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
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