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A bitmapped digital image is composed of a set of dots or squares,
called pixels (from picture elements), arranged
in a matrix of columns and rows. Each pixel has a specific color
or shade of gray, and in combination with neighboring pixels it creates
the illusion of a continuous tone image. This matrix is created
during the scanning process, in which an analog original is sampled at
regular intervals, or the color of selected points of its surface,
corresponding to each pixel, is recorded. Generally speaking, the
more samples taken from the source image, the more accurate
the resulting digital surrogate will be. Digital image files are
commonly divided into "master" and "derivative" versions. Master
files are the highest quality files available, usually the originals
created by the sampling process. It is from these that further copies
of the images are derived, and often manipulated, to create lower-quality
and more easily distributed access files.
Digital files do not have any independent material existence; rather,
they exist as data or binary code until they are
rendered by intermediary technology: application software running
on a given operating system
running on a particular hardware platform. One
effect of this is that digital image files are particularly vulnerable
to format obsolescence
and media decay, and, therefore, ensuring the longevity
of digital images can be complicated and costly. Another effect is
that a single
digital image may manifest itself differently in
different circumstances according to a number of variables. Finally,
digital images cannot
be directly located or searched; this must be done
indirectly through the information that describes themtheir metadatacreated
either
manually or with automatic indexing software. A digital image
not associated with metadata will be difficult
to find or identify and is likely to become useless very quickly.
In fact, in order for
data (in this case, the content or "essence" of a digital image file)
to have continuing value and to be worth preserving,
both it and its related metadata should be managed as a single entity,
sometimes
known as a digital object or a digital asset.
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