How Can We Reproduce Color?

Colorful graphics get the attention and the professional admiration of your viewers, but producing color graphics on the computer used to be so time consuming and expensive that it was only used for professionally published work. Now that the technology has become accessible to even casual users you may find yourself expected to produce colorful handouts, slides or reports on a regular basis.

For some purposes, it is sufficient to be able to display your graphics on screen and show them informally. In a meeting, you may need to print out a few copies as handouts. Occasionally you'll need to publish hundreds or thousands of copies to distribute more widely.

COLOR DISPLAYS

No matter how you intend to show your computer graphics, you'll see them first on a computer monitor. All monitors have limitations that you should know about before you begin.

Most color computer monitors work on the same principle as a television. The screen is composed of phosphor dots that are illuminated from behind. On a color monitor, red, green and blue dots are distributed evenly. These dots are illuminated to different brightnesses to mix the different colors you see on the screen. If you look very closely, you can see these individual dots.

Most computer display systems are made up of two components: the monitor and a video adapter card that resides in the computer itself. The quality of the display is affected by both the monitor and the video card.

Besides the size of the screen, computer display systems have two primary features that determine the quality of the image: resolution and color depth. Resolution determines the fineness of detail on the screen. The color depth determines how much control you have over the coloring of your graphics.

The video card determines how many colors can be displayed by the monitor. Since the colors are created by mixing different brightness levels for each of the three color dots, a monitor can only mix as many colors as the number of brightness combinations it can make. The number of colors that can be displayed by a video card is called its color depth and is usually specified in bits per pixel.

Color depths in commercial video cards range from black and white (one bit) to over 16 million colors (24 bits). Of course, the human eye can't distinguish that many colors, so these higher-end displays are more powerful than most people need.

When designing on the desktop, your first concern is to assure that you are seeing color on your monitor as accurately as possible. PANTONE color monitor calibration tools give you the ability to set the manufacturer's standard profile for a specific brand and model of monitor, but further lets you set up and save your personal preference for red/green/blue acuity, brightness, contrast and lighting conditions.

In many cases, the number of colors in an image will exceed the capabilities of the device used to display the image. For example, it may be necessary to present a 256-color image on a 16-color display system, or to print a scanned photograph on a low-end dot-matrix printer. In situations such as these, the image is automatically simplified to reduce the number of colors. This process is referred to as color reduction.

As a typical user, you don't need to worry about doing the color reduction. That's usually done for you by the computer system or your software application. Color reduction takes its toll on the quality of your displayed images, however, and you will probably notice these effects in your work.

Color reduction typically uses a technique called dithering. In the same way that the monitor simulates individual colors with its red, green and blue dots, even more colors can be simulated by arranging individual pixels. This technique creates a coarse image which will only look good at a distance.

Over the years, a wide variety of dithering methods (or algorithms) have been developed and implemented for use in image processing. The choice of any particular method depends on the exact nature of the image, the display system and the desired results. Dithering usually creates various distracting patterns (called artifacts or moiré patterns) in the image. Some dither patterns produce better gradations and shading than others, but may require more processing time and memory.

If you want to print your graphics on paper but only need a few copies, you need a color printer for your computer. The cost and quality of these printers has been improving dramatically since they were first introduced, leaving you with quite a few choices. The four primary printer technologies for producing color output vary in cost, resolution, color depth and paper requirements. Individual printers also vary in quality, speed, reliability and lifespan.

PANTONE Color Manager software is designed to work with popular graphic design programs like Quark XPress™, Photoshop®, Illustrator® and the like, and also provide more accurate output on a wide range of desktop printers.

Commercial Printing

If you need to produce hundreds or thousands of copies of your work, you will need to take your output to a commercial printer for a large press run. This process is somewhat demanding and expensive, but is the only way to make large numbers of copies.

Commercial printing requires quite a bit of prepress work for each job. Producing camera-ready originals is somewhat technical, so most printing houses have full-time prepress technicians who can do some or all of the work for you, depending on your experience and budget.

When you are deciding what type of printing to do, speed, cost per copy and quality of the output are some of the deciding factors. The printer's estimator can advise you about the choices available.

