Digital image technology strives to capture finer detail when
an image is either created or scanned. Capturing greater pixel depth results
in more colors delivered on output, and thus a higher quality reproduction of
the original image. Maintaining the colors of the image through a pixel spatial
resolution as close to the original as possible, results in more bits of information
being generated and saved as digital files. Increasingly, digital image creation
and capture are requiring larger volumes of data and data storage.
When selecting an image for print or Web publishing, the intended
use of the image must be considered in order to determine the proposed resolution
and the size of the file. Web publishing requires considerably less resolution
and no output concerns, unlike images intended for print publishing. Therefore,
when publishing to print it is important to understand some of the considerations
necessary as the image goes through the various stages from capture to output.
There are two types of images used in graphic arts:
vector and raster images. Vector images use mathematical formulas
to create graphic objects, lines, or shapes. When outputting to print
or Web, vector images are resolution independent and the final output
occurs using the dpi setting of the output device.
For the purposes of calculating file sizes, we are
concerned only with raster images (also called bitmap images) which
use dots per inch (dpi), lines per inch (lpi), or pixels per inch
(ppi), when creating and outputting the reproduced image. To explain
the differences between dpi, lpi, and ppi, consider the following:
- Dots per inch (dpi): A gauge used for stating the resolution
that is to be achieved from an output device such as a laser printer,
image processors, or a digital printing press. 300 dpi is a common
printer resolution while imagesetters and digital presses can achieve
significantly higher dpi outputs.
- Lines per inch (lpi): A measurement used to define the
resolution of the image to be reproduced through a lithographic
printing process. The resolution is referred to as a line screen
(also line frequency or line ruling) and is determined by the number
of lines per inch used to create the dot pattern in an image. Using
the correct lpi screen enables the four transparent ink colors for
color printing (cyan, magenta, yellow, and black) to accurately
print the reproduced image with clarity, definition, and quality.
- Pixels per inch (ppi): Video monitor or on-screen resolution
uses ppi as a measure of the resolution established by the number
of dots or pixels per linear inch. 72 ppi is a common screen resolution
with high definition screens displaying above 200 ppi. Scanners
and digital cameras may have a ppi measurement in the thousands.
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