Instruction manual

13
browsers. It is most commonly used within file formats such as TIF (described
below).
You may also see the terms “EXIF” (also “exif” or “Exif”) or “non-exif” used in
conjunction with JPF (e.g., a non-exif JPEG). The Exchangeable Image File
(EXIF) format is an international specification that lets imaging companies encode
information into the headers or application segments of a JPEG file; this
information includes shutter speed, aperture, and the date and time the photo was
captured. Cameras that store photos using EXIF-compressed files enable the
image data to be read by any application supporting JPEG, including Web
browsers and photo editing, desktop presentation, and document-creation
software programs.
JPEG is designed for compressing either full color or grayscale images of
natural, real-world scenes. It works well on photographs and for Web display, but
not so well on lettering or simple line art. It allows photos to be compressed by a
factor or 10 to 20, compared to the uncompressed original.
The JPEG format is adjustable for quality versus file size. Larger JPEGs
record enough data, even at the milder compression settings, so that artifacts
don't occur. JPEGs at the correct quality settings have no visible artifacts. Note:
If you set your camera to save at a low quality, you cannot get any of the original
quality back; this is why we recommend setting your camera’s quality at the
highest setting for CAP purposes.
A rule of thumb is that saving an uncompressed file (e.g., TIF) as a high
qualify JPEG results in a file 95% the size of the original; medium 75-85%; and
low 23%.
The file extension for JPEG is .jpg
Progressive JPEG
A regular or "baseline" JPEG file is stored as one top-to-bottom scan of the
image, whereas progressive JPEG divides the file into a series of scans. The first
scan shows the photo at the equivalent of a very low quality setting, and therefore
takes up very little space; following scans gradually improve the quality. Each
subsequent scan adds to the data already provided, so that the total storage
requirement is roughly the same as for a baseline JPEG photo of the same quality
as the final scan.
The advantage of progressive JPEG is that if a photo is being viewed (e.g., on
a computer monitor) as it is transmitted, the viewer can see an approximation of
the whole photo very quickly. As transmission continues the photo gradually
improves in quality. The disadvantage is that each scan takes about the same
amount of computation to display as a whole baseline JPEG file would. So
progressive JPEG only makes sense if one has a decoder that's fast compared to
the communication link, such as a modem-speed link (e.g., 56 kps).
Except for the ability to provide progressive display, progressive JPEG and
baseline JPEG are basically identical and both work well on the same kinds of
images. However, a progressive JPEG file is not readable by a baseline-only
JPEG decoder. It is possible to convert between baseline and progressive
representations of an photo without any quality loss, but specialized software is
needed to do this.