% language=us runpath=texruns:manuals/ontarget

% musical timestamp: 2022/07/01 while listening to Hania Rani @ https://www.youtube.com/watch?v=sp3B97N67Cw

% todo apply withprescripts -> to all components

\usemodule[math-tweaks]

\startcomponent ontarget-mathfonts

\environment ontarget-style

\startchapter[title={Dealing with math fonts}]

\startsubject[title=Introduction]

Here we will explain some of the tricks that we apply to math fonts so that they
not only work better with the \LUAMETATEX\ math engine but also look better, at
least in our opinion. We will not show specific fonts because after all, who can
complain about something that comes for free, but you can see whatever we do to
make it work in action in \CONTEXT\ where we setup these fonts. This is also a
summary of what Mikael Sundqvist and I have been doing for a while now: improve
the rendering of math, a rather enjoyable experience, also because we ran into
humorous effects with and properties of fonts. Because we consider ourselves free
from any conventions we could happily explore solution.

\stopsubject

\startsubject[title=Fences]

\startMPdefinitions
path My_left_fence    ; My_left_fence    := (1,1) -- (0,1) -- (0,-1) -- (1,-1) ;
path My_right_fence   ; My_right_fence   := (0,1) -- (1,1) -- (1,-1) -- (0,-1) ;
path My_middle_fence  ; My_middle_fence  := (0,1)                    -- (0,-1) ;

path My_right_fence_x ; My_right_fence_x   := (0,1) -- (1, 1/2) -- (1,-1/2) -- (0,-1) ;

path My_left_top      ; My_left_top      := (1, 1  ) -- (0, 1  ) -- (0, 1/2) ;
path My_left_middle   ; My_left_middle   := (0, 1/2) -- (0,-1/2) ;
path My_left_bottom   ; My_left_bottom   := (0,-1/2) -- (0,-1  ) -- (1,-1  ) ;

path My_right_top     ; My_right_top     := (0, 1  ) -- (1, 1  ) -- (1, 1/2) ;
path My_right_middle  ; My_right_middle  := (1, 1/2) -- (1,-1/2) ;
path My_right_bottom  ; My_right_bottom  := (1,-1/2) -- (1,-1  ) -- (0,-1  ) ;

path My_right_top_x    ; My_right_top_x     := (0, 1  ) -- (1, 1/2) ;
path My_right_middle_x ; My_right_middle_x  := (1, 1/2) -- (1,-1/2) ;
path My_right_bottom_x ; My_right_bottom_x  := (1,-1/2) -- (0,-1  ) ;

path My_middle_top    ; My_middle_top    := (0, 1  ) -- (0, 1/2) ;
path My_middle_middle ; My_middle_middle := (0, 1/2) -- (0,-1/2) ;
path My_middle_bottom ; My_middle_bottom := (0,-1/2) -- (0,-1  ) ;

path My_content       ; My_content       := fullcircle ;

vardef My_Fenced (expr left, middle, right, size) =
    image (
        draw My_left_fence   xyscaled (1,left  ) shifted (0,0) withcolor darkgray ;
        draw My_content      xyscaled (2,size  ) shifted (3,0) withcolor darkblue ;
        draw My_middle_fence xyscaled (1,middle) shifted (5,0) withcolor darkgray ;
        draw My_content      xyscaled (2,size  ) shifted (7,0) withcolor darkblue ;
        if left <> right :
            draw My_right_fence_x xyscaled (1,right ) shifted (9,0) withcolor darkgray ;
        else :
            draw My_right_fence   xyscaled (1,right ) shifted (9,0) withcolor darkgray ;
        fi ;
    )
enddef ;

vardef My_Extensibled (expr left, middle, right, size) =
    image (
        draw        My_left_top      xyscaled (1,left  ) shifted (0,0) withcolor darkgray ;
        draw        My_left_middle   xyscaled (1,left  ) shifted (0,0) withcolor darkgreen ;
        draw        My_left_bottom   xyscaled (1,left  ) shifted (0,0) withcolor darkgray ;
        drawpoints  My_left_middle   xyscaled (1,left  ) shifted (0,0) withpen pencircle scaled .4 withcolor white ;
        drawpoints  My_left_top      xyscaled (1,left  ) shifted (0,0) withpen pencircle scaled .2 withcolor red ;
        drawpoints  My_left_bottom   xyscaled (1,left  ) shifted (0,0) withpen pencircle scaled .2 withcolor red ;
        draw        My_content       xyscaled (2,size  ) shifted (3,0) withcolor darkblue ;
        draw        My_middle_top    xyscaled (1,middle) shifted (5,0) withcolor darkgray ;
        draw        My_middle_middle xyscaled (1,middle) shifted (5,0) withcolor darkgreen ;
        draw        My_middle_bottom xyscaled (1,middle) shifted (5,0) withcolor darkgray ;
        drawpoints  My_middle_middle xyscaled (1,middle) shifted (5,0) withpen pencircle scaled .4 withcolor white ;
        drawpoints  My_middle_top    xyscaled (1,middle) shifted (5,0) withpen pencircle scaled .2 withcolor red ;
        drawpoints  My_middle_bottom xyscaled (1,middle) shifted (5,0) withpen pencircle scaled .2 withcolor red ;
        draw        My_content       xyscaled (2,size  ) shifted (7,0) withcolor darkblue ;
        if left <> right :
            draw        My_right_top     xyscaled (1,right ) shifted (9,0) withcolor darkgray ;
            draw        My_right_middle  xyscaled (1,right ) shifted (9,0) withcolor darkgreen ;
            draw        My_right_bottom  xyscaled (1,right ) shifted (9,0) withcolor darkgray ;
            drawpoints  My_right_middle  xyscaled (1,right ) shifted (9,0) withpen pencircle scaled .4 withcolor white ;
            drawpoints  My_right_top     xyscaled (1,right ) shifted (9,0) withpen pencircle scaled .2 withcolor red ;
            drawpoints  My_right_bottom  xyscaled (1,right ) shifted (9,0) withpen pencircle scaled .2 withcolor red ;
        else :
            draw        My_right_top_x     xyscaled (1,right ) shifted (9,0) withcolor darkgray ;
            draw        My_right_middle_x  xyscaled (1,right ) shifted (9,0) withcolor darkgreen ;
            draw        My_right_bottom_x  xyscaled (1,right ) shifted (9,0) withcolor darkgray ;
            drawpoints  My_right_middle_x  xyscaled (1,right ) shifted (9,0) withpen pencircle scaled .4 withcolor white ;
            drawpoints  My_right_top_x     xyscaled (1,right ) shifted (9,0) withpen pencircle scaled .2 withcolor red ;
            drawpoints  My_right_bottom_x  xyscaled (1,right ) shifted (9,0) withpen pencircle scaled .2 withcolor red ;
        fi ;
    )
enddef ;
\stopMPdefinitions

