Image Resolution

The requirements of print differ greatly from those for video. It is necessary to work with a much higher quantity of pixels to produce satisfactory results when outputting to the printed page. On the flip side, most Web images are too small, as they have been optimized for fast download in a mosaic Web browser.

A common problem occurs when video professionals talk to their cousins in the print and Web worlds. Although all camps speak the same language, some unique terms are used that often result in confusion and extra work by both sides. The most common problem is resolution. Video makers may use the term pixels per inch (ppi); other industries use dots per inch (dpi) or lines per inch (lpi).

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Lines Per Inch (Ipi)

Talk to a print professional and you'll often hear the term lines per inch. This term derives from the traditional process where images with gradiated tones were screened for printing. This was done (traditionally) by laying film with dots over the image before the film was exposed. These days an imagesetter is used. A higher lpi means means smoother images.

Output Method

Typical lpi

Screen printing

Laser printer (coated paper)

Newsprint

Offset printing (coated paper} High-quality offset printing

35-65lpi 75-110 lpi 60-85lpi

120-150 lpi 150-300 lpi

Dots Per Inch (dpi)

Video Graphics Have a No dpi jiS^

There are no "dots per inch" (dpi) with video graphics. All that matters are the total pixels on screen.

Copyright: Krzysiekzpoczty/iStockphoto

A more common term is dots per inch (dpi), which refers to the number of dots that can fit into a one-inch by one-inch square area. Higher dpi generally means smoother images. Many printers in an average office setting can print 600 dpi or higher. This can lead to very professional-looking images. However, it is essential to remember that dots per inch is strictly a print term that refers only to a physical output. Dots per inch is not used when scanning or describing video graphics.

Samples Per Inch (spi)

When using a scanner (flatbed or slide), the correct term is samples per inch. This term refers to the number of analog samples taken in order to convert the image into a digital file. A higher number of samples will increase the output quality of the digital file. However, too many samples can surpass the quality threshold for your output devices and simply be a waste of time and disk space. The samples per inch measurement is similar in theory to the sample rate of a digital audio file. The higher the number, the more accurate the sound.

Most scanners will say dpi in their packaging and documentation. What they meant to say is spi. Fortunately, the two numbers are a pretty clean exchange. Just be sure that you look at a scanner's optical resolution (versus interpolated) when shopping. A resolution of 600X600 pixels or greater will suffice for nearly all video work.

The last measure of image resolution is pixels per inch. This measurement measures how many pixels fit on the screen. Because a video monitor is a fixed resolution, increasing the ppi will not improve quality. Rather, it will result in image detail being clipped or an overall softening as the pixels are resampled.

Pixels Per Inch (ppi)

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