% language=us engine=luatex runpath=texruns:manuals/luatex

\environment luatex-style

\startcomponent luatex-modifications

\startchapter[reference=modifications,title={Modifications}]

\startsection[title=The merged engines]

\startsubsection[title=The need for change]

\topicindex {engines}
\topicindex {history}

The first version of \LUATEX\ only had a few extra primitives and it was largely
the same as \PDFTEX. Then we merged substantial parts of \ALEPH\ into the code
and got more primitives. When we got more stable the decision was made to clean
up the rather hybrid nature of the program. This means that some primitives have
been promoted to core primitives, often with a different name, and that others
were removed. This made it possible to start cleaning up the code base. In \in
{chapter} [enhancements] we discussed some new primitives, here we will cover
most of the adapted ones.

Besides the expected changes caused by new functionality, there are a number of
not|-|so|-|expected changes. These are sometimes a side|-|effect of a new
(conflicting) feature, or, more often than not, a change necessary to clean up
the internal interfaces. These will also be mentioned.

\stopsubsection

\startsubsection[title=Changes from \TEX\ 3.1415926]

\topicindex {\TEX}

Of course it all starts with traditional \TEX. Even if we started with \PDFTEX,
most still comes from the original. But we divert a bit.

\startitemize

\startitem
    The current code base is written in \CCODE, not \PASCAL. We use \CWEB\ when
    possible. As a consequence instead of one large file plus change files, we
    now have multiple files organized in categories like \type {tex}, \type
    {pdf}, \type {lang}, \type {font}, \type {lua}, etc. There are some artifacts
    of the conversion to \CCODE, but in due time we will clean up the source code
    and make sure that the documentation is done right. Many files are in the
    \CWEB\ format, but others, like those interfacing to \LUA, are \CCODE\ files.
    Of course we want to stay as close as possible to the original so that the
    documentation of the fundamentals behind \TEX\ by Don Knuth still applies.
\stopitem

\startitem
    See \in {chapter} [languages] for many small changes related to paragraph
    building, language handling and hyphenation. The most important change is
    that adding a brace group in the middle of a word (like in \type {of{}fice})
    does not prevent ligature creation.
\stopitem

\startitem
    There is no pool file, all strings are embedded during compilation.
\stopitem

\startitem
    The specifier \type {plus 1 fillll} does not generate an error. The extra
    \quote{l} is simply typeset.
\stopitem

\startitem
    The upper limit to \prm {endlinechar} and \prm {newlinechar} is 127.
\stopitem

\startitem
    Magnification (\prm {mag}) is only supported in \DVI\ output mode. You can
    set this parameter and it even works with \type {true} units till you switch
    to \PDF\ output mode. When you use \PDF\ output you can best not touch the
    \prm {mag} variable. This fuzzy behaviour is not much different from using
    \PDF\ backend related functionality while eventually \DVI\ output is
    required.

    After the output mode has been frozen (normally that happens when the first
    page is shipped out) or when \PDF\ output is enabled, the \type {true}
    specification is ignored. When you preload a plain format adapted to
    \LUATEX\ it can be that the \prm {mag} parameter already has been set.
\stopitem

\stopitemize

\stopsubsection

\startsubsection[title=Changes from \ETEX\ 2.2]

\topicindex {\ETEX}

Being the de factor standard extension of course we provide the \ETEX\
functionality, but with a few small adaptations.

\startitemize

\startitem
    The \ETEX\ functionality is always present and enabled so the prepended
    asterisk or \type {-etex} switch for \INITEX\ is not needed.
\stopitem

\startitem
    The \TEXXET\ extension is not present, so the primitives \type
    {\TeXXeTstate}, \type {\beginR}, \type {\beginL}, \type {\endR} and \type
    {\endL} are missing. Instead we used the \OMEGA/\ALEPH\ approach to
    directionality as starting point.
\stopitem

\startitem
    Some of the tracing information that is output by \ETEX's \prm
    {tracingassigns} and \prm {tracingrestores} is not there.
\stopitem

