| 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.
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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).
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.
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.
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.
Photographic
paper | Pick
the Right Photo Paper | How
to Get the Best Prints |
Should
I Invest In A Photo Printer? | How
Color Images are Printed |
Eight
Tips For Buying A Photo Quality Printer | The
Modern Photo Printer |
What
You Need To Know When Buying A Printer |
