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From: maxtal@extro.ucc.su.OZ.AU (Tim TAL Lister)
Subject: Re: Unhalftoning Halftones
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References: <1995May3.130026.1020@csginc.com> <D8CEDy.HJ@ucc.su.OZ.AU> <3oqqph$l5e@rover.ucs.ualberta.ca>
Date: Mon, 15 May 1995 02:04:20 GMT
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Xref: glinda.oz.cs.cmu.edu comp.graphics:76556 sci.image.processing:14608

In article <3oqqph$l5e@rover.ucs.ualberta.ca> sherwood@fenris.space.ualberta.ca (System Administrator) writes:
>Tim TAL Lister (maxtal@extro.ucc.su.OZ.AU) wrote:
>
>:   an interesting problem.  the core of this problem is that the half-toned
>: imaged is 1 bit deep, and most of the original grey-scale data has already
>: been thrown away by the half-toning process.
>
>:   You might get somewhere if you could identify the exact h-t process used.
>
>
>I disagree.
>	In a halftone image the grey information is represented by the
>size of the dots.  In order to recover this information you must scan at
>a very large multiple of the dot density.  E.g.  If a typical halftone
>can represent a constrast range of 40:1, that's below 64 -- 8^2.  So to
>get most of that grey image information back, you'd need to scan at
>roughly 8 times the resolution of the image.  So if the original was
>printed at 135 dpi, you'd scan at 1000 dpi.
>	Then from that 1000 dpi, you'd take the pixels in 8 x 8 squares,
>and calculate a grey level from them, producing a grey map at 125 dpi.
.... etc.,

  Look fellas, the process you are describing is only _one_ type of
half-toning.  For a full and detailed discussion of halftoning techniques,
i refer you to:

	Digital Halftoning
	Robert Ulichney
	MIT press

  This is the best book i have ever seen on the topic; it began as Ulichney's
Masters Thesis.

	regards,
		tal

-- 
;----------------------------------------------------------------------
        TIM (TAL) LISTER,         maxtal@extro.ucc.su.oz.au
	Maxtal Pty Ltd, 81a Glebe Pt.Rd, Glebe, NSW 2037, AUSTRALIA
;------ Graphics & C++ libraries, Games & Image Processing ------------