Fences come in two variants: fixed sizes and so called extensibles that are
constructed from recipes that combine snippets that can partially overlap. Fenced
material has an optional symbol at the start, an optional one at the end and zero
or more symbols in the middle. Here we have all three:

\startlinecorrection
\startMPcode
    draw My_Fenced (2,2,2,2) scaled 5mm ;
\stopMPcode
\stoplinecorrection

Ideally these symbols scale in the same way, depending on the other context. This
means that \TEX\ first has to measure what sits in between but we will not dive
into that. The left and right symbols are normally pairs like parentheses or
braces, but any mix is possible. Ideally a font is designed with this in mind but
unfortunately we see this:

\startlinecorrection
\startMPcode
    draw My_Fenced (2,3,2,2) scaled 5mm ;
\stopMPcode
\stoplinecorrection

And even this:

\startlinecorrection
\startMPcode
    draw My_Fenced (2,3,2.5,2) scaled 5mm ;
\stopMPcode
\stoplinecorrection

It might be a side effect of the limited amount fo available slots in traditional
math fonts that also resulted in non consistent sets in \OPENTYPE\ follow|-|ups
and when one fonts does that more follow that approach.

You can find rendering like this:

\startlinecorrection
\startMPcode
    draw My_Fenced (3,2,3,2) scaled 5mm ;
\stopMPcode
\stoplinecorrection

and this

\startlinecorrection
\startMPcode
    draw My_Fenced (2,2,2,6) scaled 5mm ;
\stopMPcode
\stoplinecorrection

because a programmable language like \TEX\ can use some tricks to force sizes: we
just create some local fence which dimension is determined by some invisible
rule. In \LUAMETATEX\ we can actually enforce dimensions and in \CONTEXT\ we can
filter specific sizes.

So how does this sizing work? A fence character starts out with the normal size
but then a larger one is needed, the math engine will check if there is a larger
variant. An \OPENTYPE\ font can provide these and in the engine that works out as
following a linked list to a next size. When we run out of sizes there can be an
extension recipe present where a fence is made from snippets pasted together.
Normally that goes unnoticed because there is a little overlap between these
snippets.

\startlinecorrection
\startMPcode
    draw My_Extensibled (2,2,2,4) scaled 5mm ;
\stopMPcode
\stoplinecorrection

A larger fence will simply add more middle pieces, and it will not do as below:

\startlinecorrection
\startMPcode
    draw My_Extensibled (3,3,3,6) scaled 5mm ;
\stopMPcode
\stoplinecorrection

Because we're talking of a deliberate design you cannot simply scale snippets and
expect them to work out well visually. However, in a pure vertical case one
actually could and in practice all these extensibles have (of course) vertical
bars. Anyway, in the above example the larger middle piece actually is just
several middle pieces overlapping.

However, as we mentioned, fonts are not always consistent. First of all, when we
run over the (increasing in size) variants we have discrete steps and you're
lucky if a font has more than half a dozen. As soon as we end up with the
extensible the size can be matched well.

So how do we compensate for misbehavior? There are two parameters in \TEX\ then
determine the matching: \typ {\delimiterfactor} and \typ {\delimitershortfall}.
Plain \TEX\ set them to 901 and 5pt which works okay in most cases. In \CONTEXT\
we set them to 1000 and 0pt and instead use the parameters \typ
{\UmathDelimiterPercent} and \typ {\UmathDelimiterShortfall} that are bound fo
fonts. In addition to that we use the \typ {nooverflow} keyword with \type
{\Umiddle} which makes sure that we always stay within the size of the outer
fences. That just looks better.

In addition we can tweak the dimensions of glyphs and apply effects such as
expanding so that we get a bit more consistent visual appearance. We can also
signal that we should ignore sizes larger than a given index.

The next sequence show what happens in practice when we tell \type {\Umiddle} to
never exceed the requested size. Because we start with stepwise sizes the first
part of this sequence has no matching sizes. At some point we end up at the
extensibles.

\showmathmiddletweaks

Its is worth noticing that we tried several alternative approaches. For instance
what happens when we only use extensibles? In that case there will be no fit for
the smaller ones because the at least two parts of an extensible can seldom
completely overlap that much.

Actually, when we tested that we noticed that even in valid situations there can
be strange overlap. At that time for instance the Lucida fonts had overlapping
artifact in some curly braces which we found out when we tried to nil some of the
larger, odd looking, step variants. Latin modern and some of the gyre fonts had
unexpected jumps to larger sizes which made us decide to make the delimiter
parameters font specific so that we could more easily adapt them: they basically
became part of the math parameters of a font.

There are also inconsistencies in the perceived widths of glyphs used: often the
bars are too thin. That can be solved by applying effects like scaling
horizontally or vertically and cheating a bit with the dimensions. Another
solution is that we ignore the variants after a certain size and force
extensibles sooner but that of course needs to be tested for unwanted overlaps
too. All these tricks combined make it possible to use math fonts with imperfect
fences more or less reliable.

\startitemize[packed]
\startitem
    Provide an equal amount of fixed size larger variants for all fences: assume
    arbitrary pairing.
\stopitem
\startitem
    Because fonts have plenty room, provide some ten variants before going
    extensible.
\stopitem
\startitem
    Try to make the variants and the extensibles similar in look.
\stopitem
\startitem
    Ensure that the width of the vertical bar matches the design.
\stopitem
\startitem
    Make sure that the odd entries in a extensible recipe don't overlap badly.
\stopitem
\stopitemize

\stopsubject

\startsubject[title=Accents]

When traditional \TEX\ showed up it was not that common to have pre|-|composed
characters so when you needed something with an accept on top the way to go was
to typeset the base character and position the accent on top using either the
\type {\accent} primitive or some macro. It always was a compromise but
eventually fonts with more assembled characters showed up. In \OPENTYPE\ fonts
that operate in the \UNICODE\ domain we have even more characters but even there
characters can be composed. However, anchors that help achieving this are part of
the format. For text we use the mark features and for math we use the top anchor.
Given that, why do we need to tweak it?