\startitem
    Register management in \LUATEX\ uses the \OMEGA/\ALEPH\ model, so the maximum
    value is 65535 and the implementation uses a flat array instead of the mixed
    flat & sparse model from \ETEX.
\stopitem

\startitem
    When kpathsea is used to find files, \LUATEX\ uses the \type {ofm} file
    format to search for font metrics. In turn, this means that \LUATEX\ looks at
    the \type {OFMFONTS} configuration variable (like \OMEGA\ and \ALEPH) instead
    of \type {TFMFONTS} (like \TEX\ and \PDFTEX). Likewise for virtual fonts
    (\LUATEX\ uses the variable \type {OVFFONTS} instead of \type {VFFONTS}).
\stopitem

\startitem
    The primitives that report a stretch or shrink order report a value in a
    convenient range zero upto four. Because some macro packages can break on
    that we also provide \type {\eTeXgluestretchorder} and \type
    {\eTeXglueshrinkorder} which report values compatible with \ETEX. The (new)
    \type {fi} value is reported as \type {-1} (so when used in an \type
    {\ifcase} test that value makes one end up in the \type {\else}).
\stopitem

\stopitemize

\stopsubsection

\startsubsection[title=Changes from \PDFTEX\ 1.40]

\topicindex {\PDFTEX}

Because we want to produce \PDF\ the most natural starting point was the popular
\PDFTEX\ program. We inherit the stable features, dropped most of the
experimental code and promoted some functionality to core \LUATEX\ functionality
which in turn triggered renaming primitives.

For compatibility reasons we still refer to \type {\pdf...} commands but \LUATEX\
has a different backend interface. Instead of these primitives there are three
interfacing primitives: \lpr {pdfextension}, \lpr {pdfvariable} and \lpr
{pdffeedback} that take keywords and optional further arguments (below we will
still use the \tex {pdf} prefix names as reference). This way we can extend the
features when needed but don't need to adapt the core engine. The front- and
backend are decoupled as much as possible.

\startitemize

\startitem
    The (experimental) support for snap nodes has been removed, because it is
    much more natural to build this functionality on top of node processing and
    attributes. The associated primitives that are gone are: \orm
    {pdfsnaprefpoint}, \orm {pdfsnapy}, and \orm {pdfsnapycomp}.
\stopitem

\startitem
    The (experimental) support for specialized spacing around nodes has also been
    removed. The associated primitives that are gone are: \orm
    {pdfadjustinterwordglue}, \orm {pdfprependkern}, and \orm {pdfappendkern}, as
    well as the five supporting primitives \orm {knbscode}, \orm {stbscode}, \orm
    {shbscode}, \orm {knbccode}, and \orm {knaccode}.
\stopitem

\startitem
    A number of \quote {\PDFTEX\ primitives} have been removed as they can be
    implemented using \LUA: \orm {pdfelapsedtime}, \orm {pdfescapehex}, \orm
    {pdfescapename}, \orm {pdfescapestring}, \orm {pdffiledump}, \orm
    {pdffilemoddate}, \orm {pdffilesize}, \orm {pdfforcepagebox}, \orm
    {pdflastmatch}, \orm {pdfmatch}, \orm {pdfmdfivesum}, \orm {pdfmovechars},
    \orm {pdfoptionalwaysusepdfpagebox}, \orm {pdfoptionpdfinclusionerrorlevel},
    \orm {pdfresettimer}, \orm {pdfshellescape}, \orm {pdfstrcmp} and \orm
    {pdfunescapehex}.
\stopitem

\startitem
    The version related primitives \orm {pdftexbanner}, \orm {pdftexversion}
    and \orm {pdftexrevision} are no longer present as there is no longer a
    relationship with \PDFTEX\ development.
\stopitem

\startitem
    The experimental snapper mechanism has been removed and therefore also the
    primitives \orm {pdfignoreddimen}, \orm {pdffirstlineheight}, \orm
    {pdfeachlineheight}, \orm {pdfeachlinedepth} and \orm {pdflastlinedepth}.
\stopitem

