[XeTeX] XeTeX maintenance

Ross Moore ross.moore at mq.edu.au
Mon Apr 27 09:43:01 CEST 2015

Hi Joseph,

On 27/04/2015, at 4:19 PM, Joseph Wright wrote:

> On 27/04/2015 00:22, Ross Moore wrote:
>>> But of course that doesn't address the problem for LaTeXt users until
>>> someone writes a suitable/comparable package (maybe someone did
>>> already, I didn't try to follow).
>> I have coding for much of what is needed, using the modified pdfTeX.
>> But there is a lot that still needs to be added; e.g. PDF’s table model,
>> References, footnotes, etc. 
> Somewhat away from the original topic, but it strikes me that building a
> tagged PDF is going to be much more problematic at the macro layer than
> at the engine level: is that fair?

Certainly one needs help at the engine level, to build the tree
structures: what is a parent/child of what else.

But macros are needed to determine where new structure starts
and finishes.
Think  \section  and friends, list environments, \item  etc.

Indicators must go in at a high level, before these are decomposed
into the content:  letters, font-switches, etc.

In short, determining where structure is to be found is *much* harder 
at the engine level; but doing the book-keeping to preserve that 
structure, once known, is definitely easier when done at that level.

Philip Taylor is correct in thinking that such things can be 
better controlled in XML. But there the author has to put in 
the extra verbose markup for themselves --- hopefully with help
from some kind of interface.
However, that can involve a pretty steep learning curve anyway.

Word has had styles for decades, but how many authors actually
make proper use of them?  e.g. linking one style to another,
setting space before & after, rather than just using newlines,
and inserting space runs instead of setting tabs.
How many even know of the difference between <return>  and 
Shift-<return>  (or is it Option-<return> ) ?

The point of (La)TeX is surely to allow the human author
to not worry too much about detailed structure, but still allow
sufficient hints (via the choice of environments and macros used) 
that most things should be able to be worked out.

In particular, you need to hack into  \everypar  to determine
where the TeX mode switches from vertical to horizontal.
(LaTeX already does this, so it is delicate programming to mix
in what (La)TeX wants with what is needed for tagging.)

Doing it this way keeps things well hidden from the author,
who most likely just doesn't want to know anyway.

> Deciding what elements of a document
> are 'structure' is hard, and in 'real' documents it's not unusual to see
> a lot of input that's more about appearance than structure. That of
> course isn't limited to TeX: I suspect anyone trying to generate tagged
> output has the same concern (users do odd things).

Absolutely, as in my Word examples above.

LaTeX wants you to use a \section-like command, rather than
switching to bold-face, perhaps after inserting vertical space.
But if a human can recognise this, it should also be possible
to program TeX to recognise it. A really friendly system would
pause and question the author, perhaps with several options
available on how to proceed --- TeX can do this.
And TeX has a  \nonstopmode  to override such stoppages.

> --
> Joseph Wright

Enough on this for now.  This is surely a topic for TUG-2015.
By then we should know when the revised ADA Section 508 
will come into effect
--- or if it has been delayed or watered down. :-)



Ross Moore

Senior Lecturer
Mathematics Department  |   Level 2, E7A 
Macquarie University, NSW 2109, Australia
T: +61 2 9850 8955   |  F: +61 2 9850 8114
M: +61 407 288 255  |  http://www.maths.mq.edu.au/

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