[Asterisk-Users] Asterisk - fax - spandsp

[Steve Underwood]

Lee Howard wrote:

> 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.

Thanks Lee. I found that helpful, except the last paragraph which 
confuses me. I thought JBIG was only a bilevel compression scheme. Am I 
wrong?

Regards,
Steve