\startMPdefinitions
path My_top_accent ; My_top_accent    := (-1/2,0) -- (0,1/4) -- (1/2,0) ;

path My_content    ; My_content       := fulldiamond ;

vardef My_Accent (expr shift, accent, size, slant) =
    image (
        path p ; p := My_content
            xyscaled (2,size)
            slanted slant
        ;
        draw My_top_accent
            xyscaled (1,accent)
            shifted (0,2)
            withcolor darkgray ;
        draw p
            withcolor darkblue ;
    )
enddef ;

vardef My_Accent_X (expr shift, accent, size, slant, rotation) =
    image (
        path p ; p := My_content rotated rotation
            xyscaled (2,size)
            slanted slant
        ;
        draw My_top_accent
            xyscaled (1,accent)
            shifted point 2 of p
            shifted - (0,-1)
            withcolor darkgray ;
        draw p
            withcolor darkblue ;
    )
enddef ;

vardef My_Accent_Y (expr shift, accent, size, scale) =
    image (
        path p ; p := My_content
            xyscaled (2,size)
        ;
        draw My_top_accent xscaled scale
            xyscaled (1,accent)
            shifted (0,2)
            withcolor darkgray ;
        draw p
            withcolor darkblue ;
    )
enddef ;

vardef My_Accent_Z (expr shift, accent, size, scale) =
    image (
        path p ; p := My_content
            xyscaled (2,size)
        ;
        path q ; q := My_top_accent
            xscaled scale
            xyscaled (1,accent)
            shifted (0,2)
        ;
        draw q
            withcolor darkgray ;
        draw p
            withcolor darkblue ;
        draw outerboundingbox q
            withpen pencircle scaled 1/20
            withcolor darkred ;
        draw outerboundingbox p
            withpen pencircle scaled 1/20
            withcolor darkred ;
    )
enddef ;
\stopMPdefinitions

Here we have a base character with an accent on top. The character is upright and
the accent gets positioned in the middle.

\startlinecorrection
\startMPcode
    draw My_Accent (2.5,1,2,0) scaled 5mm ;
\stopMPcode
\stoplinecorrection

This doesn't work out well if we have a slanted or italic shape:

\startlinecorrection
\startMPcode
    draw My_Accent (2.5,1,2,.2) scaled 5mm ;
    draw My_Accent (2.5,1,2,.4) scaled 5mm shifted (2cm,0) ;
    draw My_Accent (2.5,1,2,.6) scaled 5mm shifted (4cm,0) ;
\stopMPcode
\stoplinecorrection

So we need to compensate, for instance like this:

\startlinecorrection
\startMPcode
    draw My_Accent_X (2.5,1,2,.2,0) scaled 5mm ;
    draw My_Accent_X (2.5,1,2,.4,0) scaled 5mm shifted (2cm,0) ;
    draw My_Accent_X (2.5,1,2,.6,0) scaled 5mm shifted (4cm,0) ;
\stopMPcode
\stoplinecorrection

However, what does determine the right anchor point? From this example you can
conclude that it is the top of the character. It is probably for that reason why
the semi automated construction of Latin Modern and the Gyre fonts have quite
some anchors that are rather bad: getting the anchors right is more a visual job
than something that can be automated. The topmost point is not really the best
one to focus on.

\startlinecorrection
\startMPcode
    draw My_Accent_X (2.5,1,2,.2,35) scaled 5mm ;
    draw My_Accent_X (2.5,1,2,.4,35) scaled 5mm shifted (2cm,0) ;
    draw My_Accent_X (2.5,1,2,.6,35) scaled 5mm shifted (4cm,0) ;
\stopMPcode
\stoplinecorrection

Here the topmost position is very off center. In for instance Latin Modern that
means that on digits like~7 and~4 you get very weird anchoring. And this is why
we have a tweak that just wipes all the anchors from an alphabet: most alphabets
don't need them anyway and the engine will use the center when no anchor is
defined. Just for the record: in traditional \TEX\ engines the horizontal
position is determined by the kern between a so called skew character and the
base character. The font format has no anchor field but it has kerns, so this
trick makes much sense.

We discussed vertical extensible that grow but horizontally we have accents that
can grow. There are also a few horizontal fences like braces that have
extensibles but we will cover that later. Accents are such that they only have a
fixed set of variants and one problem is that there are often not enough of them.
This means that the engine has to choose one that is reasonable.

\startlinecorrection
\startMPcode
    draw My_Accent_Y (2.5,1,2,1)   scaled 5mm ;
    draw My_Accent_Y (2.5,1,2,2)   scaled 5mm shifted (2cm,0) ;
    draw My_Accent_Y (2.5,1,2,2.5) scaled 5mm shifted (4cm,0) ;
    draw My_Accent_Y (2.5,1,2,3)   scaled 5mm shifted (6cm,0) ;
\stopMPcode
\stoplinecorrection

In the example above the first two are acceptable but the third and fourth are
not. Just imagine that there is a superscript or subscript involved. Here we
apply another cheat: we lie about the dimensions. A glyph can have left and right
margins that get subtracted when the accent analyzer tries to make a fit which
means that we can sort of enforce the second solution. The \CONTEXT\ font goodie
files set the margins for some problematic characters because (of course) these
are not part of \OPENTYPE\ math fonts. This is an optical issue mostly because the
engine will not easily put a too wide one on top.

\startlinecorrection
\startMPcode
    draw My_Accent_Z (2.5,1.0,2,1) scaled 5mm ;
    draw My_Accent_Z (2.5,1.5,2,2) scaled 5mm shifted ( 2cm,0) ;
    draw My_Accent_Z (2.5,2.0,2,3) scaled 5mm shifted ( 4cm,0) ;
\stopMPcode
\stoplinecorrection

Watch how the larger accents also have a larger bounding box. That is all right
but does interfere with a consistent makeup. The solution is simple: we use the
\CONTEXT\ dimension tweak to reposition accents and cheat with their height and
depth to make sure that we get consistent rendering. We need to tweak anyway
because sometimes accents have bad dimensions. The smallest one is actually a
text accent and therefore can have properties that are inconsistent with its
wider variants. This is typically a side effect of the fact that math accents and
text accents are not considered to be different.

\startitemize[packed]
\startitem
    Only add anchors to some (forward leaning!) italic shapes.
\stopitem
\startitem
    Make extensibles as much a possible consistent with respect to dimensions.
\stopitem
\stopitemize

\stopsubject

\startsubject[title=Kerning]

In a text font there are two mechanisms that influence the spacing between
individual characters: kerning and italic corrections. In \OPENTYPE\ text fonts
we have a more generalized relative positioning mechanism which can be seen as
kerning. The italic correction well known to \TEX\ users can be implemented as a
positional font feature but very seldom is.

