Virtism

What Is "Resolution" and How Do I Use It?

Bitmap images are composed of pixels. Image "resolution" is simply the number of "Pixels Per Inch" (PPI) in the bitmap grid. There are two aspects to every bitmap image - its size (width and height in inches) and resolution (the number of pixels per inch). These two factors alone determine the total number of pixels in an image. For example, a 2 inch by 3 inch image with a resolution of 300 pixels per inch contains (2 x 300) x (3 x 300) or 540,000 pixels.

The more pixels there are in an image, the more detail the image can be displayed with. The fewer pixels there are in an image, the less detail the image can be displayed with. There are two ways to display an image - on screen and in print. When you are preparing images you need to know what resolution to scan or size them to. There is an optimum resolution for each and it is very easy to determine.

First let us take up screen display. This is the easiest to determine. Scan or size them all to 72 PPI. Why? A monitor on a Macintosh system displays at 72 PPI. A monitor on a Windows system displays at 96 PPI. Though there is a difference between the two, the standard for screen displays is 72 PPI. Piece of cake!

Continuous Tones

Determining image resolution for printed output is a little bit different because of the way an image gets printed. A typical photograph like a snapshot or 8 x 10 glossy of your favorite movie star is known as a "continuous tone" photo. It is called a continuous tone because of the gradual changes of tones of color or shades of gray (for black and white photos).

Halftones

It is not practical to print continuous tones on a printing press so a method was developed to simulate the changes in tone using only black ink for black and white photos. For a photo to be printed on a printing press it must first be converted into a "halftone". A halftone is an image whose continuous tones have been converted to a pattern of solid dots. When viewed as a whole, this pattern of dots appears as a continuous tone, when, actually, it is not.

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		This grayscale image when printed is really composed of a pattern of dots.

Using a camera and film, you could take a continuous tone positive and expose a negative with an ordinary window screen positioned between the lens and the film and produce a halftone. The light passing through the screen will diffuse and create the halftone dots. Professional litho houses and printers have been using a method similar to this, although not with a crude window screen. With computer graphics it is done by the software and the printer. You can create your own halftones on a laser printer to get a feel for this.

A halftone is expressed in terms of the "screen frequency" - the number of "Lines Per Inch" (LPI) of the screen used to prepare it. The number of lines per inch is also referred to as the "Line Screen" (LS) of the halftone. Halftones appearing in a typical newspaper range from 65 to 85 line screen. These are coarse halftones because the paper is very porous and rough and requires a larger dot size than the smooth, coated stock used in magazines or brochures. Halftones appearing in magazines, brochures and high quality catalogs typically vary between 133 and 150 line screen.

When preparing images for printed output you must know the screen frequency of the finished halftones before scanning or sizing them. Find out from the printer what is required for the job. There are a lot of variables you need to nail down ahead of time such as the type of paper, the capability of the printing press, what kind of printing plate will be used (paper or metal) and whether the plate will be exposed from a shooting board positive or a film negative.

Once you know the screen frequency of the finished halftone, you can scan and otherwise prepare your images. There is a rule of thumb used in preparing images for printed output. Scan or size images at a resolution (PPI) of 1.5 to 2 times the screen frequency (LPI) of the finished halftone. For example, a 2 inch by 3 inch photo would be scanned or sized to a 2 inch by 3 inch image at 225 to 300 PPI for a finished halftone at 150 LPI (150 line screen). Were the same photo to be printed using a 120 line screen halftone, it would be scanned or sized to 2 inches by 3 inches at 180 to 240 PPI. This is a very workable rule and is easy to remember.

You Determine The Halftone Frequency In The Layout

Once you have prepared your images, you import them into the page layout program such as QuarkXPress™, Adobe® PageMaker®, Corel VENTURA®, Microsoft® Publisher or whatever you are using. In the picture boxes or print dialog you fill in the data fields telling the software what screen frequency to print the halftones. The laser printer or high-end imagesetter uses this information at output time and produces the halftones on paper or film from the digital data. You use grayscale images for black and white halftones and CMYK (not RGB) color images for color separations.

How does the printer or imagesetter create a halftone from a color or grayscale image? The printer creates halftone dots by means of a halftone cell (below). A halftone cell is composed of a grid of pixels. Each pixel would be either solid black or solid white. The number of shades of gray which can be represented by a finished halftone image is limited by the number of pixels available in the halftone cell. Below is an example of a 16 x 16 halftone cell.

In the example above there are 16 x 16 = 256 possible shades of gray. If all the pixels were black the halftone dot would be 100% black. If all the pixels were white the halftone dot would be 0% black. If every other pixel were black, the halftone dot would be 50% black, etc.

In the above diagram, there are 188 black pixels out of a possible 256 in the halftone cell. The dot would have a grayscale value of 73.4% black.

There is a formula for calculating the maximum screen frequency a printer can output:

Thus, for a 1200 DPI Linotronic imagesetter, the maximum screen frequency for 256 shades of gray would be 75 and the maximum screen frequency for 64 shades of gray would be 150.

In actuality, though, one can get away with as few as 32 shades of gray in most any image. Thus, for a 150 line screen halftone, you can reduce the number of grays in your images to 64 or less to achieve satisfactory output on a 1200 DPI machine.

Process Color Halftones

Both color and grayscale halftone dots are created as shades of gray. "What?", you say. "Color halftones are shades of gray?" You see, even though the halftone will print using process color, each of the four color plates (cyan, magenta, yellow and black) is prepared from a black and white halftone negative. When printed and combined with their corresponding color, the result would be a color picture (below).

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Halftone Angle or "Slope"

Part of setting up the halftone in page layout is determining its angle or "slope". If the halftone were to be displayed with a slope of 0° it would be hard on the eyes. For this reason halftone dots are placed at an angle to each other to blend in better. The dots on a simple grayscale halftone need only be angled at 45°. But the dots on a process color halftone need to be offset from each other in a pattern so they appear harmonious. If you will notice in the representative image above, the cyan, magenta, yellow and black dots appear to form a circular group called a "rosette". The default angles vary with the separation printer selected for output. The default angles for cyan, magenta, yellow and black for a Linotronic 330 are:

Line Art Resolution Rule of Thumb

In the previous tutorial a rule of thumb was given for determining the resolution of a grayscale or color image. It is always better to use an EPS vector image for line art. However, if a bitmap must be used, the rule of thumb for line art is to scan or size it to 600 PPI to avoid the "jaggies". Line art bitmaps have to be scanned or sized at a much higher resolution than color or grayscale halftones.