Re: Bug with B&W conversion

On Mon, 10 Jan 2011, Andrew Stonewall wrote:

> 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.

There is definitely a weakness in the algorithm, but it may be 
difficult to solve.  Color reduction is really intended for full-range 
photos.

> 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.

Yes, the quality of the dither can be very nice.  Slow but nice.

Both -ordered-dither and -random-threshold provide recognizable output 
from your image but the results are not as nice.

An alternative is to use local adaptive thresholding (-lat).  This 
algorithm is good for bringing out the original features of the scan. 
The best parameters to use depend on your input image and require some 
experimentation.

Here is an example:

   gm convert original.bmp -colorspace gray -lat 10x10-4% output.bmp

In this example, it computes the average value of the pixels in a 
10x10 rectangle around the target pixel.  Then it thresholds based on 
the pixel value being more than 4% different than the average.  This 
algorithm can use a lot of CPU so it is best to keep the rectangle 
size down to the smallest size which reliably works.

The end result captures all of the written/detail bits, while 
eliminating the page background.  Perhaps this is what you wanted in 
the first place?

Bob
-- 
Bob Friesenhahn
[email protected], http://www.simplesystems.org/users/bfriesen/
GraphicsMagick Maintainer,    http://www.GraphicsMagick.org/

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