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Subject: Re: [docbook-apps] tagged and accessible PDF document with DocBook

On 03/04/2017 18:07, Bob Stayton wrote:
Hi Holger,
No, you didn't miss anything.  The DocBook XSL stylesheets currently do
not provide support for tagged PDFs.

That said, my short investigation shows that implementing such support
is nontrivial.  Keep in mind that the DocBook stylesheets don't actually
create a PDF.  The stylesheets generate a FO version of the document,
and then an XSL-FO processor converts that to a PDF.  So DocBook XSL has
to generate additional markup in the FO output that an XSL-FO processor
can convert to PDF tags.

It looks like each of the XSL-FO processors commonly used with DocBook
(FOP, XEP, and Antenna House) have different extensions for implementing
the FO needed to generate the PDF accessibility tags.  For example:

FOP has fox:alt-text
XEP has rx:pdf-structure-tag
AH expects axf:pdftag

Not quite. If you produce either Tagged PDF or PDF/UA using AH Formatter
from either XSL-FO or HTML/XML+CSS, then AH Formatter will default to
generating a PDF Tag based on the type of the current element. For
example, fo:root generates 'Document' and fo:table generates 'Table'.
See https://www.antennahouse.com/product/ahf64/ahf-pdf.html#taggedpdf

@axf:pdftag is for when you want to override a default mapping. See
https://www.antennahouse.com/product/ahf64/ahf-ext.html#axf.pdftag. You
can use @axf:pdftag to set a different standard PDF Tag for that element
or you can provide your own non-standard Tag name, in which case AH
Formatter will also generate the mapping from the non-standard name to
the default PDF Tag for that element so that the requirement of the
spec for Tagged PDF for having a mapping to a known Tag is met.

This situation is similar to when PDF bookmarks were first implemented.
Each XSL-FO processor had their own extensions to implement that
feature, and DocBook XSL had to support all three extensions.  When XSL
1.1 standardized the markup for bookmarks, then all the XSL-FO
processors eventually implemented that standard and so did DocBook XSL.
I would be interested in adding PDF tagging to DocBook XSL.  It would
help if there were a clear spec for how to do so.  If I have to figure
it out for each of three XSL-FO processors, that's going to take some time.

There is no current W3C Working Group for XSL-FO. There is the Print and
Page Layout Community Group (https://www.w3.org/community/ppl/), of
which I am the Chair, but it takes more than just a venue to produce a
spec with enough impetus to convince multiple vendors/teams to implement it.


Tony Graham.
Senior Architect
XML Division
Antenna House, Inc.
Skerries, Ireland

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