[hylafax-users] About JPEG (Color Fax) and JBIG compression support

[Lee Howard <faxguy@xxxxxxxxxxxxxxxx>]

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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