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Re: [hylafax-users] Receiving a color fax
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.html)
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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