Re: Bug with B&W conversion

Hi Bob,
This bug goes back to the ImageMagick base-code for the 'riemersma' dither routine.  I raised this on the ImageMagick forums some time back (http://www.imagemagick.org/discourse-server/viewtopic.php?f=3&t=17251) but had a hard time convincing their admin that it was a real problem.
The main reason I'm using ImageMagick (and now GraphicsMagick) is that this routine provides an exceptional B&W conversion on some very 'noisy' documents that I'm processing at the moment so using another dither isn't an option.
Regards,
Phillip

> Date: Fri, 7 Jan 2011 08:36:45 -0600
> From: [email protected]
> To: [email protected]
> CC: [email protected]
> Subject: RE: [GM-bugs] Bug with B&W conversion
> 
> On Fri, 7 Jan 2011, Andrew Stonewall wrote:
> > 
> > "c:\Program Files\GraphicsMagick-1.3.12-Q16\gm.exe" convert -monochrome -format bmp original.bmp error.bmp
> > 
> > on the attached ZIP-ed BMP files you'll see that you get a blank image - these images have no compression or
> > alpha channels - just plain 24-bit BMP.
> 
> Hmmm.  I am able to recreate this issue here.  This time I don't have 
> an answer (yet) for why the issue occurs but it clearly has something 
> to do with using the color reduction algorithm and some property of 
> the input image.
> 
> This permutation works but only uses simple thresholding (key is to 
> add +dither):
> 
>    gm convert original.bmp +dither -type bilevel crap.bmp
> 
> Bob
> -- 
> Bob Friesenhahn
> [email protected], http://www.simplesystems.org/users/bfriesen/
> GraphicsMagick Maintainer,    http://www.GraphicsMagick.org/

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