An \OPENTYPE\ math font has both kerning and italic correction. The kerning at
the left top, left bottom, right top and right bottom of a glyph can be specified
as a staircase and is used to position scripts. The italic correction is bit more
curious and is applied is some cases. Keep in mind that in math a sequence of
alphabetic characters does not make a word but represents a multiplication of
ordinary symbols and thereby specific inter|-|atom spacing rules apply.

A traditional \TEX\ font has kerns and italic correction and an \OPENTYPE\ font has
staircase kerns and italic correction. For practical (space and time) reasons the
widths of italic shapes in traditional math fonts are such that when you add the
correction they kind of match the bounding box.

So, one way to suit both kind of fonts is to add the italic correction (often
absent in \OPENTYPE\ glyphs but very present in traditional ones) as well as the
staircase kerns (present in \OPENTYPE\ fonts and unknown to traditional fonts).

There are however some complications: what if a glyph has both? Which one is to
be preferred? Should we try our luck? Even worse: in the traditional the italic
correction is {\em always} added to the box that wraps a glyph but in some cases
that correction gets removed. \footnote {In \LUAMETATEX\ we never remove but
compensate so that we can track what happens.} But should we also remove a
staircase kern? When we started with \LUAMETATEX\ it was a bit of a gamble
because the specification only showed up later and improved over time. \footnote
{This is true for much of \OPENTYPE\ which has the danger that bugs and side
effects become features. Keep that in mind when you criticize solutions that
early adopters came up with!}

In \LUATEX\ we often have to code paths. That was done after other attempts to
deal with this weak aspect of fonts worked for one font and not for the other.
For instance at the time of this writing some fonts have italic corrections for
upright characters! In \LUAMETATEX\ that model was changed into a detailed
control model at the font as well as engine level. Later, when Mikael and I went
over the fonts, usage of characters in math, atoms and spacing, we decided to
kick out that detailed control and use more general control mechanisms assuming
that we use \OPENTYPE\ fonts without bad italics. Whenever we had bad ones we
could correct that in a so called font goodie file. In other words: no heuristics
in the engine but fixed fonts. For that we always take Cambria as reference.

{\em To be considered: should we finally turn italic correction into top and
bottom kerns? Basically: model after Cambria? Then we can kick out code! We just
assume font goodies.}

\startMPdefinitions
vardef My_Character (expr shape, sx, sy, slant, left, right, side) =
    image (
        path p ; p := shape
            xyscaled (sx, sy)
            slanted slant
        ;
        draw p
            withcolor darkgray ;
        draw (outerboundingbox p) leftenlarged side rightenlarged side
            withpen pencircle scaled 1/20
            withcolor darkgreen ;
        draw (outerboundingbox p) leftenlarged left rightenlarged right
            withpen pencircle scaled 1/20
            withcolor darkred ;
    )
enddef ;

vardef My_Character_K (expr shape, sx, sy, slant, left, right, lt, lb, rt, rb) =
    image (
        path p ; p := shape
            xyscaled (sx, sy)
            slanted slant
        ;
        draw p
            withcolor darkgray ;
        draw (outerboundingbox p) leftenlarged left rightenlarged right
            withpen pencircle scaled 1/20
            withcolor darkred ;
        draw (ulcorner outerboundingbox p) shifted (lt,0)
            withpen pencircle scaled 1/2
            withcolor darkgreen ;
        draw (llcorner outerboundingbox p) shifted (lb,0)
            withpen pencircle scaled 1/2
            withcolor darkgreen ;
        draw (urcorner outerboundingbox p) shifted (rt,0)
            withpen pencircle scaled 1/2
            withcolor darkgreen ;
        draw (lrcorner outerboundingbox p) shifted (rb,0)
            withpen pencircle scaled 1/2
            withcolor darkgreen ;
    )
enddef ;
\stopMPdefinitions

\startlinecorrection
\startMPcode
    draw My_Character (fulldiamond, 2, 4, 0, 1/4, 1/4, 0) scaled 5mm ;
    draw My_Character (fullcircle,  2, 4, 0, 1/4, 1/4, 0) scaled 5mm shifted (3cm,0) ;
    draw My_Character (fullsquare,  2, 4, 0, 1/4, 1/4, 0) scaled 5mm shifted (6cm,0) ;
\stopMPcode
\stoplinecorrection

Here we have three shapes and as usual they have some space at the left and right.
A text font like Lucida is designed in such a way that no kerns between glyphs
are needed but most text fonts have kerns. These compensate for these average
acceptable side bearings.

\startlinecorrection
\startMPcode
    draw My_Character (fulldiamond, 2, 4, 0.3, 1/4, 1/4, 0) scaled 5mm ;
    draw My_Character (fullcircle,  2, 4, 0.3, 1/4, 1/4, 0) scaled 5mm shifted (3cm,0) ;
    draw My_Character (fullsquare,  2, 4, 0.3, 1/4, 1/4, 0) scaled 5mm shifted (6cm,0) ;
\stopMPcode
\stoplinecorrection

In these slanted versions we get wider shapes but not always. For some shapes the
amount of perceived spacing at the left top and right bottom increases and this is
where we start thinking in terms of italic correction:

\startlinecorrection
\startMPcode
    draw My_Character (fulldiamond, 2, 4, 0.3,  1/4,  1/4, 0  ) scaled 5mm ;
    draw My_Character (fullcircle,  2, 4, 0.3,  1/4,  1/4, 0  ) scaled 5mm shifted (3cm,0) ;
    draw My_Character (fullsquare,  2, 4, 0.3,  1/4, -1/2, 1/4) scaled 5mm shifted (6cm,0) ;
    draw My_Character (fullsquare,  2, 4, 0.3, -1/2, -1/2, 1/4) scaled 5mm shifted (9cm,0) ;
\stopMPcode
\stoplinecorrection

That way, when we put characters next to each other on the average words look
better. But how about math: a superscript should be outside that bounding box and
a subscript inside, assuming that we have a shape like this. It is unfortunate
that the widely used $f$ is the perfect candidate. When using that one as test
case things can look great but kick in a $g$ and it gets worse. The $v$ and $w$
are also a fertile playground. It is hard to come up with logic that satisfied
all which is why glyph specific engine control was implemented (and later
dropped). In the previous graphic the fourth shape also cheats on the left and
yes, there are some fonts that do just that, which the of course interferes with
prescripts.

