Jpg files. What, When & Why.
What is a "JPG" or "JPEG" file?
The name JPEG stands for Joint Photographic Experts Group,
the name of the joint ISO/CCITT committee which created the
jpeg format or, more accurately, jpeg standard.
Without getting too technical, jpg files are image files
(usually photographic images as line drawings and other vector
graphics are less suited to the jpeg format) that are compressed
usually to enable large files to be stored or transmitted
as smaller files, like dehydrated food or clothes stored in
vaccum bags. For possibly more information than you could
possibly want, check out a detailed analysis of what jpg files
are on this Wikipedia page: http://en.wikipedia.org/wiki/JPEG
Essentially, what happens to an image file that is saved
as a jpg file is that instead of storing information about
the colour and density of each pixel (the basic unit of an
image file), a file that is saved as a jpg image has a number
of adjoining pixels averaged out into one block of colour.
So instead of having a file that has, say, a million pixels
described as different colours, a jpg file may have only 100,000
or 500,000 descriptions of those pixels making up the image.
The resulting file may look essentially the same but will
be much smaller in totaly bytes or megabytes. The physical
dimensions of the image are unchanged, just the amount of
information which is stored is reduced.
When an image is saved as a jpg file it will be assigned
the extension ".jpg" or ".jpeg". There
are other extensions as well but for simplicities sake and
in the real world of internet use, the ".jpg" extension
is almost always seen.
Will jpg compression damage the image file?
Only in extreme cases where high compression is applied to
a low resolution image there will be a loss of sharpness and
possible colour fringing or other "artifacts". Jpg
compression is a "lossy" process where information
is removed by the process of averaging pixel information.
 This
image shows an extreme example of jpg compression. You can
see the tones in the ocean that have been averaged out into
a few shades of blue and are now visible as blocks of colour,
giving a chequered appearance to the image. Also, colour fringing
can be seen on the body of the whale. Such a highly compressed
image is barely suitable for the web and of no use for any
reproduction work.
If this same image was saved with very low compression, there
would be no visible indication of compression artifacts. Only
extreme enlargement of the image would show any pixel averaging
which would be invisible in the final printed product.
Are jpg files only used on web pages?
No. Usually when a web page has an photograph displayed it
is in the jpg format. However that is not the only use for
jpg files.
Are jpg files only "low resolution" and useless
for any serious image use?
No. While jpg files found on web pages are essentially useless
for anything else (except maybe a powerpoint presentation
which most people sleep through anyway) a jpg file may be
used for any purpose, as long as the quality of the file is
high enough for the intended use.
When can a jpg file be used for press work or reproduction?
When it has enough information to be reproduced without displaying
image degredation from the jpg compression process. It also
needs to have a higher dpi (dots per inch or pixels per inch)
than an image used on the web.
Typically an image used for the web will have 72 dpi, a file
intended for a newspaper may have 100 or 150 dpi, a file for
a colour print from an inkjet or similar printer may need
150-250 dpi or even 300 dpi while an image for a magazine
or book will need 300 dpi. Some high grade printing might
use 600 dpi.
How compressed should a jpg be for my use?
As little as possible. When a file is saved in jpg format,
for example in Photoshop, part of the saving process allows
you to stipulate how much compression to apply. An image used
on a web page may be sized at 600 x 400 pixels at 72 dpi (a
fairly large image for a web page) and when saved as a jpg
it may be saved at a high compression level or Quality setting
of 3 or 4 as shown below. The size of the file that is transmitted
over the internet to your computer will be only about 75 kb
(depending on the number of colours in the image and other
factors).
When used in a magazine or book, the same image
will be sized in inches or mm such as 297mm x 210mm (A4 or
full magazine page size) at 300 dpi. The full size of that
file will be around 24 -25 megabytes, a much, much larger
file. While downloading a file that size may be fine if you
have a fast broadband connection, the space taken up on the
server and the time taken to upload and download will be much
reduced by applying jpg compression, especially when more
than one or many files need to be transmitted.
Such a file would be saved in jpg format at
level 10-12. The higher numbers denote a much smaller amount
of compression. The A4 file would be reduced, when saved at
level 10, to a 1-2 megabyte file, much quicker to upload and
download, not to mention storage space on the server. At level
12 the same file may be around 3-5 megabytes, still much smaller
than the original file and with no reduction in image quality.
Any compression "artifacts" or blurring
of detail or colour smudging would be invisible or non existant
at those low compression levels.
So what's the bottom line?
Let's liken files saved in jpg compression format
to cars. A low resolution (72 dpi) file saved at a high compression
rate could be like a rusty old four cylinder car limping along
the road. The same file saved at 300 dpi with the lowest compression
setting is like a formula 1 racing car. They are both cars,
but you know which one will win the race.
Suffice to say, all Marine Themes image files
intended for reproduction which are delivered as jpg files
are saved at very low compression levels.
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