\startitem
    The experimental primitives \lpr {primitive}, \lpr {ifprimitive}, \lpr
    {ifabsnum} and \lpr {ifabsdim} are promoted to core primitives. The \type
    {\pdf*} prefixed originals are not available.
\stopitem

\startitem
    Because \LUATEX\ has a different subsystem for managing images, more
    diversion from its ancestor happened in the meantime. We don't adapt to
    changes in \PDFTEX.
\stopitem

\startitem
    Two extra token lists are provided, \orm {pdfxformresources} and \orm
    {pdfxformattr}, as an alternative to \orm {pdfxform} keywords.
\stopitem

\startitem
    Image specifications also support \type {visiblefilename}, \type
    {userpassword} and \type {ownerpassword}. The password options are only
    relevant for encrypted \PDF\ files.
\stopitem

\startitem
    The current version of \LUATEX\ no longer replaces and|/|or merges fonts in
    embedded \PDF\ files with fonts of the enveloping \PDF\ document. This
    regression may be temporary, depending on how the rewritten font backend will
    look like.
\stopitem

\startitem
    The primitives \orm {pdfpagewidth} and \orm {pdfpageheight} have been removed
    because \lpr {pagewidth} and \lpr {pageheight} have that purpose.
\stopitem

\startitem
    The primitives \orm {pdfnormaldeviate}, \orm {pdfuniformdeviate}, \orm
    {pdfsetrandomseed} and \orm {pdfrandomseed} have been promoted to core
    primitives without \type {pdf} prefix so the original commands are no longer
    recognized.
\stopitem

\startitem
    The primitives \lpr {ifincsname}, \lpr {expanded} and \lpr {quitvmode}
    are now core primitives.
\stopitem

\startitem
    As the hz and protrusion mechanism are part of the core the related
    primitives \lpr {lpcode}, \lpr {rpcode}, \lpr {efcode}, \lpr
    {leftmarginkern}, \lpr {rightmarginkern} are promoted to core primitives. The
    two commands \lpr {protrudechars} and \lpr {adjustspacing} replace their
    prefixed with \type {\pdf} originals.
\stopitem

\startitem
    The hz optimization code has been partially redone so that we no longer need
    to create extra font instances. The front- and backend have been decoupled
    and more efficient (\PDF) code is generated.
\stopitem

\startitem
    When \lpr {adjustspacing} has value~2, hz optimization will be applied to
    glyphs and kerns. When the value is~3, only glyphs will be treated. A value
    smaller than~2 disables this feature. With value of 1, font expansion is
    applied after \TEX's normal paragraph breaking routines have broken the
    paragraph into lines. In this case, line breaks are identical to standard
    \TEX\ behavior (as with \PDFTEX).
\stopitem

\startitem
    The \lpr {tagcode} primitive is promoted to core primitive.
\stopitem

\startitem
    The \lpr {letterspacefont} feature is now part of the core but will not be
    changed (improved). We just provide it for legacy use.
\stopitem

\startitem
    The \orm {pdfnoligatures} primitive is now \lpr {ignoreligaturesinfont}.
\stopitem

\startitem
    The \orm {pdfcopyfont} primitive is now \lpr {copyfont}.
\stopitem

\startitem
    The \orm {pdffontexpand} primitive is now \lpr {expandglyphsinfont}.
\stopitem

\startitem
    Because position tracking is also available in \DVI\ mode the \lpr {savepos},
    \lpr {lastxpos} and \lpr {lastypos} commands now replace their \type {pdf}
    prefixed originals.
\stopitem

\startitem
    The introspective primitives \type {\pdflastximagecolordepth} and \type
    {\pdfximagebbox} have been removed. One can use external applications to
    determine these properties or use the built|-|in \type {img} library.
\stopitem

\startitem
    The initializers \orm {pdfoutput} has been replaced by \lpr {outputmode} and
    \orm {pdfdraftmode} is now \lpr {draftmode}.
\stopitem

