% language=us

% This feature has been removed because we have different control now in the
% reworked engine so this chapter cnanot be processed any longer.

\endinput

\startcomponent onandon-fences

\environment onandon-environment

% avoid context defaults:
%
% \mathitalicsmode   \plusone   % default in context
% \mathdelimitersmode\plusseven % optional in context

\def\UseMode#1{\appendtoks\mathdelimitersmode#1\to\everymathematics}

\startchapter[title={Tricky fences}]

Occasionally one of my colleagues notices some suboptimal rendering and asks me
to have a look at it. Now, one can argue about \quotation {what is right} and
indeed there is not always a best answer to it. Such questions can even be a
nuisance; let's think of the following scenario. You have a project where \TEX\
is practically the only solution. Let it be an \XML\ rendering project, which
means that there are some boundary conditions. Speaking in 2017 we find that in
most cases a project starts out with the assumption that everything is possible.

Often such a project starts with a folio in mind and therefore by decent tagging
to match the educational and esthetic design. When rendering is mostly automatic
and concerns too many (variants) to check all rendering, some safeguards are used
(an example will be given below). Then different authors, editors and designers
come into play and their expectations, also about what is best, often conflict.
Add to that rendering for the web, and devices and additional limitations show
up: features get dropped and even more cases need to be compensated (the quality
rules for paper are often much higher). But, all that defeats the earlier
attempts to do well because suddenly it has to match the lesser format. This in
turn makes investing in improving rendering very inefficient (read: a bottomless
pit because it never gets paid and there is no way to gain back the investment).
Quite often it is spacing that triggers discussions and questions what rendering
is best. And inconsistency dominates these questions.

So, in case you wonder why I bother with subtle aspects of rendering as discussed
below, the answer is that it is not so much professional demand but users (like
my colleagues or those on the mailing lists) that make me look into it and often
something that looks trivial takes days to sort out (even for someone who knows
his way around the macro language, fonts and the inner working of the engine).
And one can be sure that more cases will pop up.

All this being said, let's move on to a recent example. In \CONTEXT\ we support
\MATHML\ although in practice we're forced to a mix of that standard and
\ASCIIMATH. When we're lucky, we even get a mix with good old \TEX-encoded math.
One problem with an automated flow and processing (other than raw \TEX) is that
one can get anything and therefore we need to play safe. This means for instance
that you can get input like this:

\starttyping
f(x) + f(1/x)
\stoptyping

or in more structured \TEX\ speak:

\startbuffer
$f(x) + f(\frac{1}{x})$
\stopbuffer

\typebuffer

Using \TeX\ Gyre Pagella, this renders as: {\UseMode\zerocount\inlinebuffer}, and
when seeing this a \TEX\ user will revert to:

\startbuffer
$f(x) + f\left(\frac{1}{x}\right)$
\stopbuffer

\typebuffer

which gives: {\UseMode\zerocount \inlinebuffer}. So, in order to be robust we can
always use the \type {\left} and \type {\right} commands, can't we?

\startbuffer
$f(x) + f\left(x\right)$
\stopbuffer

\typebuffer

which gives {\UseMode\zerocount \inlinebuffer}, but let's blow up this result a
bit showing some additional tracing from left to right, now in Latin Modern:

\startbuffer[blownup]
\startcombination[nx=3,ny=2,after=\vskip3mm]
    {\scale[scale=4000]{\hbox{$f(x)$}}}
        {just characters}
    {\scale[scale=4000]{\ruledhbox{\showglyphs \showfontkerns \showfontitalics$f(x)$}}}
        {just characters}
    {\scale[scale=4000]{\ruledhbox{\showglyphs \showfontkerns \showfontitalics \showmakeup$f(x)$}}}
        {just characters}
    {\scale[scale=4000]{\hbox{$f\left(x\right)$}}}
        {using delimiters}
    {\scale[scale=4000]{\ruledhbox{\showglyphs \showfontkerns \showfontitalics$f\left(x\right)$}}}
        {using delimiters}
    {\scale[scale=4000]{\ruledhbox{\showglyphs \showfontkerns \showfontitalics \showmakeup$f\left(x\right)$}}}
        {using delimiters}
\stopcombination
\stopbuffer

