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% language=us runpath=texruns:manuals/evenmore
\environment evenmore-style
\startcomponent evenmore-inserts
\startchapter[title=Inserts]
{\em As in other chapters, this is mostly a wrapup of some developments and nota
manual. What is described here might actually evolve. Normally \CONTEXT\ users
will not notice these details, but it might help them to get an idea of what
complications we're dealing with.}
\startsection[title=Notes]
Quite some \TEX\ users love footnotes. These typographical elements have two
properties:
\startitemize[n]
\startitem
A blob of text, often placed at the bottom of a page or at the end of a
section, chapter or document. A note can be a short one|-|liner but also
a paragraph of text.
\stopitem
\startitem
A symbol in the running text that serves as reference to that blob. It can be
a number of just some unique symbol.
\stopitem
\stopitemize
When such a note is placed at the bottom of a page, the page builder needs to
take its dimensions into account when calculating an optimal page break. It might
even have to split a note over pages. Preferably the reference and note are on
the same page. In order to achieve this there is a special mechanism that deals
with this: inserts. In this chapter we only discuss them in the perspective of
notes but they can also be used for, for instance, top or bottom figures, tables,
etc.
\stopsection
\startsection[title=Global and local]
Normally notes are placed in the text flow. We call these global inserts. When
they are attached to a specific typographical element we tend to call them local
and there is a good reason for that. For instance, when used in a table, one
might want to keep them with that table when it is moved to a place where it
fits. When notes are packaged {\em with} the table they are flushed on demand and
not handled by the page builder. Actually they don't use inserts then. One
problem with global notes is that the \TEX\ engine has some limitations with
respect to inserts. In traditional \TEX, the following example will only result
in note \quote {a} being placed. The other two are buried too deeply.
\startbuffer[insert]
test \footnote {a} \hbox {test \footnote{b} \hbox {test \footnote {c}} test} test
\stopbuffer
\typebuffer[insert]
There are a few places where \TEX\ will bubble up the notes: when it constructs
lines during a breaking a paragraph into lines, and when constructing alignment
cells. In all other cases they are simply discarded.
In \CONTEXT\ \MKII\ there is a mechanism that can postpone such notes. Normally a
user will not use boxes directly so when some mechanism is used that does box
its content, \CONTEXT\ can collect the notes and flush them in a spot that
exposes them. This works reasonable well be it that we loose some of the
synchronization with the page builder: flushing can happen after a page break.
Already early in the \CONTEXT\ \MKIV\ development a more advanced mechanism was
introduced, using \LUA\ callbacks: auto migration. There (optionally) inserts are
bubbled up to a level where they are visible but even that has its limitations. Users
explicitly need to enable automigration in \MKIV.
In \CONTEXT\ \LMTX\ we do it a bit different because the \LUAMETATEX\ engine has
a feature that helps. More about that next.
\stopsection
\startsection[title=Pre and post migration]
There is another mechanism that moves material from the spot: adjusts. Here is an
example (watch the interline spacing):
\startbuffer[adjust]
\strut test \vadjust pre {\red \strut before} test \vadjust {\red \strut after}
\stopbuffer
\typebuffer[adjust]
This gives:
\blank {\bf\showstruts \getbuffer[adjust]} \blank
The \type {pre} variant was introduced in \PDFTEX\ at a time that I actually has
use for it (think of marginal notes) but in \MKIV\ we don't use this adjust
mechanism.
Inserts on the other hand always end up \quote {post} where they are injected.
Some day I might think about what good there is in injecting them \quote {pre}
where they are injected. It might become an option.
\blank {\bf \showboxes \getbuffer[insert]} \blank
In this already shown example the footnotes (that are implemented using inserts)
result indeed in text showing up at the bottom of the page, even when they are
wrapped in boxes. The reason is that we have enabled a mechanism that
automatically bubble them up:
\starttyping
\automigrationmode = 3 % \numexpr 1 + 2 \relax
\stoptyping
Here bit~1 controls migration in lists and bit~2 controls migration in the main
vertical list (the page flow). In \CONTEXT\ \LMTX\ we enable both.
\startbuffer
h: \setbox0\hbox{box \footnote{h: box}}\setbox2\hbox{\box 0}\box2\quad
h: \setbox0\hbox{copy \footnote{h: copy}}\setbox2\hbox{\copy 0}\box2\quad
h: \setbox0\hbox{unbox \footnote{h: unhbox}}\setbox2\hbox{\unhbox 0}\box2\quad
h: \setbox0\hbox{uncopy \footnote{h: unhcopy}}\setbox2\hbox{\unhcopy0}\box2\quad
v: \setbox0\hbox{box \footnote{v: box}}\setbox2\vbox{\box 0}\box2\quad
v: \setbox0\hbox{copy \footnote{v: copy}}\setbox2\vbox{\copy 0}\box2\quad
v: \setbox0\hbox{unbox \footnote{v: unhbox}}\setbox2\vbox{\unhbox 0}\box2\quad
v: \setbox0\hbox{uncopy \footnote{v: unhcopy}}\setbox2\vbox{\unhcopy0}\box2\quad
\stopbuffer
\typebuffer
This is a kind of torture test:
\blank \hbox{\bf \getbuffer} \blank
It might look kind of trivial to accomplish this but it actually took some
experimenting to get it right. It also must be noted that because in \LUATEX\ and
even more in \LUAMETATEX\ the nodes that store a box are much larger than in
traditional \TEX\ and that the price for adding some additional overhead for
dealing with pre- and post material had not that much impact on the performance
of \LUAMETATEX.
On the one hand we need to make sure that the inserts are kept with the box but
on the other hand they should really migrate when the boxed material gets boxed
itself. This has to be combined with unboxing and copying. It has to work in
regular boxes but also in the main vertical list (the page flow).
\stopsection
\startsection[title=Playground]
You can play a bit with this mechanism by getting and setting the pre and post
material than relates to a box.
\startbuffer
\setbox0\ruledhbox{\strut content \footnote {footnote}}
[flush pre\prelistbox 0] % \prelistcopy 0
[flush post\postlistbox 0] % \postlistcopy 0
[set pre\setprelistbox 0\ruledhbox{\strut before}]
[set post\setpostlistbox0\ruledhbox{\strut after}]
\blank \box0 \blank
\stopbuffer
\typebuffer
\getbuffer
Of course there can be side effects but so far this mechanism works reasonable
well, given what it has to deal with.
\stopsection
\startsection[title=Marks]
The low level marks mechanism can use the same pre and post mechanisms but I
didn't test that yet. We have a different approach to marks but eventually might
revert to using the build in mechanism, be it in adapted form.
\stopsection
% \startsection[title=Maybe]
%
% Inserts are implemented in a bit special way. A class of inserts needs a box,
% dimen, count and skip register. So, the macro package has to allocate these
% en|-|block. This puts some constraints on the register allocation macros. On the
% agenda is to investigate if a dedicated compound register makes sense. However,
% this means that the interfaces change, but because a \CONTEXT\ user doesn't use
% the low level inserts directly this is not really a problem. Adapting the
% implementation can have the side effect of using a different insert chaining
% which can give a better interface to \LUA\ because the current interface is not
% really useable.
%
% \stopsection
\stopchapter
\stopcomponent
|