\startitem
    The pixel multiplier dimension \orm {pdfpxdimen} lost its prefix and is now
    called \lpr {pxdimen}.
\stopitem

\startitem
    An extra \orm {pdfimageaddfilename} option has been added that can be used to
    block writing the filename to the \PDF\ file.
\stopitem

\startitem
    The primitive \orm {pdftracingfonts} is now \lpr {tracingfonts} as it
    doesn't relate to the backend.
\stopitem

\startitem
    The experimental primitive \orm {pdfinsertht} is kept as \lpr {insertht}.
\stopitem

\startitem
    There is some more control over what metadata goes into the \PDF\ file.
\stopitem

\startitem
    The promotion of primitives to core primitives as well as the separation of
    font- and backend means that the initialization namespace \type {pdftex} is
    gone.
\stopitem

\stopitemize

One change involves the so called xforms and ximages. In \PDFTEX\ these are
implemented as so called whatsits. But contrary to other whatsits they have
dimensions that need to be taken into account when for instance calculating
optimal line breaks. In \LUATEX\ these are now promoted to a special type of rule
nodes, which simplifies code that needs those dimensions.

Another reason for promotion is that these are useful concepts. Backends can
provide the ability to use content that has been rendered in several places, and
images are also common. As already mentioned in \in {section}
[sec:imagedandforms], we now have:

\starttabulate[|l|l|]
\DB \LUATEX \BC \PDFTEX \NC \NR
\TB
\NC \lpr {saveboxresource}             \NC \orm {pdfxform}           \NC \NR
\NC \lpr {saveimageresource}           \NC \orm {pdfximage}          \NC \NR
\NC \lpr {useboxresource}              \NC \orm {pdfrefxform}        \NC \NR
\NC \lpr {useimageresource}            \NC \orm {pdfrefximage}       \NC \NR
\NC \lpr {lastsavedboxresourceindex}   \NC \orm {pdflastxform}       \NC \NR
\NC \lpr {lastsavedimageresourceindex} \NC \orm {pdflastximage}      \NC \NR
\NC \lpr {lastsavedimageresourcepages} \NC \orm {pdflastximagepages} \NC \NR
\LL
\stoptabulate

There are a few \lpr {pdffeedback} features that relate to this but these are
typical backend specific ones. The index that gets returned is to be considered
as \quote {just a number} and although it still has the same meaning (object
related) as before, you should not depend on that.

The protrusion detection mechanism is enhanced a bit to enable a bit more complex
situations. When protrusion characters are identified some nodes are skipped:

\startitemize[packed,columns,two]
\startitem zero glue \stopitem
\startitem penalties \stopitem
\startitem empty discretionaries \stopitem
\startitem normal zero kerns \stopitem
\startitem rules with zero dimensions \stopitem
\startitem math nodes with a surround of zero \stopitem
\startitem dir nodes \stopitem
\startitem empty horizontal lists \stopitem
\startitem local par nodes \stopitem
\startitem inserts, marks and adjusts \stopitem
\startitem boundaries \stopitem
\startitem whatsits \stopitem
\stopitemize

Because this can not be enough, you can also use a protrusion boundary node to
make the next node being ignored. When the value is~1 or~3, the next node will be
ignored in the test when locating a left boundary condition. When the value is~2
or~3, the previous node will be ignored when locating a right boundary condition
(the search goes from right to left). This permits protrusion combined with for
instance content moved into the margin:

\starttyping
\protrusionboundary1\llap{!\quad}«Who needs protrusion?»
\stoptyping

\stopsubsection

\startsubsection[title=Changes from \ALEPH\ RC4]

\topicindex {\ALEPH}

Because we wanted proper directional typesetting the \ALEPH\ mechanisms looked
most attractive. These are rather close to the ones provided by \OMEGA, so what
we say next applies to both these programs.

