Newsgroups: comp.graphics,sci.image.processing
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From: myers.r@applelink.apple.com
Subject: Re: Unhalftoning Halftones
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Message-ID: <myers.r-1105950737160001@rmyers1.atg.apple.com>
Date: Thu, 11 May 1995 15:37:16 GMT
References: <3o756l$eot@mack.rt66.com> <1995May3.130026.1020@csginc.com> <D8CEDy.HJ@ucc.su.OZ.AU> <grunes.208.800121134@news.nrl.navy.mil>
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Xref: glinda.oz.cs.cmu.edu comp.graphics:76337 sci.image.processing:14547

One technique I've used successfully is to supersample the halftoned
image. I suggest using sampling resolutions of double the halftone
frequency (to satisfy the Nyquist limit) or better. Many images in
magazines are printed at 133 or 150 lpi, so sampling should be at 266 or
300 dpi. I typically use 600 dpi. This image will have, in theory, only 8
colors (16 really, but 8 of these are blends with black and are often
indistinguishable).

To blend these 8 colors into a continuous tone image, the supersampled
image is then blurred using a gaussian blur with the radius set to the
size of the halftone (stated in pixels of the supersampled image).

The image is then subsampled using a nearest neighbor algorithm. Usually
the image is reduced to 50% or 25% of the supersampled size. The image is
now continuous tone, but the edges are a bit indistinct. So an unsharp
mask is applied to sharpen the edges.

This technique has been applied to many images with good success using
Photoshop.


Please use this method only for personal use. Images in magazines and
other printed materials are copyrighted.


Robin Myers
