Sign & Digital Graphics

October '14

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48 • October 2014 • S I G N & D I G I T A L G R A P H I C S DIGITAL PRINTING AND FINISHING DIGITAL GRAPHICS Figure 4: When you con- vert an image color to the Indexed mode, the color information uses a color look-up table to specify a specific palette. image is also composed of a single chan- nel, but each pixel contains eight bits of information—eight on/off switches. The combinations of the eight switches being either on or off, produces greater tonal potential. Do the math! Two to the eighth power (2 8 ) equals 256, which is the number of shades of gray displayed by an eight-bit grayscale image Each pixel is assigned a brightness level from zero (black) to 255 (white). Darker tones have lower numbers and lighter tones have higher numbers. Midtone gray has a value of 128 (half of 256). When you look at a grayscale image close up, you can see the tonal varia- tions of pixels (see Figure 3). Indexed Color Mode Indexed color mode uses a minimum of two colors to a maxi- mum of 256 colors to display full-color images. When you convert an image color to the indexed mode, the color infor- mation uses a color look-up table to spec- ify colors (see Figure 4). Converting an RGB image to index color reduces its bit depth. A specific palette can be selected to display the image to match the colors as closely as possible to the original. Because it contains fewer colors, indexed color creates smaller file sizes, which is why it is often used when pub- lishing files to the Web or to multimedia applications where it will be displayed on a monitor or mobile device. Red Green Blue (RGB) RGB is the system used by devices such as digital cameras, scanners and monitors to acquire or display color. Because RGB produces color with light (the more light that is added the brighter the color becomes) it is referred to as an additive color mode. The tonal information is passed through red, green or blue filters of the Figure 5: The color information of an RGB image is stored as three separate color channels.

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