\startitemize

\startitem
    The extended 16-bit math primitives (\orm {omathcode} etc.) have been
    removed.
\stopitem

\startitem
    The \OCP\ processing has been removed completely and as a consequence, the
    following primitives have been removed: \orm {ocp}, \orm {externalocp}, \orm
    {ocplist}, \orm {pushocplist}, \orm {popocplist}, \orm {clearocplists}, \orm
    {addbeforeocplist}, \orm {addafterocplist}, \orm {removebeforeocplist}, \orm
    {removeafterocplist} and \orm {ocptracelevel}.
\stopitem

\startitem
    \LUATEX\ only understands 4~of the 16~direction specifiers of \ALEPH: \type
    {TLT} (latin), \type {TRT} (arabic), \type {RTT} (cjk), \type {LTL} (mongolian).
    All other direction specifiers generate an error. In addition to a keyword
    driven model we also provide an integer driven one.
\stopitem

\startitem
    The input translations from \ALEPH\ are not implemented, the related
    primitives are not available: \orm {DefaultInputMode}, \orm
    {noDefaultInputMode}, \orm {noInputMode}, \orm {InputMode}, \orm
    {DefaultOutputMode}, \orm {noDefaultOutputMode}, \orm {noOutputMode}, \orm
    {OutputMode}, \orm {DefaultInputTranslation}, \orm
    {noDefaultInputTranslation}, \orm {noInputTranslation}, \orm
    {InputTranslation}, \orm {DefaultOutputTranslation}, \orm
    {noDefaultOutputTranslation}, \orm {noOutputTranslation} and \orm
    {OutputTranslation}.
\stopitem

\startitem
    Several bugs have been fixed and confusing implementation details have been
    sorted out.
\stopitem

\startitem
    The scanner for direction specifications now allows an optional space after
    the direction is completely parsed.
\stopitem

\startitem
    The \type {^^} notation has been extended: after \type {^^^^} four
    hexadecimal characters are expected and after \type {^^^^^^} six hexadecimal
    characters have to be given. The original \TEX\ interpretation is still valid
    for the \type {^^} case but the four and six variants do no backtracking,
    i.e.\ when they are not followed by the right number of hexadecimal digits
    they issue an error message. Because \type{^^^} is a normal \TEX\ case, we
    don't support the odd number of \type {^^^^^} either.
\stopitem

\startitem
    Glues {\it immediately after} direction change commands are not legal
    breakpoints.
\stopitem

\startitem
    Several mechanisms that need to be right|-|to|-|left aware have been
    improved. For instance placement of formula numbers.
\stopitem

\startitem
    The page dimension related primitives \lpr {pagewidth} and \lpr {pageheight}
    have been promoted to core primitives. The \prm {hoffset} and \prm {voffset}
    primitives have been fixed.
\stopitem

\startitem
    The primitives \type {\charwd}, \type {\charht}, \type {\chardp} and \type
    {\charit} have been removed as we have the \ETEX\ variants \type
    {\fontchar*}.
\stopitem

\startitem
    The two dimension registers \lpr {pagerightoffset} and \lpr
    {pagebottomoffset} are now core primitives.
\stopitem

\startitem
    The direction related primitives \lpr {pagedir}, \lpr {bodydir}, \lpr
    {pardir}, \lpr {textdir}, \lpr {mathdir} and \lpr {boxdir} are now core
    primitives.
\stopitem

\startitem
    The promotion of primitives to core primitives as well as removing of all
    others means that the initialization namespace \type {aleph} that early
    versions of \LUATEX\ provided is gone.
\stopitem

\stopitemize

The above let's itself summarize as: we took the 32 bit aspects and much of the
directional mechanisms and merged it into the \PDFTEX\ code base as starting
point for further development. Then we simplified directionality, fixed it and
opened it up.

\stopsubsection

\startsubsection[title=Changes from anywhere]

The \type {\partokenname} and \type {\partokencontext} primitives are taken from
the \PDFTEX\ change file posted on the implementers list. They are explained in
the \PDFTEX\ manual and are classified as \ETEX\ extensions.

