25 Jan
2006
25 Jan
'06
3:46 p.m.
In article <006301c62205$66b0c700$5b01a8c0 at pc1>,
"a.carlini at ntlworld.com" <arcarlini at iee.org> writes:
I've not found a good solution for colour text and
line drawings.
JPEG still is not the right answer (it still hates those high
frequency edges) but other formats I've tried all eat up
inordinate amounts of disk space. (The original TIFF scans
of some pages of some RSX-11 manauls I did weigh in at 24MB/page!).
My theory on why this is so...
The scanner is returning a continuous range of colors instead of a
single block of solid color. So while everything "looks" like a
single shade of turquoise to your eye, its actually a smearing of
values that are similar but not identical. The smearing of color
values (i.e. turquoise +/- 5%, or whatever) gives bad compression for
things like PNG and so-on.
The trick is to take your raw scans and put them through a color
quantization step before compression them. The idea is to take that
spread of colors that is approximately turquoise and quantize them to
exactly turquoise. Then when you compress the resulting image, you
should have large solid blocks of constant color and should once again
get good results from a format like PNG or GIF or TIFF RLE.
--
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