Light, like you get from the sun or from a flashlight, can be broken down into three colors — red, green, and blue. These colors are referred to as the primary colors of light. They are also the colors that correspond to the three types of receptors, or cones, inside your eyes. Each cone, red, green, or blue, in your eyes senses the amount of red, green, and blue light, respectively. When that information reaches your brain, it is translated into millions of different colors. I mean millions literally. And I don't just mean a couple million, either. When you mix just these three colors, you can produce up to 16.7 million colors.
If you have ever visited a children's museum or science center, they always have a display where the kids can shine three spotlights, red, green, and blue into the same spot. They are amazed to find that the resulting color is white. Turn off all the lights, and the color is black. Move the light up or down and create blue, green, orange, and every color in between. By moving the light, the intensity of the light is being varied. In the digital world, we call the intensity of the light brightness value. Each color — red, green, and blue — contains 256 (from 0 to 255) levels of brightness. These brightness values are contained in what we call channels.
Think of channels as holding tanks of color information. Getting down to the nitty-gritty, each individual pixel in the channel in your image is assigned its own brightness value ranging from 0 (black) to 255 (white). When you view them separately, as shown in Figure 3-5, each channel appears as a grayscale (black and white) image. But when you view the pixels from each channel together, the pixels mix to form colors, which, in turn, provide you with a composite, or full-color image.
^jfjAMW A value of 255 from all three channels produces white while a value of 0
from all three channels produces black. Different values produce all the other colors. Because RGB colors combined at 255 create white, they are known as additive colors. If your digital image is comprised of these three colors, it is called, not surprisingly, an RGB image.
Digital cameras, scanners, computer monitors, and even television screens all display colors in RGB. Many desktop inkjet printers, such as Epson, also print RGB images beautifully. RGB is the primary color mode (with Indexed Color being secondary) to use on any images to be viewed on-screen, whether on the Web, or in any kind of multimedia presentation such as slide shows and video CDs.
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