\stopsubsection

\startsubsection[title=Changes from standard \WEBC]

\topicindex {\WEBC}

The compilation framework is \WEBC\ and we keep using that but without the
\PASCAL\ to \CCODE\ step. This framework also provides some common features that
deal with reading bytes from files and locating files in \TDS. This is what we do
different:

\startitemize

\startitem
    There is no mltex support.
\stopitem

\startitem
    There is no enctex support.
\stopitem

\startitem
    The following encoding related command line switches are silently ignored,
    even in non|-|\LUA\ mode: \type {-8bit}, \type {-translate-file}, \type
    {-mltex}, \type {-enc} and \type {-etex}.
\stopitem

\startitem
    The \prm {openout} whatsits are not written to the log file.
\stopitem

\startitem
    Some of the so|-|called \WEBC\ extensions are hard to set up in non|-|\KPSE\
    mode because \type {texmf.cnf} is not read: \type {shell-escape} is off (but
    that is not a problem because of \LUA's \type {os.execute}), and the paranoia
    checks on \type {openin} and \type {openout} do not happen. However, it is
    easy for a \LUA\ script to do this itself by overloading \type {io.open} and
    alike.
\stopitem

\startitem
    The \quote{E} option does not do anything useful.
\stopitem

\stopitemize

\stopsubsection

\stopsection

\startsection[reference=backendprimitives,title=The backend primitives]

\startsubsection[title={Less primitives}]

\topicindex {backend}
\topicindex {\PDF+backend}

In a previous section we mentioned that some \PDFTEX\ primitives were removed and
others promoted to core \LUATEX\ primitives. That is only part of the story. In
order to separate the backend specific primitives in de code these commands are
now replaced by only a few. In traditional \TEX\ we only had the \DVI\ backend
but now we have two: \DVI\ and \PDF. Additional functionality is implemented as
\quote {extensions} in \TEX\ speak. By separating more strickly we are able to
keep the core (frontend) clean and stable and isolate these extensions. If for
some reason an extra backend option is needed, it can be implemented without
touching the core. The three \PDF\ backend related primitives are:

\starttyping
\pdfextension command [specification]
\pdfvariable  name
\pdffeedback  name
\stoptyping

An extension triggers further parsing, depending on the command given. A variable is
a (kind of) register and can be read and written, while a feedback is reporting
something (as it comes from the backend it's normally a sequence of tokens).

\stopsubsection

\startsubsection[title={\lpr{pdfextension}, \lpr {pdfvariable} and \lpr {pdffeedback}},reference=sec:pdfextensions]

In order for \LUATEX\ to be more than just \TEX\ you need to enable primitives. That
has already been the case right from the start. If you want the traditional \PDFTEX\
primitives (for as far their functionality is still around) you now can do this:

\starttyping
\protected\def\pdfliteral             {\pdfextension literal}
\protected\def\pdflateliteral         {\pdfextension lateliteral}
\protected\def\pdfcolorstack          {\pdfextension colorstack}
\protected\def\pdfsetmatrix           {\pdfextension setmatrix}
\protected\def\pdfsave                {\pdfextension save\relax}
\protected\def\pdfrestore             {\pdfextension restore\relax}
\protected\def\pdfobj                 {\pdfextension obj }
\protected\def\pdfrefobj              {\pdfextension refobj }
\protected\def\pdfannot               {\pdfextension annot }
\protected\def\pdfstartlink           {\pdfextension startlink }
\protected\def\pdfendlink             {\pdfextension endlink\relax}
\protected\def\pdfoutline             {\pdfextension outline }
\protected\def\pdfdest                {\pdfextension dest }
\protected\def\pdfthread              {\pdfextension thread }
\protected\def\pdfstartthread         {\pdfextension startthread }
\protected\def\pdfendthread           {\pdfextension endthread\relax}
\protected\def\pdfinfo                {\pdfextension info }
\protected\def\pdfcatalog             {\pdfextension catalog }
\protected\def\pdfnames               {\pdfextension names }
\protected\def\pdfincludechars        {\pdfextension includechars }
\protected\def\pdffontattr            {\pdfextension fontattr }
\protected\def\pdfmapfile             {\pdfextension mapfile }
\protected\def\pdfmapline             {\pdfextension mapline }
\protected\def\pdftrailer             {\pdfextension trailer }
\protected\def\pdfglyphtounicode      {\pdfextension glyphtounicode }
\protected\def\pdfrunninglinkoff      {\pdfextension linkstate 1 }
\protected\def\pdfrunninglinkon       {\pdfextension linkstate 0 }
\stoptyping

