Datasheet

resolution of like files—those that are uncompressed, the same color mode, and the same
file type—and before processing that will add image elements (layers, vectors, alpha chan-
nels, etc.). File size is likely the least-used measure of resolution.
Beyond file size, physical pixel dimension, and ppi, there are some interrelated terms
for resolution that you will hear from time to time used both correctly and incorrectly
(often in the same sentence). For example, dpi is a commonly used term, often used
casually as a universal term for resolution. However, it is really an output term specific to
printing and the number of dots a printer uses in representing an image. Other measures
of resolution include spi, ppi, and lpi. The various resolution-related terms can be tricky
to use correctly and consistently, but you should know what they mean and use them
properly when you mean something specific.
spi (samples per inch) Capture resolution. The number of scanning and digital capture
samples per inch.
ppi (pixels per inch) Digital file resolution. The assigned number of digital pixel elements
to be used in printing or displaying an image.
dpi (dots per inch) Printer resolution. The number of bitmap dots (smallest printing com-
ponent) an output can create per inch.
lpi (lines per inch) A measure of halftone dot size. Halftone dots are made of multiple
printer dots. The number of rows of halftone dots per inch.
It would seem that you would always want high resolution in your images if you con-
sider detail important. But that’s not always the case. Correct resolution depends not only
on what size you want the result to be but also on what display or print medium you will
be using and how the image information is applied.
For example, if you don’t have enough resolution to meet the needs of print output,
images won’t look as sharp as they could; they might appear a little soft, fuzzy, or blocky
(again, have a good look at Figure 1.1). If you have too much resolution in your image, file
sizes are unnecessarily large; processing will take longer than it needs to; you’ll take up
excess storage; and the results will not improve. On the Web, images without enough res-
olution will be too small; those with too much resolution will be too large and will take
longer to download. You can’t just guess how much image information you need when
applying an image; you have to know the amount you really need and work within those
parameters. Understanding what resolution is and how it is used is the only way to use it
correctly.
To simplify with better accuracy, use spi when speaking of capture (scan sampling), ppi when
discussing digital files, dpi when considering output resolution, and lpi in the context of
halftone dot size.
6 ■ chapter 1: Resolution: The Cornerstone of Image Detail
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