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



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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