There are two different ways color can be applied to paper in color printing: spot color and process color. Spot color is a method of applying a premixed color of ink directly to the page. Process color applies four or more standard ink colors (the basic four are cyan, magenta, yellow and black) in very fine screens so that many thousands of colors are created. Spot color is usually used when a few exact colors are needed. Process color is more useful for printing photographs, paintings and very complex colored images.

In some cases, both spot color and process color can be used on the same document. For example, a company brochure may include color photos (process color) and a corporate logo (spot color). Spot color applies a premixed ink to the page. This color is usually identified by a color system such as the PANTONE MATCHING SYSTEM. Spot color is useful for documents that require only a few colors, such as newsletters, brochures and stationery. Spot color is also used to match specific colors very closely.

The cost of printing color documents is related to the number of ink colors used. As process color requires four or more inks, spot color can be cheaper if you use fewer than four colors. Spot color also has the advantage of printing a wider range of clean, bright colors.

Look around you for an example of spot color printing. If a color seems smooth and even no matter how closely you look, it's probably printed with spot color.

Process color is a method used to create thousands of colors using four or more standard inks. The colors used in four-color process are the three subtractive primaries (cyan, magenta and yellow) plus black.

The original image is separated into its cyan, yellow, magenta and black components. A film is made for each separation and then a plate is produced from the film. The paper is run through the four stations of a four-color press to accept layers of ink from each plate. When all four colors are printed together, the illusion of continuous color is complete.

Take a look around you for a full-color newspaper, book or magazine. If you look very closely at a color photograph, you can make out the halftone dots of the four inks.

If you are printing in process color, your image will require a plate for each of the cyan, magenta, yellow and black inks. As each color of ink used is laid down on the paper individually, a different plate must be created for each ink. Spot colors each require their own plate as well.

Separations can be created in quite a few different ways. You may be asked to provide a full color printout to be optically separated. Your image will either be scanned or run through a separator, which separates the image using filters. On the other hand, you may be asked to provide a disk containing the graphics file. An imagesetter, which is essentially a very high-resolution printer, will create the separations directly from the graphics file.

Halftoning is the most common of the many ways printers create different shades of color from just one ink. A finely etched screen is used when making each plate. This screen changes the darker and lighter areas of the original into areas of larger and smaller dots. When printed, the larger dots will appear darker than the smaller dots, due to greater ink coverage. When multiple colors of ink are printed together, the different apparent shades will combine to simulate far more colors than are actually used.

Halftoning is done with a very fine screen when printing on glossy paper and for higher quality documents. Coarser screens are used for rough paper such as newsprint. Newspapers use coarse screens, so it's fairly easy to make out the individual dots in newspaper photographs. The fineness of halftone screens is determined by the number of lines of halftone dots per inch. This is called the "lines per inch" or the LPI.

Lithography means "stone-writing." Invented in 1799 by Aloys Senefelder in Germany, this process relies on the fact that water and grease repel each other. A lithograph stone is prepared by drawing the image to be printed on polished limestone with a greasy crayon. In commercial offset lithography, the lithography stone is replaced by thin metal plate that wraps around a printing cylinder.

The imaging areas on the plate are water repellent and accept ink, while the non-imaging areas accept water and reject ink. The ink is offset from the metal plate onto a rubber blanket and then onto the paper, preventing excess wear of the plate. Offset printing is well-suited for color printing, because a typical press can handle six colors with a single pass, including four process and two spot colors

WHY WYS IS NOT WYG

A common acronym in computer graphics is WYSIWYG. It stands for "What you see is what you get." Unfortunately, a common problem in reproducing color graphics is that what you see on the screen is not what you get when you print. Several effects come together to cause this problem:

1. Monitors and output devices have limitations. Each device has a range of colors it can reproduce, called its color gamut. These vary with the type and model. The printer type, ink and paper quality and the printer's condition also affect the results.
2. Equipment can easily become miscalibrated and require very expensive, specialized accessories to keep them standardized to a predictable performance.
3. Printers that dither can only create a limited number of colors. If you attempt to print a color which does not fall within its abilities, it will produce the nearest match. The printer's resolution is important to its dithering ability, so higher resolution printers usually print higher quality color.

Color management systems are available to help solve these problems if precise color matching is important to you.

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