[Asterisk-Users] Asterisk - fax - spandsp

[Lee Howard]

Steve Underwood wrote:

> Jean-Yves Avenard wrote:
>
>> Is there any future support for fax in colours? just wondering, I 
>> don't think I've ever used it before anyway :)
>
>
> Priorities:
>    1. T.38
>    2. A more complete T.30
>    3. ECM (which is really a part of 2)
>    4. Things like colour FAX (which in part is part of 2, but requires 
> all the compress/decompression and other stuff needed for grey scale 
> and colour).


The next HylaFAX release (current CVS HEAD already has it) will have 
color fax receiving support in Class 1/1.0... provided that the 
prerequisites are met.

As HylaFAX depends on libtiff for it's image file handling color fax 
support with HylaFAX requires that libtiff also support color fax image 
type.  However, color fax is of a CIELAB JPEG compression, and most 
image readers are not going to be able to make sense of the colorspace, 
neither can an unpatched libtiff.  Libtiff depends on libjpeg for its 
JPEG compression/decompression capabilities, and libjpeg doesn't get 
CIELAB JPEG compression either.  So libjpeg must be patched also.  
Patches exist for both libtiff and libjpeg for CIELAB JPEG, but as 
libjpeg is pretty much at a developmental lock-up, it may be a while 
before native releases support it.

As the JPEG itself and the session parameters contain coded information 
regarding the page size and formatting, and because ECM must be used for 
JPEG communication, it is not necessary for the fax receiver to actually 
decode the JPEG image like is required with MMR compression to count the 
number of lines.  You can just receive the data and save it to disk - 
ECM virtually guarantees that the data is intact.  (However, in the case 
of color JPEG, if you expect people to be able to view it, you'll need 
to convert it from CIELAB to something more common.)

In sending JPEG faxes the trick is in generating the tagline without 
decoding and then reencoding, as JPEG is lossy... you don't want to 
corrupt the image.  There are libjpeg patches available to assist in 
that endeavor, but as HylaFAX doesn't yet support color fax sending, I 
haven't approached that issue.

The other option to color faxing is JBIG compression.  The problem 
there, is that JBIG is fairly heavily patented still.  Thus many fax 
machines just aren't supporting it yet.

Lee.