Now think of using shapes in formulas: some are put in sequence, in which case
inter atom spacing is added so there is less danger of touching due to too a
narrow width and \TEX ies somehow accepted that adding thin spaces every now and
then is fine \footnote {We don't think so which is why we came up with a more
granular inter atom spacing mode.} But there's more than sequences: shapes end up
in scripts, as degrees in radicals, above and below fraction rules, as fences and
accents. Just assume the worst possible scenarios!

In these examples the italic correction at the right is the difference between
the red and green box. There is no left italic correction and that is why \OPENTYPE\
math (driven by Cambria) has these four sets of kerns.

\startlinecorrection
\startMPcode
    draw My_Character_K (fulldiamond, 2, 4, 0.3, 1/4, 1/4, 1/4, -1/4, 1/2, -1/4) scaled 5mm ;
    draw My_Character_K (fullcircle,  2, 4, 0.3, 1/4, 1/4, 1/8,  0,   0,   -1/8) scaled 5mm shifted (3cm,0) ;
    draw My_Character_K (fullsquare,  2, 4, 0.3, 1/4, 1/4, 1/8, -1/2, 1/2, -1/8) scaled 5mm shifted (6cm,0) ;
\stopMPcode
\stoplinecorrection

Here we show some possibilities for better anchoring but it will be clear that it
is a compromise. Staircase kerns as in \OPENTYPE\ therefore have a set of kerns
going up or down for not only the base character but also the one that ends up in
a script as that one itself can have a \quote {problematic} shape.

Our solution to this problem is to tweak dimensions and italic correction of
problematic characters. We also can add four kerns that roughly compensate for
tricky corrections needed. We therefore have more glyph properties than the
official \OPENTYPE\ specification provides (cheaper and easier than staircase
kerns that work on pairs of characters) . But we have more font parameters
anyway, and with \TEX\ being one of the main math renderers and \TEX ies always
being to tune and tweak we think this is okay. It is better to have more control
than to rely on (hard to fight) heuristics.

We're stuck with the fonts we have and (in the case of \TEX\ fonts) the chosen
traditional approach of dimensions and italic corrections (no staircase kerns) but
the least we can ask is:

\startitemize[packed]
    \startitem
        Be consistent in dimensions and italic corrections. Get rid of limitations
        imposed by the 8 bit font era: we have plenty slots available and some more
        glyph properties as well. And if you compromise: make that clear.
    \stopitem
    \startitem
        Don't just take the old properties but assume that \OPENTYPE\ math fonts
        are used in a modern \OPENTYPE\ \TEX\ engine.
    \stopitem
    \startitem
        Assume that any character is used in any combination: that is what made it
        hard to satisfy all needs at the engine end. Better play safe.
    \stopitem
\stopitemize

\stopsubject

\startsubject[title=Scripts and primes]

We start with showing a few shapes (in \TEX\ speak: nuclei) with a different
perceived spacing and with some items attached to the corners: scripts.

\startMPdefinitions
vardef My_Character_P (expr shape, sx, sy, slant, left, right, prime, index) =
    image (
        path p ; p := shape
            xyscaled (sx, sy)
            slanted slant
        ;
        draw p
            withcolor darkgray ;
        draw (outerboundingbox p) leftenlarged left rightenlarged right
            withpen pencircle scaled 1/20
            withcolor darkred ;
        draw (fullsquare shifted (ulcorner outerboundingbox p)) shifted (-1,0)
            withpen pencircle scaled 1/5
            withcolor darkgreen ;
        draw (fullsquare shifted (llcorner outerboundingbox p)) shifted (-1,0)
                withpen pencircle scaled 1/5
                withcolor darkgreen ;
        if prime:
            draw (fullsquare xyscaled(1/2,1) shifted (urcorner outerboundingbox p)) shifted (.75,1/2)
                withpen pencircle scaled 1/5
                withcolor darkblue ;
            draw (fullsquare shifted (urcorner outerboundingbox p)) shifted (1.75,0)
                withpen pencircle scaled 1/5
                withcolor darkgreen ;
        else :
            draw (fullsquare shifted (urcorner outerboundingbox p)) shifted ( 1,0)
                withpen pencircle scaled 1/5
                withcolor darkgreen ;
        fi
        if index:
            draw (fullsquare shifted (lrcorner outerboundingbox p)) shifted (if prime: 3 else : 2 fi,0)
                withpen pencircle scaled 1/5
                withcolor darkyellow ;
        else :
            draw (fullsquare shifted (lrcorner outerboundingbox p)) shifted ( 1,0)
                withpen pencircle scaled 1/5
                withcolor darkgreen ;
        fi ;
    )
enddef ;
\stopMPdefinitions

\startlinecorrection
\startMPcode
    draw My_Character_P (fulldiamond, 2, 4, 0, 1/4, 1/4, false, false) scaled 5mm ;
    draw My_Character_P (fullcircle,  2, 4, 0, 1/4, 1/4, false, false) scaled 5mm shifted ( 5cm,0) ;
    draw My_Character_P (fullsquare,  2, 4, 0, 1/4, 1/4, false, false) scaled 5mm shifted (10cm,0) ;
\stopMPcode
\stoplinecorrection

The green ones represent the super- and sub-, pre- and postscripts. According to what we
discussed previously the anchoring depends on the shape

\startlinecorrection
\startMPcode
    draw My_Character_P (fulldiamond, 2, 4, .3, 1/4, 1/4, false, false) scaled 5mm ;
    draw My_Character_P (fullcircle,  2, 4, .3, 1/4, 1/4, false, false) scaled 5mm shifted ( 5cm,0) ;
    draw My_Character_P (fullsquare,  2, 4, .3, 1/4, 1/4, false, false) scaled 5mm shifted (10cm,0) ;
\stopMPcode
\stoplinecorrection

But we will not take that into account here because this time we focus on support
for primes and indices. A prime normally is a narrow character that is positioned
after the nuclues and sits in a similar position as a superscript. When it is
present a subscript doesn't move. Here the blue rectangle represents the prime.

\startlinecorrection
\startMPcode
    draw My_Character_P (fulldiamond, 2, 4, 0, 1/4, 1/4, true, false) scaled 5mm ;
    draw My_Character_P (fullcircle,  2, 4, 0, 1/4, 1/4, true, false) scaled 5mm shifted ( 5cm,0) ;
    draw My_Character_P (fullsquare,  2, 4, 0, 1/4, 1/4, true, false) scaled 5mm shifted (10cm,0) ;
\stopMPcode
\stoplinecorrection

An index, here yellow, is a subscript that applies to the whole so that one does
move. We cannot have a subscript and an index at the same time. The prescripts
are not affected by primes and indices other than that the engine has to do more
work in getting it right.

