Brian writes:

What is the best compression (1 - 100%) to use when saving a scanned
colour photo in jpeg?

It sounds like the compression factor in the program you're using is
reversed from the -quality switch in the cjpeg program I use, so switch 0%
and 100% in the information below, from cjpeg's manual page.

I personally find quality 75 to work well even with photos that have
lots of high-contrast transitions.

The -quality switch lets you trade off compressed file size against
quality of the reconstructed image: the higher the quality setting, the
larger the JPEG file, and the closer the output image will be to the
original input. Normally you want to use the lowest quality setting
(smallest file) that decompresses into something visually indistin-
guishable from the original image. For this purpose the quality set-
ting should be between 50 and 95; the default of 75 is often about
right. If you see defects at -quality 75, then go up 5 or 10 counts at
a time until you are happy with the output image. (The optimal setting
will vary from one image to another.)

-quality 100 will generate a quantization table of all 1's, minimizing
loss in the quantization step (but there is still information loss in
subsampling, as well as roundoff error). This setting is mainly of
interest for experimental purposes. Quality values above about 95 are
not recommended for normal use; the compressed file size goes up dra-
matically for hardly any gain in output image quality.

In the other direction, quality values below 50 will produce very small
files of low image quality. Settings around 5 to 10 might be useful in
preparing an index of a large image library, for example. Try -quality
2 (or so) for some amusing Cubist effects. (Note: quality values below
about 25 generate 2-byte quantization tables, which are considered
optional in the JPEG standard. cjpeg emits a warning message when you
give such a quality value, because some other JPEG programs may be
unable to decode the resulting file. Use -baseline if you need to
ensure compatibility at low quality values.)



--

http://ourdoings.com/ Let your digital photos organize themselves.
Sign up today for a 7-day free trial.