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[hylafax-users] About JPEG (Color Fax) and JBIG compression support
HylaFAX users,
(This content is also available on the HylaFAX HOWTO at:
http://hylafax.sourceforge.net/howto/install.php#ss2.5)
The usage of JPEG and JBIG compression schemes are generally considered
"advanced" fax features and may not be available to all fax machines.
JPEG compression is most commonly used for sending and receiving *color*
facsimile. JBIG compression is most commonly used for sending and
receiving *monochrome* facsimile, but with a tighter compression (and
thus faster send-times) than MMR. Both JPEG and JBIG facsimile
compression require the usage of error-correction protocol (ECM).
*_JPEG_*
Currently JPEG support is only available in Class 1/1.0 receiving. JPEG
send support is not yet available. The Class 2.1 specification does
provide for JPEG support, but HylaFAX is not yet capable of sending or
receiving JPEG facsimile in Class 2.1.
HylaFAX JPEG receive support is entirely dependent upon CIELAB JPEG
support being available in libtiff, and libtiff gets some of its CIELAB
JPEG support from libjpeg. Unfortunately, as of libjpeg version 6b
released by the Independent JPEG Group (http://www.ijg.org/) libjpeg
does not support CIELAB JPEG. This will require libjpeg to be patched,
rebuilt, and reinstalled. The patch to libjpeg can be found here:
http://bugs.hylafax.org/bugzilla/show_bug.cgi?id=224#c7
Then libtiff will need to be patched, rebuilt, and reinstalled. The
libtiff patch can be found here:
http://bugzilla.remotesensing.org/show_bug.cgi?id=736
At this point, then, color fax reception support can be enabled in the
modem config file by setting:
Class1ColorJPEGSupport: yes
Most TIFF viewers will not be capable of displaying the JPEG-compressed
TIFF pages with the proper color scheme (they would need to support
CIELAB JPEG, which is mostly specific to fax). So to view
JPEG-compressed TIFF images, convert them first using |tiff2ps| and then
|ps2pdf| (or the |bin/tiff2pdf| HylaFAX script, as libtiff's |tiff2pdf|
doesn't seem to work the same right now), or convert them to another
color-supporting TIFF compression.
*_JBIG_*
HylaFAX supports both sending and receiving JBIG-compressed facsimile in
Class 1/1.0. The Class 2.1 specification does not include JBIG, and so
JBIG is not likely to be available with Class 2/2.0/2.1.
Like JPEG, HylaFAX JBIG receive support is entirely dependent upon JBIG
support being available in libtiff. Most recent libtiff releases include
the ability to write JBIG-compressed TIFF files. However, doing anything
with the TIFF file from there will be difficult as TIFF viewers usually
do not support JBIG compression. So usually the JBIG-compressed TIFF
file requires conversion to PDF or a non-JBIG compression for viewing.
This will require the installation of the JBIG-KIT library and libtiff
to be patched, rebuilt, and reinstalled. The patch can be found here:
http://bugzilla.remotesensing.org/show_bug.cgi?id=896
At this point, then, JBIG fax reception support can be enabled in the
modem config file by setting:
Class1JBIGSupport: yes
HylaFAX JBIG send support utilizes the |ModemSoftRTFCC| feature. Thus
sending JBIG-compressed facsimile requires that |ModemSoftRTFCC| be
enabled (which is the default setting). If the JBIG-KIT libraries were
found when HylaFAX was compiled then the |ModemSoftRTFCC| feature will
also convert to JBIG when appropriate (and |Class1JBIGSupport| will also
default to "yes"). JBIG send support does not require libtiff to be
patched.
*_A word of caution about installing patched versions of libjpeg and
libtiff:_* On most systems several other packages will likely have been
built against libjpeg and perhaps to a lesser extent also libtiff.
Problems may likely arise with those other packages due to the library
(libjpeg and/or libtiff) being altered without rebuilding that package.
In this case these packages that also depend upon the library will also
need to be rebuilt. Yes, it's not fun.
The libtiff JBIG patch was paid-for work. Thanks in part goes to iFAX
Solutions, Inc. for the funding of that work.
Thanks,
Lee.
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