\startlinecorrection
\startMPcode
    draw My_Character_P (fulldiamond, 2, 4, 0, 1/4, 1/4, false, true) scaled 5mm ;
    draw My_Character_P (fullcircle,  2, 4, 0, 1/4, 1/4, false, true) scaled 5mm shifted ( 5cm,0) ;
    draw My_Character_P (fullsquare,  2, 4, 0, 1/4, 1/4, false, true) scaled 5mm shifted (10cm,0) ;
\stopMPcode
\stoplinecorrection

Of course we can have primes and indices at the same time. A prime actually belongs to the
nuclues so that combination determines quite a bit of the vertical and horizontal spacing.

\startlinecorrection
\startMPcode
    draw My_Character_P (fulldiamond, 2, 4, 0, 1/4, 1/4, true, true) scaled 5mm ;
    draw My_Character_P (fullcircle,  2, 4, 0, 1/4, 1/4, true, true) scaled 5mm shifted ( 5cm,0) ;
    draw My_Character_P (fullsquare,  2, 4, 0, 1/4, 1/4, true, true) scaled 5mm shifted (10cm,0) ;
\stopMPcode
\stoplinecorrection

The problem with primes is that they were never part of the concept and
\OPENTYPE\ math inherited that. Thanks to \UNICODE\ (and the math community not
being convincing enough) we ended up with a prime being equivalent to minutes and
double primes being minutes. The fact that we have triple and quadruple primes as
well as reverse primes is a consequence of not having symbols for sub second
indicators. And even the seconds are overloaded by meaning: time related and
geographical.

What has this to do with primes? Well, it means that a font has a prime character
that is already positioned at a certain distance from the baseline. But in most
math fonts the script and scriptscript sizes aren't. It looks as if the
assumption is that primes are treated like superscripts. But which one then gets
superscripted at the text level? The text one or the script one? Unfortunately
fonts are somewhat inconsistent in the sizing and positioning of primes and the
math community not being convincing enough. It was a tough decision to make: in
some fonts using the text prime have nice output and in other fonts the script
one.

So, because in the traditional approach primes had to be manually positioned, be
able deal with super and subscripts the old school approach was to make them
active characters and pick up what follows in order to deal with this situation.
It probably is the main reason why we have a somewhat special active character
mechanism in math mode. In \CONTEXT\ \MKIV\ we followed a different approach,
also because we wanted to deal with collapsing multiple primes to their rightful
\UNICODE\ slot.

In \LMTX\ and \LUAMETATEX\ we (need to) go a step further because we want proper
inter|-|atom space as well as more fine tuned positioning. Primes became elements
bound to a nucleus! We also wanted to solve this issue once and for all (in
\MKIV\ we has several methods but in \LMTX\ we only have one left). We not only
added a native prime element to nuclei but also introduced font parameters
similar to those of superscripts. Just keep in mind that in \LUAMETATEX\ we don't
need to worry about having a few more fields in a math node. There are not that
many nodes involved and the amount of extra memory used can be neglected. Of
course there is plenty of code added to deal with it so the binary definitely is
larger due to it and the code way more complex. In fact we already introduced
some missing parameters for the spacing related to prescripts. Furthermore we can
tune the prime spacing relative to the scripts.

In the goodie files you can add fixes that relate to primes and most of them
involved quite a bit of experimental. We can safely say that we spent quite some
prime time on this issue.

\startitemize[packed]
\startitem
    It would be nice if fonts at least has prime shapes that are consistent
    because currently script one can look quite different (tilt and shape).
\stopitem
\startitem
    Because primes are also minutes and seconds we probably have to accept the
    current situation and deal with it in the goodie files forever.
\stopitem
\stopitemize

\stopsubject

% Yet another section, this time I got in the mood after listening to Cindy
% Blackman Santana’s "Give The Drummer Some" ... here we go:

\startsubject[title=Two choices]

Imagine a situation like this: a sequence of characters with a leading left fence
but nothing at the right. The fence is not really a fence but something that can
grow and have its own super and subscripts, either after it or on top and below.
We leave these scripts (aka limits) out of the discussion.

\startMPdefinitions
vardef My_Character (expr shape, sx, sy, c) =
    image (
        draw
            shape xyscaled (sx, sy)
            withcolor c ;
    )
enddef ;
\stopMPdefinitions

\startlinecorrection
\startMPcode
    draw My_Character (fulldiamond, 2, 4, darkblue) scaled 5mm ;
    for i = 1 upto 5 :
        draw My_Character (fullcircle, 2, 2 randomized 1, darkgray ) scaled 5mm shifted (i * 2cm,0) ;
    endfor ;
\stopMPcode
\stoplinecorrection

There is a visual aspect here, which can be illustrated from a variant:

\startlinecorrection
\startMPcode
    draw My_Character (fulldiamond, 2, 4, darkblue)                  scaled 5mm ;
    for i = 1 upto 3 :
        draw My_Character (fullcircle, 2, 2 randomized 1, darkgray ) scaled 5mm shifted (i * 2cm,0) ;
    endfor ;
    draw My_Character (fulldiamond, 2, 6, darkblue)                  scaled 5mm shifted (9cm,0) ;
    for i = 1 upto 3 :
        draw My_Character (fullcircle, 2, 3 randomized 1, darkgray ) scaled 5mm shifted (9cm + i * 2cm,0) ;
    endfor ;
\stopMPcode
\stoplinecorrection

One can argue that the larger characters in the second sequence rightfully trigger a larger
blue variant. But for consistency one might actually go for:

\startlinecorrection
\startMPcode
    draw My_Character (fulldiamond, 2, 6, darkblue)                  scaled 5mm ;
    for i = 1 upto 3 :
        draw My_Character (fullcircle, 2, 2 randomized 1, darkgray ) scaled 5mm shifted (i * 2cm,0) ;
    endfor ;
    draw My_Character (fulldiamond, 2, 6, darkblue)                  scaled 5mm shifted (9cm,0) ;
    for i = 1 upto 3 :
        draw My_Character (fullcircle, 2, 3 randomized 1, darkgray ) scaled 5mm shifted (9cm + i * 2cm,0) ;
    endfor ;
\stopMPcode
\stoplinecorrection

Now imagine that you only have two choices? When do you go for the larger one?
This situation occurs with so called large operators like integrals and
summations. The original \TEX\ fonts have two sizes of these characters. The
smaller one is used in normal (text) math and the larger one in display math.

