Page 22 - CC 2019 ID Look Inside

Understanding Resolution

Raster image quality depends directly on the fi le’s resolution; Image Resolution
when you work with raster image fi les, you need to When an output device creates halftone dots, it calculates
understand the resolution requirements from the very the average value of a group of pixels and generates a
beginning of the process: spot of appropriate size. An image’s resolution controls the
• Pixels per inch (ppi) is the number of pixels in one quantity of pixel data that the printer can read. Regardless of
horizontal or vertical inch of a digital raster fi le. the source — camera, scanner, or fi les created from scratch
in a program such as Photoshop — images need to have
• Lines per inch (lpi) is the number of halftone dots suffi cient resolution for the output device to generate enough
produced in a linear inch by an output device to simulate halftone dots to create the appearance of continuous tone.
the appearance of continuous-tone color.
• Dots per inch (dpi) or spots per inch (spi) is the The relationship between pixels and halftone dots defi nes the
number of dots produced by an output device in a single rule of resolution for raster-based images — the resolution of
line of output. Dpi is sometimes used interchangeably a raster image (ppi) should be two times the screen ruling (lpi)
(although incorrectly) with pixels per inch. that will be used for printing.
When reproducing a photograph on a printing press, the In the image below, each white square represents a pixel. The
image must be converted into a set of different-size dots highlighted area shows the pixel information used to generate
that fool the eye into seeing continuous tones. The result of a halftone dot. Ideally, the printer has four pixels for each
this conversion process is a halftone image; the dots used to halftone dot created.
simulate continuous tone are called halftone dots. Light
tones in a photograph are represented as small halftone dots;
dark tones are represented as large halftone dots. (In pre-
digital design processes, images were converted to halftones
by photographing the image through a screen to create
halftone dots; different screens produced different numbers of
dots in an inch — hence the term “dots per inch”.)

Screen Ruling
The screens used with old graphic-arts cameras had a fi nite 72 ppi 300 ppi
number of available dots in a horizontal or vertical inch. That If an image only has 72 pixels per inch, the output device
number was the screen ruling, or lines per inch. A screen has to generate four halftone dots per pixel, resulting in poor
ruling of 133 lpi means that in a square inch there are 133 1 printed quality.
133 (17,689) possible locations for a halftone dot. If the
screen ruling is decreased, there are fewer total halftone dots, In the images below, the same raster image is reproduced
producing a grainier image; if the screen ruling is increased, at 300 ppi (left) and 72 ppi (right); notice the obvious
there are more halftone dots, producing a clearer image. degradation in quality in the 72-ppi version.

Line screen is a fi nite number based on a combination of the
intended output device and paper. You can’t randomly select
a line screen. Ask your printer what line screen will be used
before you begin creating your images. If you can’t fi nd out
ahead of time or are unsure, follow these general guidelines:
• Newspaper or newsprint: 85–100 lpi
• Magazine or general commercial printing: 133–150 lpi
• Premium-quality-paper jobs (such as art books): For line art, the general rule is to scan the image at the
150–175 lpi (some specialty jobs use 200 lpi or more)
same resolution as the output device. Many laser printers
If you fi nd this information a bit confusing, don’t worry. As and digital presses image at 600–1,200 dots per inch (dpi);
a general rule for preparing commercial print layouts, most imagesetters used to make printing plates for a commercial
raster images should have twice the pixel resolution at 100% press typically output at much higher resolution, possibly
size as the line screen that will be used. 2,400 dpi or more.

32 Project 1: Letterhead Design