\startlinecorrection
\UseMode\zerocount
\switchtobodyfont[modern]\getbuffer[blownup]
\stoplinecorrection

When we visualize the glyphs and kerns we see that there's a space instead of a
kern when we use delimiters. This is because the delimited sequence is processed
as a subformula and injected as a so|-|called inner object and as such gets
spaced according to the ordinal (for the $f$) and inner (\quotation {fenced} with
delimiters $x$) spacing rules. Such a difference normally will go unnoticed but
as we mentioned authors, editors and designers being involved, there's a good
chance that at some point one will magnify a \PDF\ preview and suddenly notice
that the difference between the $f$ and $($ is a bit on the large side for simple
unstacked cases, something that in print is likely to go unnoticed. So, even when
we don't know how to solve this, we do need to have an answer ready.

When I was confronted by this example of rendering I started wondering if there
was a way out. It makes no sense to hard code a negative space before a fenced
subformula because sometimes you don't want that, especially not when there's
nothing before it. So, after some messing around I decided to have a look at the
engine instead. I wondered if we could just give the non|-|scaled fence case the
same treatment as the character sequence.

Unfortunately here we run into the somewhat complex way the rendering takes
place. Keep in mind that it is quite natural from the perspective of \TEX\
because normally a user will explicitly use \type {\left} and \type {\right} as
needed, while in our case the fact that we automate and therefore want a generic
solution interferes (as usual in such cases).

Once read in the sequence \type {f(x)} can be represented as a list:

\starttyping
list = {
 {
  id = "noad", subtype = "ord", nucleus = {
   {
    id = "mathchar", fam = 0, char = "U+00066",
   },
  },
 },
 {
  id = "noad", subtype = "open", nucleus = {
   {
    id = "mathchar", fam = 0, char = "U+00028",
   },
  },
 },
 {
  id = "noad", subtype = "ord", nucleus = {
   {
    id = "mathchar", fam = 0, char = "U+00078",
   },
  },
 },
 {
  id = "noad", subtype = "close", nucleus = {
   {
    id = "mathchar", fam = 0, char = "U+00029",
   },
  },
 },
}
\stoptyping

The sequence \type {f \left( x \right)} is also a list but now it is a tree (we
leave out some unset keys):

\starttyping
list = {
 {
  id = "noad", subtype = "ord", nucleus = {
   {
    id = "mathchar", fam = 0, char = "U+00066",
   },
  },
 },
 {
  id = "noad", subtype = "inner", nucleus = {
   {
    id = "submlist", head = {
     {
      id = "fence", subtype = "left", delim = {
       {
        id = "delim", small_fam = 0, small_char = "U+00028",
       },
      },
     },
     {
      id = "noad", subtype = "ord", nucleus = {
       {
        id = "mathchar", fam = 0, char = "U+00078",
       },
      },
     },
     {
      id = "fence", subtype = "right", delim = {
       {
        id = "delim", small_fam = 0, small_char = "U+00029",
       },
      },
     },
    },
   },
  },
 },
}
\stoptyping

So, the formula \type {f(x)} is just four characters and stays that way, but with
some inter|-|character spacing applied according to the rules of \TEX\ math. The
sequence \typ {f \left( x \right)} however becomes two components: the \type {f}
is an ordinal noad,\footnote {Noads are the mathematical building blocks.
Eventually they become nodes, the building blocks of paragraphs and boxed
material.} and \typ {\left( x \right)} becomes an inner noad with a list as a
nucleus, which gets processed independently. The way the code is written this is
what (roughly) happens:

