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From: flaig@cit-vlsi.UUCP
Newsgroups: comp.graphics
Subject: Re: reverse dithering
Message-ID: <4579@cit-vax.Caltech.Edu>
Date: Wed, 18-Nov-87 15:41:19 EST
Article-I.D.: cit-vax.4579
Posted: Wed Nov 18 15:41:19 1987
Date-Received: Sat, 21-Nov-87 07:03:59 EST
References: <1525@ssc-vax.UUCP> <34159@sun.uucp>
Sender: news@cit-vax.Caltech.Edu
Reply-To: flaig@cit-vlsi.UUCP (Charles M. Flaig)
Organization: California Institute of Technology
Lines: 31
Keywords: dithering, halftone, gray scale

In article <34159@sun.uucp> cmcmanis@sun.UUCP (Chuck McManis) writes:
>In article <1525@ssc-vax.UUCP> dickey@ssc-vax.UUCP (Frederick J Dickey) writes:
>>My question is the following: Given the image in the dithered form,
>>is there any reasonable way of reconstructing the original gray scale
>>image?
>
>If you know the region of the dither (like 4 X 4 squares or something) you
>can just average all of the pixels in the region and multiply that by the 
>a normalizing factor.
>--Chuck McManis

Unfortunately, while the dithering _pattern_ may be 4x4, most dithering is
done on a pixel by pixel basis.  Information (in the form of more bits of
color resolution) is thrown away when a picture is dithered, and cannot
be recovered (maybe that pattern of dots _was_ in the original picture).

There are two exceptions where information can be recovered:

1) If the dithering wasn't done on a pixel by pixel basis (usually this
   is only done with low resolution pixtures, and the result shows it).

2) If you can identify large areas in the dithered picture which were a
   constant untextured shade in the original, then averaging pixels
   within that area will give the original color.  But most interesting
   pictures have textures or shading which will not allow this.

Note:  I am not an expert in the above topic, but I have taken classes
in graphics and information theory and done dithering myself.

--Charles Flaig
  flaig@vlsi.caltech.edu