Re: [hylafax-users] Config Files and Color Faxing for HylaFax-4.2.2rc5

[Lee Howard <faxguy@xxxxxxxxxxxxxxxx>]

Frank Peters wrote:

> Ooops.  After doing some quick searching, I discover that the
> color fax capability does not refer to JPEG images but to a
> particular kind of JPEG-in-TIFF compression, which is different
> from an ordinary JPEG format file.
>  

Well, the data that comes over-the-wire is raw JPEG.  The TIFF wrapper 
is done by HylaFAX... with reason.  JPEG doesn't support a multi-page 
(in one image file) feature... so in order to get multiple pages into a 
single file you have to wrap the JPEG up with something like TIFF or PDF.

In fact, the same goes for MH, MR, and MMR.  Although those are 
generally considered native "TIFF" formats, you can actually remove the 
TIFF wrapper from them and put a PDF wrapper around them instead and you 
still end up with a valid image data file.  Those image data formats are 
also not natively multi-page supporting without the wrapper.  When the 
fax communication occurs the only data that is transmitted is the raw 
MH/MR/MMR data... there is not any TIFF-specific portion to the fax 
protocol.

> The standard that Hylafax-4.2.2 uses for JPEG-in-TIFF transmission
> is T.30-E (from hylafax-config man page).  These LittleCMS color
> profiles refer to a T.42 fax standard.  But I think that T.30-E
> also requires a profile.
>  

T.30-E refers to T.42.  And T.42 refers to a bunch of other ITU 
documents, too.

> This is as far as my current understanding goes.  What are the
> nature of these two standards?  Why two standards for color
> images?

They're not different standards, T.30-E just limits or specifies the 
usage of T.42... it's like a wrapper spec around another spec.

T.30-E discusses the fax protocol-specific ways in which T.42 should be 
interpreted and implemented.  T.42 discusses the CIELAB colorspace 
specifics for yet other JPEG documents.

So if you're a fax (HylaFAX) developer you generally would only need to 
refer to T.30-E; however, if you're a image library (libjpeg) developer 
you'd probably only refer to T.42 to understand the differences between 
CIELAB JPEG and other JPEG types.

Lee.


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