\startitemize
\startitem
    A formula starts; normally this is triggered by one or two dollar signs.
\stopitem
\startitem
    The \type {f} becomes an ordinal noad and \TEX\ goes~on.
\stopitem
\startitem
    A fence is seen with a left delimiter and an inner noad is injected.
\stopitem
\startitem
    That noad has a sub|-|math list that takes the left delimiter up to a
    matching right one.
\stopitem
\startitem
    When all is scanned a routine is called that turns a list of math noads into
    a list of nodes.
\stopitem
\startitem
    So, we start at the beginning, the ordinal \type {f}.
\stopitem
\startitem
    Before moving on a check happens if this character needs to be kerned with
    another (but here we have an ordinal|-|inner combination).
\stopitem
\startitem
    Then we encounter the subformula (including fences) which triggers a nested
    call to the math typesetter.
\stopitem
\startitem
    The result eventually gets packaged into a hlist and we're back one level up
    (here after the ordinal \type {f}).
\stopitem
\startitem
    Processing a list happens in two passes and, to cut it short, it's the second
    pass that deals with choosing fences and spacing.
\stopitem
\startitem
    Each time when a (sub)list is processed a second pass over that list
    happens.
\stopitem
\startitem
    So, now \TEX\ will inject the right spaces between pairs of noads.
\stopitem
\startitem
    In our case that is between an ordinal and an inner noad, which is quite
    different from a sequence of ordinals.
\stopitem
\stopitemize

It's these fences that demand a two-pass approach because we need to know the
height and depth of the subformula. Anyway, do you see the complication? In our
inner formula the fences are not scaled, but this is not communicated back in the
sense that the inner noad can become an ordinal one, as in the simple \type {f(}
pair. The information is not only lost, it is not even considered useful and the
only way to somehow bubble it up in the processing so that it can be used in the
spacing requires an extension. And even then we have a problem: the kerning that
we see between \type {f(} is also lost. It must be noted that this kerning is
optional and triggered by setting \type {\mathitalicsmode=1}. One reason for this
is that fonts approach italic correction differently, and cheat with the
combination of natural width and italic correction.

Now, because such a workaround is definitely conflicting with the inner workings
of \TEX, our experimenting demands another variable be created: \type
{\mathdelimitersmode}. It might be a prelude to more manipulations but for now we
stick to this one case. How messy it really is can be demonstrated when we render
our example with Cambria.

\startlinecorrection
\UseMode\zerocount
\switchtobodyfont[cambria]\getbuffer[blownup]
\stoplinecorrection

If you look closely you will notice that the parenthesis are moved up a bit. Also
notice the more accurate bounding boxes. Just to be sure we also show Pagella:

\startlinecorrection
\UseMode\zerocount
\switchtobodyfont[pagella]\getbuffer[blownup]
\stoplinecorrection

When we really want the unscaled variant to be somewhat compatible with the
fenced one we now need to take into account:

\startitemize[packed]
\startitem
    the optional axis|-|and|-|height|/|depth related shift of the fence (bit 1)
\stopitem
\startitem
    the optional kern between characters (bit 2)
\stopitem
\startitem
    the optional space between math objects (bit 4)
\stopitem
\stopitemize

Each option can be set (which is handy for testing) but here we will set them
all, so, when \type {\mathdelimitersmode=7}, we want cambria to come out as
follows:

\startlinecorrection
\UseMode\plusseven
\switchtobodyfont[cambria]\getbuffer[blownup]
\stoplinecorrection

When this mode is set the following happens:

\startitemize
\startitem
    We keep track of the scaling and when we use the normal size this is
    registered in the noad (we had space in the data structure for that).
\stopitem
\startitem
    This information is picked up by the caller of the routine that does the
    subformula and stored in the (parent) inner noad (again, we had space for
    that).
\stopitem
\startitem
    Kerns between a character (ordinal) and subformula (inner) are kept,
    which can be bad for other cases but probably less than what we try
    to solve here.
\stopitem
\startitem
    When the fences are unscaled the inner property temporarily becomes
    an ordinal one when we apply the inter|-|noad spacing.
\stopitem
\stopitemize

Hopefully this is good enough but anything more fancy would demand drastic
changes in one of the most sensitive mechanisms of \TEX. It might not always work
out right, so for now I consider it an experiment, which means that it can be
kept around, rejected or improved.

In case one wonders if such an extension is truly needed, one should also take
into account that automated typesetting (also of math) is probably one of the
areas where \TEX\ can shine for a while. And while we can deal with much by using
\LUA, this is one of the cases where the interwoven and integrated parsing,
converting and rendering of the math machinery makes it hard. It also fits into a
further opening up of the inner working by modes.

