primary subtractive colors are cyan, magenta, and yellow, and the model is sometimes known as the CMY model. Usually, however, black is included in the mix, for reasons that will become clear shortly. When black is added, this color system becomes the CMYK model (black is represented by its terminal character, k, rather than b to avoid confusion with the additive primary blue). Figure 6.4 shows the subtractive color model in a fanciful representation, retaining the color filter motif I started out with in describing the additive color system. (You couldn't overlap filters to produce the colors shown, although you could print with inks to create them.)

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