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Others have explained why and when on this, but I thought I should add
some background:
We refer to JPEG and GIF formats as "lossy" because they discard pixel
information during compression. These formats assume that the viewer
won't notice the discarded information, any more than a newspaper reader
will notice that the rendition of a color photograph in a newspaper
isn't the same as the photograph itself.
JPEG format uses a complex, sophisticated algorithm to reduce the file
size. The true color value of each pixel is retained, but adjacent
pixels with very similar color values may be combined into one pixel. If
you want a thorough and overwhelming description of the algorithm, see
the Wikipedia article for *JPEG*.
JPEG has advantages for images that contain a wide range of colors that
shade from one to the next. The true color values are kept, but the
resolution of the image decreases. This is why JPEG is favored for
photographs.
The GIF mechanism is to reduce the total number of color values in the
file to 256. These color values are then mapped onto each pixel in the
image. The set of color values is called the *palette*. For each pixel,
GIF stores the palette position rather than a real color value. This has
great advantages for an image that does not contain many different
colors, since the entire range of colors is kept while the overall color
depth is reduced. This is why GIF is favored for diagrams and screen
shots.
Because they greatly compress the image's file size, both JPEG and GIF
are favored as Internet file formats.
In contrast, TIFF uses non-lossy compression. Basically, TIFF uses a
compression algorithm that attemps to save space without losing
information. TIFF results in very large files, but these files retain
all the original information. TIFF is used by scanners, where the
initial image itself usually isn't kept.
BMP format is *not compressed*. The main difference between BMP and RAW
format is that BMP uses a color palette while RAW is simple the bitmap
image.
As far as I know, RAW has no advantages over TIFF. You don't lose
information with TIFF, and so either one is good to use as the initial
format for an image.
If you're creating a diagram, chart, or screenshot, I think you might as
well save the result in GIF. These images don't use many different
colors (unless they contain a photo or art), so you don't lose any
important info. I always save screenshots in GIF.
When I take photographs, I save to either JPEG or TIFF, depending on the
quality I want.
Resizing is *certain* to affect the apparent quality of an image. But
this e-mail is too long anyway, so I'll save that...
Joe Malin
Technical Writer
(408)625-1623
jmalin -at- tuvox -dot- com
www.tuvox.com
The views expressed in this document are those of the sender, and do not
necessarily reflect those of TuVox, Inc.
-----Original Message-----
From: techwr-l-bounces+jmalin=tuvox -dot- com -at- lists -dot- techwr-l -dot- com
[mailto:techwr-l-bounces+jmalin=tuvox -dot- com -at- lists -dot- techwr-l -dot- com] On Behalf
Of obair81 -at- comcast -dot- net
Sent: Monday, May 01, 2006 1:51 PM
To: techwr-l -at- lists -dot- techwr-l -dot- com
Subject: graphics
Can anyone answer these basic graphics questions (or point me to a place
where I can get answers)?
- If one is given a jpeg from a photo shoot, if it is accidentally
converted to a bmp instead of a tiff, can one open the bmp and save it
as a tiff (assuming the jpeg is no longer available), and still get the
various advantages offered by the tiff format?
Thanks for any help.
Paul
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