[pdftex] Tagged PDF generated from LaTeX using unpatched pdftex

Ross Moore ross.moore at mq.edu.au
Tue Nov 22 03:52:22 CET 2016 

Hi all,

I’ve recently returned to tackle the task of generating Tagged PDF using pdftex,
in particular for PDF 2.0, PDF/A-1a  and  PDF/UA  format specifications.

This is using ordinary pdftex, not the one with extra primitives specially for the
tagging structures.
So far I’ve had more success than I’d originally thought possible.

The attached document below is an example that fully conforms to PDF/A-1a.
It also passes all the Accessibility tests in Acrobat Pro DC.

However there is some smallish issues that would be good to have implemented.

Using  \pdfliteral  for the tagging, to insert material before and after textual content,
the page-content stream looks like:

>>> 36 0 obj
>>> <<
>>> /Length 2718
>>> >>
>>> stream
>>> 1 0 0 1 108.737 686 cm
>>> /T <</MCID 0 >> BDC
>>> 1 0 0 1 -108.737 -686 cm
>>> BT
>>> /F15 10.9091 Tf 108.737 686 Td [(Here)]TJ/F17 1 Tf( )Tj/F15 10.9091 Tf 25.788 0 Td [(is)]TJ/F17 1 Tf( )Tj/F15 10.9091 Tf 10.97 0 Td [(a)]TJ/F17 1 Tf( )Tj/F15 10.9091 Tf 9.091 0 Td [(paragraph)]TJ/F17 1 Tf( )Tj/F15 10.9091 Tf 52.182 0 Td [(b)28(y)]TJ/F17 1 Tf( )Tj/F15 10.9091 Tf 15.151 0 Td [(itself.)]TJ
>>> ET
>>> 1 0 0 1 252.586 686 cm
>>> EMC
>>> …

Note
   1.  the use of interword spaces between words;
   2.  the coordinate space adjustments prior to tags an BT textual content:

1 0 0 1 108.737 686 cm
/T <</MCID 0 >> BDC
1 0 0 1 -108.737 -686 cm
BT

Further down this becomes excessive, for just a single line of text with styling:

>>> 1 0 0 1 -179.576 -10.095 cm
>>> /T <</MCID 4 >> BDC
>>> 1 0 0 1 -108.737 -563.655 cm
>>> BT
>>> /F15 10.9091 Tf 108.737 563.655 Td [(And)]TJ/F17 1 Tf( )Tj/F15 10.9091 Tf 23.94 0 Td [(another)]TJ/F17 1 Tf( )Tj/F15 10.9091 Tf 40.03 0 Td [(with)]TJ/F17 1 Tf( )Tj/F15 10.9091 Tf 24.849 0 Td [(some)]TJ
>>> ET
>>> 1 0 0 1 224.889 563.655 cm
>>> EMC
>>> /T <</MCID 5 >> BDC
>>> 1 0 0 1 -224.889 -563.655 cm
>>> BT
>>> /F16 10.9091 Tf/F17 1 Tf( )Tj/F16 10.9091 Tf 224.889 563.655 Td [(b)-32(old)]TJ/F17 1 Tf( )Tj/F16 10.9091 Tf 28.227 0 Td [(text)]TJ
>>> ET
>>> 1 0 0 1 275.245 563.655 cm
>>> EMC
>>> /T <</MCID 6 >> BDC
>>> 1 0 0 1 -275.245 -563.655 cm
>>> BT
>>> /F15 10.9091 Tf/F17 1 Tf( )Tj/F15 10.9091 Tf 275.245 563.655 Td [(.)]TJ
>>> ET
>>> 1 0 0 1 283.124 563.655 cm
>>> EMC


The length of the output can be reduced (by approx 15–20%) using   \pdfliteral direct ….
But there is a drawback, since BT … ET  and  BDC … EMC  operators
must be correctly nested, else the PDF is malformed.

viz.
>>> 36 0 obj
>>> <<
>>> /Length 2232
>>> >>
>>> stream
>>> …
>>> ...
>>> BT
>>> /F17 1 Tf 108.737 563.655 Td [( )]TJ
>>> /T <</MCID 4 >> BDC
>>> /F15 10.9091 Tf 0 0 Td [(And)]TJ/F17 1 Tf( )Tj/F15 10.9091 Tf 23.94 0 Td [(another)]TJ/F17 1 Tf( )Tj/F15 10.9091 Tf 40.03 0 Td [(with)]TJ/F17 1 Tf( )Tj/F15 10.9091 Tf 24.849 0 Td [(some)]TJ
>>> EMC
>>> /T <</MCID 5 >> BDC
>>> /F16 10.9091 Tf/F17 1 Tf( )Tj/F16 10.9091 Tf 27.333 0 Td [(b)-32(old)]TJ/F17 1 Tf( )Tj/F16 10.9091 Tf 28.227 0 Td [(text)]TJ
>>> EMC
>>> /T <</MCID 6 >> BDC
>>> /F15 10.9091 Tf/F17 1 Tf( )Tj/F15 10.9091 Tf 22.129 0 Td [(.)]TJ
>>> EMC
>>> ET

Note here that  \pdffakespace  is used immediately before the first  \pdfliteral direct
otherwise one gets incorrect nesting as:

>>> /T <</MCID 4 >> BDC
>>> BT
>>> /F15 10.9091 Tf 0 0 Td [(And)]TJ/F17 1 Tf( )Tj/F15 10.9091 Tf 23.94 0 Td [(another)]TJ/F17 1 Tf( )Tj/F15 10.9091 Tf 40.03 0 Td [(with)]TJ/F17 1 Tf( )Tj/F15 10.9091 Tf 24.849 0 Td [(some)]TJ
>>> EMC
>>> /T <</MCID 5 >> BDC
>>> ...

However that “fake space” is viewed as content, for Accessibility purposes.
It must therefore be within tags — but that cannot be achieved this way.


So here are my requests.

  1.   please add a new mode to  \pdfliteral
        e.g.    \pdfliteral text {….}
        which checks whether we have    pdf_doing_text   as true.
If so,  just do what   \pdfliteral direct   does;
otherwise do
    pdf_print_ln("BT");
    pdf_doing_text := true;
then place the contents literally.
When used correctly, textual content would follow, without needing to change   pdf_doing_text
nor include the initial “BT”.

Presumably there’ll need to be an adjustment to

       procedure pdf_begin_text; {begin a text section}

to not do   pdf_print_ln("BT”);  when  pdf_doing_text  is already true.



2.
It would be great to be able to do away with the  \pdfinterwordspaceon/off  for every word.
That is, generate shorter output with explicit spaces (when the font has it in slot 32 ) such as:

  /T <</MCID 4 >> BDC
 /F15 10.9091 Tf 0 0 Td [(And )<num>(another )<num>(with )<num>(some )]TJ
 EMC

where each <num> is calculated using the width of the space character in the font.

Not only does this reduce the (uncompressed) size considerably, but it would also
allow for the “Reflow” effect in Adobe Reader and Acrobat Pro (and other ?) PDF readers.



All the best,

        Ross




Dr Ross Moore
Mathematics Dept | Level 2, S2.638 AHH
Macquarie University, NSW 2109, Australia

T: +61 2 9850 8955  |  F: +61 2 9850 8114
M:+61 407 288 255  |  E: ross.moore at mq.edu.au

http://www.maths.mq.edu.au


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