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JPEG
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The most commonly used
digital image format is JPEG (Joint Photographic Experts Group).
Universally compatible with browsers, viewers, and image editing
software, it allows photographic images to be compressed by a factor
10 to 20 compared to the uncompressed original with very little
visible loss in image quality.
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| The Theory in a Nutshell
In a nutshell, JPEG
rearranges the image information into color and detail information,
compressing color more than detail because our eyes are more
sensitive to detail than to color, making the compression less
visible to the naked eye. Secondly, it sorts the detail information
into fine and coarse detail and discards the fine detail first
because our eyes are more sensitive to coarse detail than to fine
detail. This is achieved by combining several mathematical and
compression methods which are beyond the scope of this glossary but
explained in detail in 123di.
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| A Practical Example
JPEG allows
you to make a trade-off between image file size and image quality. JPEG
compression divides the image in squares of 8 x 8 pixels which are
compressed independently. Initially these squares manifest themselves
through "hair" artifacts around the edges. Then, as you increase the
compression, the squares themselves will become visible, as shown in the
examples below, which are magnified by a factor 2. |
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100% Quality JPEG is very hard to
distinguish from the uncompressed original which would
typically take up 6 times more storage space.
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80% Quality JPEG looks still very
good, especially when bearing in mind that this crop is 2
times enlarged and that the file size is typically 10 times
smaller than the uncompressed original. Notice some
deterioration along the edges of the yellow crayon. Most
digital cameras will use a higher quality level than 80% as
their highest quality JPEG setting.
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60% Quality JPEG. If you
look carefully, you will notice some of the JPEG
squares and "hair" artifacts around the edges.
However, the unmagnified crop shows that the
quality is sufficient for websites.
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It is a great trade-off because the file
size is typically 20 times smaller than
the uncompressed original.
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10% Quality JPEG shows serious image
degradation with very visible 8 x 8 JPEG squares. The only
benefit of this low quality level is that it illustrates
what JPEG is doing in a more subtle way at higher quality
levels. It is unlikely you will ever compress this
aggressively. The
example also shows that compression is most visible around
the edges.
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Practical Tips
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When editing an image in several sessions,
it is recommended to save the intermediate image in an
uncompressed format such as TIFF or the editing program's native
format (e.g. PSD for Adobe Photoshop or PSP for Paintshop Pro).
If you save for instance an image in JPEG, close it, open it
again and save it again in JPEG with the same quality setting,
the file size will not reduce further, but quality will have
degraded further. So only compress after
all editing is done.
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Cameras usually have different JPEG
quality settings, such as FINE, NORMAL, BASIC, etc. Unless you
shoot in RAW or TIFF, it is recommended to shoot in the hightest
available JPEG quality setting. Note however that some cameras
will compress more than others, even at their highest JPEG
quality setting.
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| The compression article shows some numerical
examples of file sizes. |
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