But what if, as in \OPENTYPE\ fonts there are more? There is a font parameter \typ
{DisplayOperatorMinHeight} that tells what size to use (as a minimum) in display
mode. Because the \TEX\ engines never had to make a decision the value of that
variable in the Latin Modern and Gyre fonts is somewhat arbitrary. It is one of
the variables that we need to adapt in the goodie files: we roughly bump from
1300 to 1800 to get what we are accustomed to in display mode.

So what are these large operators anyway? In traditional \TEX\ they are just
that: larger variants of smaller ones. In \OPENTYPE\ math however, they are
extensibles. That means that when there are more variants and an extensible
recipe it can grow on demand. That conflicts with the expectations of two sizes.

In order to support these two conflicting demands \LUAMETATEX\ provides so called
left operators that either act upon their own, in which case we talk of \quote
{auto} mode, or they can be told to have a specific size, or they can adapt, in
which case they have the usual bogus right companion that ends the subformula.
And, because they are implemented as fences but are not really fences there are
some provisions for the scripts: they bind to the left fence and not the right
one.

As side note: when we were experimenting with the usage of these (new) mechanisms
we again ran into the race condition that is imposed by the algorithm that
determines the size of fences: the delimiters target height is either a fraction
of the total height of the sublist or the total height diminished by some
constant amount. In \LUAMETATEX\ we therefore added the already discussed \typ
{\UmathDelimiterPercent} and \typ {\UmathDelimiterShortfall} but these only kick
in when we have a wrapped integral or such.

\startitemize[packed]
\startitem
    We can't be too picky but it would be nice if fonts have \typ
    {DisplayOperatorMinHeight} set to a more reasonable value. We have to tweak
    most font for this now (and probably forever).
\stopitem
\stopitemize

\stopsubject

\startsubject[title=To the point]

A nice feature of math fonts is that they have so called extensible characters. Fences
can grow vertically and accents horizontally. When we run out of discrete steps in size,
we end up with a recipe that build large characters from snippets as we discussed before.

The limitations in the size of fonts and the fact that the engine also had to
impose some limits in the amount of used fonts and complexity of their usage made
for some exceptions to the rule and, no pun intended, it might be why \TEX\ has a
concept of rules. Lines over and under something, arrows, fractions and radicals
all use rules. It is interesting to notice that when \OPENTYPE\ fonts showed up
the converted \TEX\ fonts didn't make arrows and bars use the extension
mechanism, thereby forcing the old school construction of them. This doesn't hurt
\CONTEXT\ much because we can implement proper characters using the virtual
character mechanism and we did things different anyway (for instance we can kick
in \METAFUN\ replacements).

Nevertheless there is this curious in|-|between: radical. It comes in a set of
fixed sized often sloped characters and then switches to an upright one. In all
cases there is that horizontal rule attached that covers the nucleus. There is no
mix of horizontal and vertical extensibles so there is no fancy end of rule
thingie possible (the little hook that we learned to draw at school): it's just a
stupid rule.

\startlinecorrection
\startMPcode
    for i = 1 upto 5 :
        draw image (
            draw fullsquare xyscaled(2, 4+i)
                withcolor darkblue ;
            fill fullsquare xyscaled(1/2, 1/2+i)
                shifted (1,2)
                withcolor darkblue ;
            draw fullsquare xyscaled(1, 1)
                shifted (1+i/100,2+i/2+i/100)
                withcolor darkgray ;
        ) scaled 5mm shifted (i*2cm, 0) ;
    endfor ;
\stopMPcode
\stoplinecorrection

Instead of a rule, these examples show a simple rectangle attached to another
one. The idea is that the center of that small one snaps onto the upper right
corner of the large one. In these examples we show how it looks when that is not
exact. At small sizes it goes unnoticed but at larger ones it becomes visible.
Actually, when you start scaling up math you often see artifacts in shapes, which
makes one wonder if high resolution screens are really used for proofing.

\startlinecorrection
\startMPcode
    for i = 1 upto 5 :
        draw image (
            draw fullsquare xyscaled(2, 4+i)
                withcolor darkblue ;
            draw fullsquare xyscaled(1, 1)
                shifted (1+i/100,2+i/2+i/100)
                withcolor darkgray ;
        ) scaled 5mm shifted (i*2cm, 0) ;
    endfor ;
\stopMPcode
\stoplinecorrection

In the first example the effect is small because we make sure that the right top
corner is square but in the second one we don't do that. Now it really becomes
annoying, even if we remove the small errors in positioning.

\startlinecorrection
\startMPcode
    for i = 1 upto 5 :
        draw image (
            draw fullsquare xyscaled(2, 4+i)
                withcolor darkblue ;
            draw fullsquare xyscaled(1, 1)
                shifted (1,2+i/2)
                withcolor darkgray ;
        ) scaled 5mm shifted (i*2cm, 0) ;
    endfor ;
\stopMPcode
\stoplinecorrection

We have seen different effects. The most common was an inaccurate font dimension
that determines what the height of the rule is. That we can fix in the goodie
file and we do that. Other artifacts were (in the stepwise sizes) characters that
were so sloped that there was not possibility for properly attaching the rule (we
noticed this best when checking the Lucida and Latin Modern fonts): one can cheat
and move a little to the left but the thickness of the sloped line was not enough
for that, so it could qualify as a design error but a design cannot simply be
changed in such case: one has to compromise and assume no scaling (keep in mind
that we scale on screen while on print one has to take a magnifying glass and
reading math that way is not much fun).

Some of these extensible radicals have rather thin vertical lines but we can
assume that their height is never such that this becomes too annoying. One
complication in designing these shapes is that when one uses a font creator
program it is unlikely to provide a math composer for testing. The same is true
for overlapping the end glyphs of e.g.\ curly braces: the middle piece can get
into the way when we have the smallest extensible and have that one kick in too
soon due to the lack of stepwise sizes. It's hard to check that without an engine
applying them to the extreme cases.

\startitemize[packed]
\startitem
    Benefit from your high resolution screen and zoom in on the results. Take no
    risk.
\stopitem
\startitem
    Play safe and create an top right corner that has some overlap and already
    starts out horizontally.
\stopitem
\startitem
    If the design complicates rule attachments, move to an extensible sooner
    because that one can have a horizontal attachment point as part of the
    design.
\stopitem
\stopitemize

\startsubject[title=Removing slack]

In the process of optimizing the spacing we ran into several cases where a bit of
spacing was added that eventually made that the resulting formula had small quite
visible whitespace at the edges. Take for instance the little bit of kerning
between a nuclues and a superscript.

