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Re: COMP:Scanning question




Chris Darlington wrote:
> 
> Hi iris fans,
> 
> I have a question for the anyone who is scanning their photos to send to
> the list or for web pages.  Question is , what is a good DPI to scan
> photos so that they'll look resonably sharp without taking up to much
> memory .  When I scan at 75 DPI which is supposedly all one needs for
> internet purposes , my photos look  unexceptably soft .  When I used 300
> DPI this afternoon to scan a photo , the computer warned me that it was
> running out of disc space.  I'm sure people would be irritated with me
> if I sent a 5 mb attached photo to the list.  Maybe there is something
> I'm doing wrong.

Hi Chris,

Scanning, color quality, and image management are all complicated issues
with lots of interactions.

Lets start first with the scanning part. 75 DPI is th maximum you need
to scan at for computer monitor presentation, whether it is email or
webpage - because that is the highest resolution you will every have on
99.9999% of the computer monitors. Anything more than that is a waste of
storage and just ends up making the picture appear bigger on the screen.

However the quality of the scanner and the bit depth you use can make a
big difference. 8-bit color (i.e., 256 color) will give you a smaller
file but a lower quality image (because the computer only has 256 colors
to choose from - and that is not many when you consider all the hues and
blending colors that are needed to dither an image). 24 or 32-bit color
(meaning millions of colors) gives you a much better picture even at 75 DPI.

We will not concern ourselves with color representation from one monitor
to another, I sent an email about that not too long ago.

Compression. -  JPEG is a compression standard. It works by building a
table of the colors that are used in an image and assigning a code to
each color that takes less than the number of bits typically used to
specify the color. The routines do some other things that are beyond
discussion here (remember the professors that used to say "the remainder
is left as an exercise by the reader"? - This is the same sort of
dodge... <grin>)  The point of this all is that the greater the
compression, the less the reconstructed image (after decompression)
looks exactly the same as the original. Use less compression for a
higher quality (bigger storage requirement) image and vice versa. At
high compression JPEG is somewhat "lossy".

If you think your scanner is not doing a good enough job, scan at high
resolution, then use a high quality image manipulation program (like
Photoshop) to reduce the dpi on the image, change its size, and specify
the JPEG compression ratio.



John                     | "There be dragons here"
                         |  Annotation used by ancient cartographers
                         |  to indicate the edge of the known world.
________________________________________________

USDA zone 8/9 (coastal, bay) 
Fremont, California, USA 
Visit my website at:
http://members.home.net/jijones

President, Westbay Iris Society
Director, Region 14 of the AIS
Chairman, AIS Committee for Electronic Member Services

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