The introspective primitives can be defined as:

\starttyping
\def\pdftexversion    {\numexpr\pdffeedback version\relax}
\def\pdftexrevision           {\pdffeedback revision}
\def\pdflastlink      {\numexpr\pdffeedback lastlink\relax}
\def\pdfretval        {\numexpr\pdffeedback retval\relax}
\def\pdflastobj       {\numexpr\pdffeedback lastobj\relax}
\def\pdflastannot     {\numexpr\pdffeedback lastannot\relax}
\def\pdfxformname     {\numexpr\pdffeedback xformname\relax}
\def\pdfcreationdate          {\pdffeedback creationdate}
\def\pdffontname      {\numexpr\pdffeedback fontname\relax}
\def\pdffontobjnum    {\numexpr\pdffeedback fontobjnum\relax}
\def\pdffontsize      {\dimexpr\pdffeedback fontsize\relax}
\def\pdfpageref       {\numexpr\pdffeedback pageref\relax}
\def\pdfcolorstackinit        {\pdffeedback colorstackinit}
\stoptyping

The configuration related registers have become:

\starttyping
\edef\pdfcompresslevel            {\pdfvariable compresslevel}
\edef\pdfobjcompresslevel         {\pdfvariable objcompresslevel}
\edef\pdfrecompress               {\pdfvariable recompress}
\edef\pdfdecimaldigits            {\pdfvariable decimaldigits}
\edef\pdfgamma                    {\pdfvariable gamma}
\edef\pdfimageresolution          {\pdfvariable imageresolution}
\edef\pdfimageapplygamma          {\pdfvariable imageapplygamma}
\edef\pdfimagegamma               {\pdfvariable imagegamma}
\edef\pdfimagehicolor             {\pdfvariable imagehicolor}
\edef\pdfimageaddfilename         {\pdfvariable imageaddfilename}
\edef\pdfpkresolution             {\pdfvariable pkresolution}
\edef\pdfpkfixeddpi               {\pdfvariable pkfixeddpi}
\edef\pdfinclusioncopyfonts       {\pdfvariable inclusioncopyfonts}
\edef\pdfinclusionerrorlevel      {\pdfvariable inclusionerrorlevel}
\edef\pdfignoreunknownimages      {\pdfvariable ignoreunknownimages}
\edef\pdfgentounicode             {\pdfvariable gentounicode}
\edef\pdfomitcidset               {\pdfvariable omitcidset}
\edef\pdfomitcharset              {\pdfvariable omitcharset}
\edef\pdfomitinfodict             {\pdfvariable omitinfodict}
\edef\pdfomitmediabox             {\pdfvariable omitmediabox}
\edef\pdfpagebox                  {\pdfvariable pagebox}
\edef\pdfminorversion             {\pdfvariable minorversion}
\edef\pdfuniqueresname            {\pdfvariable uniqueresname}

\edef\pdfhorigin                  {\pdfvariable horigin}
\edef\pdfvorigin                  {\pdfvariable vorigin}
\edef\pdflinkmargin               {\pdfvariable linkmargin}
\edef\pdfdestmargin               {\pdfvariable destmargin}
\edef\pdfthreadmargin             {\pdfvariable threadmargin}
\edef\pdfxformmargin              {\pdfvariable xformmargin}