\startlinecorrection
\startMPcode
    draw image (
        draw fullsquare xyscaled(2, 4)
            withcolor darkblue ;
        draw fullsquare xyscaled(1/2, 1)
            shifted (2-1/4,-1.5)
            withcolor darkred ;
        draw fullsquare xyscaled(1, 1)
            shifted (3,2)
            withcolor darkgray ;
    ) scaled 5mm shifted (2cm, 0) ;
\stopMPcode
\stoplinecorrection

Normally one can predict this situation but when components are glued together
that are typeset independently the (in this case red) spacing remains. The engine
is capable to remove that kind of slack at the left and right of the formula so
that one gets a proper tight bounding box. There is a bit more logic on board now.

The fact that all kind of spacing gets added is one of the reasons for the more
granular inter atom spacing: that way we could remove for instance the spacing
added to the left and right of fractions (which is buried in the constructed
box).

\startitemize[packed]
\startitem
    There is not much we can advice here because it doesn't relate to fonts.
    However, given that Lucida text fonts have no kerning could make one wonder
    to what extend math fonts needs all these compensations.
\stopitem
\startitem
    Some of the inter atom|/|script kerning is probably there as safeguard for
    italic shapes where staircase kerns are lacking. The question then is how the
    related font parameters themselves relate to staircase kerns. We suppose that
    this puts some stress on the font designer.
\stopitem
\stopitemize

\stopsubject

\page

\startsubject[title=Initializing fonts]

\startsubsubject[title=introduction]

When a math font is initialized there are (as with any font) features applies.
The only official one is the {ssty} feature that defines the substitution sets
for script and scripscript size. In traditional \TEX\ one has to set up multiple
fonts because one can only have 256 characters and because one might want to map
alphabets on the regular \ASCII\ slots. In an \OPENTYPE\ font all these alphabets
are on the same font so basically one can do with three instances per size: text,
script and scriptscript. What actually is done depends on the philosophy of the
macro package but we will not discuss that here. So let's assume three instances
in the default case. In \LUAMETATEX\ we can scale on the fly and therefore we can
stick to one instance: as long as we know what the slots of the (optionally
provided) smaller sizes are. Supporting this had quite some consequences for the
engine as scaling happens all over the place (also think of font parameters) and
we're talking two independent dimensions here: horizontal and vertical sizing.

In addition to this size related font feature \CONTEXT\ provide some additional
ones (and always had). Some are set up as regular features and some are driven by
the font goodie features which organizes a lot of features under one umbrella.
Font goodies always have been part of \MKIV\ and \LMTX.

{\em also mention the non tweak related ones}

\stopsubsubject

\startsubsubject[title=wipeitalics]

This tweak wipes the italics from one or more characters, most noticeable upright
characters. Because locating the individual characters took too much time, we
added the option complete alphabets.

\starttyping
example
\stoptyping

\stopsubsubject

\startsubsubject[title=wipeanchors]

This tweak removes the top anchors from one or more characters, most noticeable
upright characters. As with wiping italics, we often wipe complete alphabets.

\starttyping
example
\stoptyping

\stopsubsubject

\startsubsubject[title=accentdimensions]

Configure the position of overbar, underbar, overbrace, underbrace, overparent,
underparent, overbracket and underbracket. More? Added 220307.

\stopsubsubject

\startsubsubject[title=addrules]

I think we did not get overbars for some fonts (kpfonts and erewhon come to
mind).

\stopsubsubject

\startsubsubject[title=addscripts]

Maybe something done for Ton? (\typ{math-act})

\stopsubsubject

\startsubsubject[title=checkspacing]

I don't know how it works. Seems there is not really any settings. But you once
wrote

\starttyping
$\liminf$ \quad
$\limsup$ \quad [\sixperemspace] \quad
$x\sixperemspace x$

\setbox0\hbox{$x\sixperemspace x$} \showbox0
\stoptyping

in an email (220118).

\stopsubsubject

\startsubsubject[title=dimensions]

We use this to change anchor points of accents, to raise/lower them, and of
course to change bounding boxes and italic correction for many letters/symbols.
We use it in bonum to resize the whole lower case fraktur alphabet (and maybe
more should be added)

\stopsubsubject

\startsubsubject[title=fixprimes]

Maybe explained elsewhere? But it could belong here.

\stopsubsubject

\startsubsubject[title=fixradicals]

I don't know what it does. Could be: In some fonts the radicals are just
positioned wrongly, and this moves them up/down to have something uniform to work
with. (\typ{math-act})

\stopsubsubject

\startsubsubject[title=kerns]

A bit like staircase kerning. The \quote{topleft} and \quote{bottomright} come in
handy, in particular the last one when it comes to the position of subscripts.

\stopsubsubject

\startsubsubject[title=margins]

We fake the width of some characters. Useful, for example, not to get too
large/too small accents when using \tex{widehat} (also to get a uniform size over
some alphabets or glyphs that often go togeter).

\stopsubsubject

\startsubsubject[title=variants]

Some fonts (lucida, xits, stixtwo) have calligraphic (chancery) and script
(roundhand) alphabets. Of course not the same as default, and of course not the
same ss0X.

\stopsubsubject

\startsubsubject[title=wipecues]

Only added to cambria and dejavu. Added 220327. I think it has to do with
characters like 2061, 2062 and 2063. From the mails I think it has why does 2061
render a shape in some fonts so we need an 'wipe char' tweak (basically making it
a zero dimensions symbol)

\starttyping
\mathspacingmode2
\showmakeup[mathglue]
$ \sin                 \Uchar"2061  (y) $ \quad
$ x                    \Uchar"2062   y  $ \quad
$ x                    \Uchar"2063   y  $ \par
$ \sin      \mathghost{\Uchar"2061} (y) $ \quad
$ x         \mathghost{\Uchar"2062}  y  $ \quad
$ x         \mathghost{\Uchar"2063}  y  $ \par
\stoptyping

\stopsubsubject

\startsubsubject[title=bigslots]

Fences are chosen automatically to match what they surround. However, in traditional
engines a fenced sub formula won't break across lines. In \CONTEXT\ we have sveral st

1, 3, 5, 7 that could fit here, if we find a font where the linking is not
present.

\stopsubsubject

\startsubsubject[title=parameters]

I don't know if all of them are parameters that typically moved from document to
font level.

\stopsubsubject

\stopsubject

\stopchapter

\stopcomponent

