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10.1 HOW COLOR IMAGES ARE PRINTED | ||
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Color printers create images by dividing a page into thousands, or even millions, of tiny dots, each of which can be addressed by the computer. As the printer moves across and down the page, it can print a dot of color, print two or three colors on top of each other, or leave the spot blank (white). To understand digital printing, you need to know a little about the colors that are used and the patterns in which they are printed.
As you've seen, color displays use three colors, red, green, and blue (RGB) to create color images on the screen. This process is referred to as additive color because adding all three colors together forms white. Color printers use a different process, called subtractive color. This process uses three subtractive primaries—cyan, magenta and yellow. When two of these are overprinted, they form red, green, or blue. When all three are overprinted, they form black. Most printers include a separate black color to provide a deeper black than that formed by combining the primaries. This is useful, not only for richer blacks in photographs, but also when printing text. These four colors give the color system its name—CMYK (C for cyan, M for magenta, Y for yellow, and K for black). If your browser has a Shockwave plug-in, you can assemble a full-color image from the three basic colors at Konica's site in Japan.
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When you use cyan, magenta, and yellow inks or pigments, you create subtractive colors. |
By leaving a spot blank, or by using one or more of the three subtractive primaries on a single dot, the printer can create eight primary colors as follows:
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On most printers (dye-sub is an exception), each printed dot has the same density of color. If the printer only combined these solid colors, it would be limited to the eight primary colors describe above. To get the millions of colors in a photograph, the printer has to "fake" it by generating a pattern of small dots that the eye blends to form the desired shade. This process is called halftoning or dithering and designing printer software that does it well is as much art as it is science. As a result, printers vary widely in the methods used and the results obtained. One sign of halftoning being well done is when a smooth gradation of color in the original looks smooth in the print. If the process isn't well done, these smooth transitions will be made up of distinct bands of color and may also include moiré or doily patterns. Halftoning is done by arranging printable dots into grid-like groups, called cells, and then using these larger dots as a single unit to print pixels with. Each cell may be 5 by 5 or 8 by 8 dots in size. The three or four primary colors are printed combined in a pattern of dots in these cells, and the eye perceives them as intermediate hues. For example, to print purple the printer uses a combination of magenta and cyan dots. For less saturated hues, the printer leaves some dots unprinted and hence white in color.
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The printed forms on the left in each picture are actually made up of square pixels as shown in the right-hand objects. Courtesy of Tektronix |
Halftoning has long been used in the conventional printing industry and you can see it by looking at a magazine photo with a magnifying glass. It is also embedded in page description languages such as Adobe's PostScript Level 2. However, printer manufactures try to improve on these standards with their own proprietary systems that are better matched to their printers.
Color gamut refers to the range of colors that can be reproduced by any device. Unfortunately for photographer's mother nature's color gamut is a lot larger than any we can reproduce with light, inks, dyes, or pigments. The best we can do is get as close to the original scene as possible. Your first experience with the limitations of color gamut is if you shoot both slide and print film. The slides are always richer and brighter than the prints because slide film has a larger color gamut.
One way to think of color gamut is to imagine that you have a set of dull inks. You're asked to use them to print bright colors. It can't be done because the color gamut of the inks is smaller than the color gamut you're asked to create. On the other hand, if you have a set of bright inks, you can reproduce the colors of the dull inks because they fall within the color gamut of the inks you're using.
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Dull inks with a small gamut only let you create dull colors. |
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Bright inks with a large gamut allow you to create both dull and bright colors. |
When reproducing colors on the screen or printed page, we use what are called color models. One model, called Lab, has the largest gamut. Within the color model can be found all of the colors of the two most popular color models: RGB and CMYK. The RGB gamut includes only those colors that can be displayed on a computer screen. Some colors, such as pure cyan or pure yellow, can’t be displayed accurately on a monitor. The CMYK model, used for printing, has the smallest gamut. When colors in an image can not be displayed or printed because they aren't in a device's gamut, they are called out-of-gamut colors.
Until recently, there were no inexpensive color printers but great strides have been made over the past few years. There are now a variety of printers at a variety of price points. When you choose a printer to print photographs, it's not always the most expensive kinds that give the best looking results. Let's take a look at some of the ways printers transfer images to the page.