From mboxrd@z Thu Jan  1 00:00:00 1970
To: 9fans@cse.psu.edu
From: steve.simon@snellwilcox.com
Subject: Re:[9fans] image resampling
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Date: Mon, 15 Apr 2002 10:39:32 +0100
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Hi,

To downconvert an image you must first filter it and then resample it - If you
don't filter first then
you get aliasing (similar to the effect of cart-wheels going backwards in
movies, though this is
temporal not spatial aliasing).

The filtering is often done with a few (6 or 7) fixed lowpass filters used to
cover a wide
range of downconversion ratios. (a "good enough" solution) - really one should
use a
fixed filter per downconversion ratio.

For simple down conversion ratios (2, 4, etc) the sub-sampling is easily done
using pixle dropping, however
for arbitary ratios you really need a set of polyphase filters & a phase
accumulator - not difficult but needs
some thinking about.

A lot depends on how "good" your results need to be.

-Steve



____________________Reply Separator____________________
Subject:    [9fans] image resampling
Author: 9fans@cse.psu.edu
Date:       12/04/02 16:38

Hi, friends,

can anyone advise me how to resample (scale down) a big bitmap image?
I browsed through iconv(1), crop(1), jpg(1) ... still having no idea.
Just wanna look at my pictures without going to linux 'convert' :-((

TIA,
++pac.



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