LZW, libtiff, & data corruption

For many years, libtiff has not included LZW support due to patent 
issues.  The LZW support for libtiff has been made available as a 
replacement source file.  As of GraphicsMagick 1.1.3, LZW is now 
enabled by default.  When LZW is enabled in GraphicsMagick, it will 
attempt to use LZW in libtiff since there seems to be no good way to 
know if the installed libtiff supports LZW compression.

If a TIFF file is written requesting LZW compression and libtiff 
doesn't support it, a warning is issued and the output file is junk. 
This is particularly dangerous with the 'mogrify' command since it 
replaces the input file.  The image provided to 'mogrify' may be 
destroyed.

If you are using TIFF, please make sure that either libtiff includes 
LZW compression, or LZW is disabled entirely in GraphicsMagick.  When 
in doubt, use the 'convert' command instead.  With the 'convert' 
command, you can verify that the output file is ok before replacing 
the input file.

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


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