\startbuffer[simple]
\dontleavehmode
\scale
    [scale=3000]
    {\ruledhbox
        {\showglyphs
         \showfontkerns
         \showfontitalics
         $f(x)$}}
\stopbuffer

\startbuffer[fenced]
\dontleavehmode
\scale
    [scale=3000]
    {\ruledhbox
        {\showglyphs
         \showfontkerns
         \showfontitalics
         $f\left(x\right)$}}
\stopbuffer

\def\TestMe#1%
  {\bTR
       \bTD[width=35mm,align=middle,toffset=3mm] \switchtobodyfont[#1]\UseMode\zerocount\getbuffer[simple] \eTD
       \bTD[width=35mm,align=middle,toffset=3mm] \switchtobodyfont[#1]\UseMode\zerocount\getbuffer[fenced] \eTD
       \bTD[width=35mm,align=middle,toffset=3mm] \switchtobodyfont[#1]\UseMode\plusseven\getbuffer[simple] \eTD
       \bTD[width=35mm,align=middle,toffset=3mm] \switchtobodyfont[#1]\UseMode\plusseven\getbuffer[fenced] \eTD
   \eTR
   \bTR
       \bTD[align=middle,nx=2] \type{\mathdelimitersmode=0} \eTD
       \bTD[align=middle,nx=2] \type{\mathdelimitersmode=7} \eTD
   \eTR
   \bTR
       \bTD[align=middle,nx=4] \switchtobodyfont[#1]\bf #1 \eTD
   \eTR}

\startbuffer
\bTABLE[frame=off]
    \TestMe{modern}
    \TestMe{cambria}
    \TestMe{pagella}
\eTABLE
\stopbuffer

Another objection to such a solution can be that we should not alter the engine
too much. However, fences already are an exception and treated specially (tests
and jumps in the program) so adding this fits reasonably well into that part of
the design.

In the following examples we demonstrate the results for Latin Modern, Cambria
and Pagella when \type {\mathdelimitersmode} is set to zero or one. First we show
the case where \type {\mathitalicsmode} is disabled:

\startlinecorrection
    \mathitalicsmode\zerocount\getbuffer
\stoplinecorrection

When we enable \type {\mathitalicsmode} we get:

\startlinecorrection
    \mathitalicsmode\plusone  \getbuffer
\stoplinecorrection

So is this all worth the effort? I don't know, but at least I got the picture and
hopefully now you have too. It might also lead to some more modes in future
versions of \LUATEX.

\startbuffer[simple]
\dontleavehmode
\scale
    [scale=2000]
    {\ruledhbox
        {\showglyphs
         \showfontkerns
         \showfontitalics
         $f(x)$}}
\stopbuffer

\startbuffer[fenced]
\dontleavehmode
\scale
    [scale=2000]
    {\ruledhbox
        {\showglyphs
         \showfontkerns
         \showfontitalics
         $f\left(x\right)$}}
\stopbuffer

\def\TestMe#1%
  {\bTR
       \dostepwiserecurse{0}{7}{1}{
           \bTD[align=middle,toffset=3mm] \switchtobodyfont[#1]\UseMode##1\getbuffer[simple] \eTD
        }
   \eTR
   \bTR
       \dostepwiserecurse{0}{7}{1}{
           \bTD[align=middle,toffset=3mm] \switchtobodyfont[#1]\UseMode##1\getbuffer[fenced] \eTD
        }
   \eTR
   \bTR
       \dostepwiserecurse{0}{7}{1}{
           \bTD[align=middle]
              \tttf
              \ifcase##1\relax
              \or ns       % 1
              \or    it    % 2
              \or ns it    % 3
              \or       or % 4
              \or ns    or % 5
              \or    it or % 6
              \or ns it or % 7
              \fi
           \eTD
       }
   \eTR
   \bTR
       \bTD[align=middle,nx=8] \switchtobodyfont[#1]\bf #1 \eTD
   \eTR}

\startbuffer
\bTABLE[frame=off,distance=2mm]
    \TestMe{modern}
    \TestMe{cambria}
    \TestMe{pagella}
\eTABLE
\stopbuffer

\startlinecorrection
\getbuffer
\stoplinecorrection

In \CONTEXT, a regular document can specify \type {\setupmathfences
[method=auto]}, but in \MATHML\ or \ASCIIMATH\ this feature is enabled by default
(so that we can test it).

We end with a summary of all the modes (assuming italics mode is enabled) in the
table below.

\stopcomponent
