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How Color Photographs are Printed

 
Click to explore how cyan, magenta and yellow can combine to create full color images.
 
 
 
Line art contains just solid colors.
 
 
Saturation or density controlled by varying the number of dots printed in each cell. Fewer dots look less saturated. Courtesy of Tektronix.
 
 
Shapes can be created by varying the pattern of dots in each cell. The printed forms on the left are actually made up of square pixels as shown in the right-hand objects. Courtesy of Tektronix.
Color printers create images by dividing a page into a grid containing thousands, or even millions, of tiny cells, each of which can be addressed by the computer. The printer then sprays or otherwise uses dots of color arranged in patterns to create the image.

CMYK Colors

As you've seen, color displays use three transmitted colors, red, green, and blue (RGB) to create color images on the screen. This process is referred to as additive color because adding varying amounts forms all colors. Printers use reflected light and the three colors cyan, magenta, and yellow (CMY) to create images on the printed page. This process is called subtractive color because printed colors subtract from the light hitting them so only red, green and blue are reflected to our eyes. When all three colors are overprinted at full strength, they form black-called processed black. Printers also include a separate black ink because process black tends to be too dull and muddy to print type and the rich blacks needed in photos. These four colors give the color system its name—CMYK (C for cyan, M for magenta, Y for yellow, and K for black).

As the three basic CMYK colors visually mix, they form other colors. Combining magenta and cyan creates blue (left), combining yellow and cyan creates green (middle), and combining yellow and magenta creates red (right). Note that the printer doesn't print white-that is formed by the paper color showing through the ink layer. If you on't print on white paper, your print won't have any white in it.

Continuous Tone Printers

Photographs are continuous tone images, meaning they have smooth gradations between lighter and darker tones without any noticeable transitions from one tone to the next. These images are different from type, line drawings, or woodcuts, all of which have distinct areas of solid color without gradations. The role of a photo printer is to reproduce a photograph's continuous tones as accurately as possible. The only printer processes that can actually print continuous tones are dye-sublimation or dye-sub, thermoautochrome, zink and printers such as the Lightjet and Lambda that expose silver-halide paper. All of these processes are discussed later in this chapter.

On other printers, including inkjets, each printed dot usually has the same size and density of color. To reproduce the millions of colors found in a continuous tone photograph, these printers use a process called dithering to generate a pattern of small dots that the eye blends to form the desired color and shade. In effect dithering produces an illusion of colors that don't actually exist as separate inks. Dithering is the digital equivalent of a technique called halftoning long used in the conventional printing industry and you can see it by looking at a magazine or newspaper photo with a magnifying glass. What appear to be continuous tones at a distance turn out, when magnified, to be made up of colored dots, each of a separate solid color.

Designing printer software that prints smooth gradations 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 dithering being well done is when a smooth gradation of color in the original looks smooth in the print, and changes tones at the same rate as the original. If the process isn't well done, these smooth transitions will be shifted somewhat, made up of distinct bands of color, and may also include moiré or doily patterns.

Dithering is done by arranging printable dots into grid-like groups, called halftone cells or screen cells, and then using these larger dots as a single unit to print pixels. The dots printed in each cell control both the color and density of that cell.
  • To control color, various combinations of dots from the available colors are printed in each cell. For example, to print purple the printer prints a combination of magenta and cyan dots.
  • To control density, the printer varies the number of dots printed in a cell. The more area of a cell that's covered, the darker the cell will look. For less saturated hues, the printer leaves some dots unprinted so the color of the paper, usually white, shows through. On the latest printers dilute inks are also be used for lighter colors.

Printer Drivers

All printers are supplied with software drivers that control the conversion of RGB to CMYK data. The driver for your printer is usually available from an application's Print dialog box when you click a button, sometimes labelled Properties or Setup button. Using this dialog box you specify such things as the type, size and orientation of the paper, the desired output quality, and the number of copies. You also specify the output profile, often specific to a paper. Based on the setting you make, the driver determines the most efficient way to apply ink drops to the page.

The printer driver dialog box for the Epson 2400 printer.
 
Don't confuse a printer's dots per inch (dpi) with an image's pixels per inch (ppi). Each pixel is printed using many of the printer's dots. If you know your printer's dpi, you can divide it by your image's ppi to calculate how many dots are used per pixel.


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