Notes about GM 1.2 file writing philosophy

I just posted the following note in a bug report as to why the command

   gm convert -compress fax input.tiff output.tiff

does not always produce an output file which uses the requested 
compression.  There is a paradigm shift in 1.2 from the old 
ImageMagick way of doing things in order to put more control in the 
hands of the user and assure that performance is predictable.

"GraphicsMagick 1.2 does work a bit different than previous versions 
of GraphicsMagick.  Prior to GraphicsMagick 1.2, if someone requested 
FAX compression and the image was not bi-level (pure black and white 
pixels) then (depending on the dither setting) an excruciatingly slow 
reduction algorithm might be used which takes minutes or even hours 
depending on the size of the image and CPU speed.  For GraphicsMagick 
1.2 I decided that performance when writing files should always be 
predictable.  It is not acceptable that sometimes a file is saved in 
one second while other times it takes half an hour simply because it 
contains one color pixel.  Based on this, if the requested compression 
is not suitable for the image's existing type, then the requested 
compression is ignored.

In order to successfully always produce FAX-compressed output you 
should use an algorithm to ensure that the image is already suitable 
for compression using FAX.  This could be done using -threshold, 
-ordered-dither, or color reduction to two colors in a gray colorspace 
(the previous defauult). This requirement to process the image in 
advance may seem a bit annoying but it puts control over the result in 
the hands of the user and assures consistent performance."

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


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