Re: [hylafax-users] Receiving a color fax

[Blas Rodriguez Somoza <blas@xxxxxxxxxxxxx>]

Hello Lee

   Another looong mail, although I hope it helps and don't tire too much.

Lee Howard wrote:

> Blas Rodriguez Somoza wrote:
>
> >   Excuse me if the mail are too long, but I expend hours trying to 
> > analyze the problem I and wish to explain the process, perhaps others 
> > can get some valuable information. ;-)
>
> I actually enjoy these kinds of discussions very much.  I only wish 
> that I knew more about JPEG.
Fine

> > After this result which don't answer the question I've found that the 
> > color space for a fax is not CIELab nor ICCLab but ITULab and so it 
> > is coded the tif.
>
>
> Initially I assumed that color JPEG faxes were ITULab because of the 
> "ITU" involvment there.  However, I assumed incorrectly.  ITU-T.30 
> Table 2 Note 35 (in the 2003 revision) clearly indicates that color 
> JPEG faxes use CIELab color space.  This was further reinforced by my 
> discussions with those on the IJG mailing lists, in particular with 
> the author of the CIELAB libjpeg patch, John Barber.  His libjpeg 
> patches are here:
The question is that the CIELab color space is not the same that CIELAB 
encoding. CIEL*a*b have at least three encodings CIELab, ICC(Lab) and 
ITU(Lab).

The CIELAB encoding, which is the first to be used, was defined to be 
able to encode with 8/16 bits the l*a*b values which are unbounded. 
(uses L unsigned, a*b signed)
ICCLab modify CIELAB encoding to match the encoding used by the ICC 
profile specification. (uses L*a*b unsigned)
ITULab modify CIELAB encoding and adds a gamut range which allows to 
better represent the colors for different image sources.
-- The ITULAB gamut range can be in tiff with the optional tiff tag 
"Decode" (http://www.awaresystems.be/imaging/tiff/tifftags/decode.php)

I can give you two pages about that refers to the color space and it 
three encodings: CIELab, ICC(Lab) and ITU(Lab)

http://davengrace.com/cgi-bin/cspace.pl
http://www.inventoland.net/imaging/cii/ei02.pdf (page 34)

** About the ITU standards

The T.30 Annex E Standard (version 04_99, page 147) mention CIELAB color 
space but refers to T.4 Annex about encoding.

In T.4 Annex (version 07_03, page 41) ITU defines a encoding for L*a*b 
values which the one called usually ITULab encoding.  You can verify 
that with the formulas in the previous link.

The full definition of the color space/encoding are in T.42.

** About CIE L*a*b codings in tiff

- The TIFF 6.0 Specification only mentions one encoding (CIELAB encoding)
- The TIFF 6.0 Specification suplement 1 add ICCLab.
- The TIFF 6.0 Specification suplement 2 is dedicated only to make 
details about CIELab and ICCLab.
- And finally ITULab appears only in TIFF-FX (RFC 2301 File Format for 
Internet Fax)

In the RFC2301 (chapter 6.2.1) there is very clear reference to the 
encoding used in faxes :

PhotometricInterpretation(262) = 10. SHORT Base color fax mode requires 
pixel values to be stored using the CIE L*a*b* encoding defined in ITU-T 
Rec. T.42. This encoding is indicated by the PhotometricInterpretation 
value 10, referred to as ITULAB

> And we will need to use more of them in order to get color fax 
> *sending* going (if we intend to get the tagline encoded in a lossless 
> manner).
>
> All this said, on a purely theoretical and technical level the sending 
> fax machine could encode the image in whatever format it chooses.  It 
> could use CIELab or ITULab or whatever format it wanted.  As we've 
> seen many, many times before fax machines do not always adhere to the 
> standard (which appears to be CIELAB).  The purest, and most correct 
> method to determine which type of colorspace is used by the fax image 
> would be to look inside the JPEG image data itself, looking for the 
> JPEG markers which will indicate which type of colorspace is used.  
> And, as a fax receiver, we would need to be able to identify this and 
> use the information to set the appropriate TIFF tag.  

Yes, it can be in 1-bit RGB, 1-bit CMY, 1-bit CMYK, indexed (palette) 
and CIELAB/ITULab

I'll give a try to get the correct value from the jpeg and put it in the 
tiff.

About standards, at least for tiff-fx it seems some fax manufacturers 
make efforts to assure compliance and interoperability,

http://www.ietf.org/IESG/Implementations/rfc2301-implementation-report.txt

> > The question about the DataFormat remains unanswered.
>
> The JPEG and JBIG DataFormat listing comes from the FaxDCS TIFF tag.  
> Do a 'tiffinfo' on my file and compare it against the 'tiffinfo' 
> output on your file.  There was an enhancement to the faxinfo utility 
> that parsed the FaxDCS tag to interpret the JPEG and JBIG formats from 
> it.
>
> > The only difference that seems significative between your log and 
> > mine which I can see is that I don't get any CallID and a line in 
> > yours I don't get
> >
> > RECV/CQ: Found Define Number of Lines (DNL) Marker, lines: 2112
> This requires that you have Copy Quality checking and correction 
> logging turned on in your SessionLogging.
>
> If, after enabling the logging, you still do not see DNL markers 
> logged it must mean that the fax machine does not use DNL markers ... 
> which would mean that it does not transmit JPEG pages of variable 
> length - that the page length is predetermined before the fax begins.
Thanks, I'll try to find the issue.

> Lee.


Blas

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