\edef\pdfpagesattr                {\pdfvariable pagesattr}
\edef\pdfpageattr                 {\pdfvariable pageattr}
\edef\pdfpageresources            {\pdfvariable pageresources}
\edef\pdfxformattr                {\pdfvariable xformattr}
\edef\pdfxformresources           {\pdfvariable xformresources}
\edef\pdfpkmode                   {\pdfvariable pkmode}

\edef\pdfsuppressoptionalinfo     {\pdfvariable suppressoptionalinfo }
\edef\pdftrailerid                {\pdfvariable trailerid }
\stoptyping

The variables are internal ones, so they are anonymous. When you ask for the
meaning of a few previously defined ones:

\starttyping
\meaning\pdfhorigin
\meaning\pdfcompresslevel
\meaning\pdfpageattr
\stoptyping

you will get:

\starttyping
macro:->[internal backend dimension]
macro:->[internal backend integer]
macro:->[internal backend tokenlist]
\stoptyping

The \prm {edef} can also be a \prm {def} but it's a bit more efficient to expand
the lookup related register beforehand.

The backend is derived from \PDFTEX\ so the same syntax applies. However, the
\type {outline} command accepts a \type {objnum} followed by a number. No
checking takes place so when this is used it had better be a valid (flushed)
object.

In order to be (more or less) compatible with \PDFTEX\ we also support the option
to suppress some info but we do so via a bitset:

\starttyping
\pdfvariable suppressoptionalinfo \numexpr
        0
    +   1   % PTEX.FullBanner
    +   2   % PTEX.FileName
    +   4   % PTEX.PageNumber
    +   8   % PTEX.InfoDict
    +  16   % Creator
    +  32   % CreationDate
    +  64   % ModDate
    + 128   % Producer
    + 256   % Trapped
    + 512   % ID
\relax
\stoptyping

In addition you can overload the trailer id, but we don't do any checking on
validity, so you have to pass a valid array. The following is like the ones
normally generated by the engine. You even need to include the brackets here!

\starttyping
\pdfvariable trailerid {[
    <FA052949448907805BA83C1E78896398>
    <FA052949448907805BA83C1E78896398>
]}
\stoptyping

Although we started from a merge of \PDFTEX\ and \ALEPH, by now the code base as
well as functionality has diverted from those parents. Here we show the options
that can be passed to the extensions. The \type {shipout} option is a compatibility
feature. Instead one can use the \type {deferred} prefix.

\starttexsyntax
\pdfextension literal
    [shipout] [ direct | page | raw ] { tokens }
\stoptexsyntax

\starttexsyntax
\pdfextension dest
    num integer | name { tokens }!crlf
    [ fitbh | fitbv | fitb | fith| fitv | fit |
      fitr <rule spec> | xyz [ zoom <integer> ]
\stoptexsyntax

\starttexsyntax
\pdfextension annot
    reserveobjnum | useobjnum <integer>
    { tokens }
\stoptexsyntax

\starttexsyntax
\pdfextension save
\stoptexsyntax

\starttexsyntax
\pdfextension restore
\stoptexsyntax

\starttexsyntax
\pdfextension setmatrix
    { tokens }
\stoptexsyntax

\starttexsyntax
[ \immediate ] \pdfextension obj
    reserveobjnum
\stoptexsyntax

\starttexsyntax
[ \immediate ] \pdfextension obj
    [ useobjnum <integer> ]
    [ uncompressed ]
    [ stream  [ attr { tokens } ] ]
    [ file ]
    { tokens }
\stoptexsyntax

\starttexsyntax
\pdfextension refobj
    <integer>
\stoptexsyntax

\starttexsyntax
\pdfextension colorstack
    <integer>
    set { tokens } | push { tokens } | pop | current
\stoptexsyntax

\starttexsyntax
\pdfextension startlink
    [ attr { tokens } ]
    user { tokens } | goto | thread
    [ file { tokens } ]
    [ page <integer> { tokens } | name { tokens } | num  integer ]
    [ newwindow  | nonewwindow ]
\stoptexsyntax

\starttexsyntax
\pdfextension endlink
\stoptexsyntax

\starttexsyntax
\pdfextension startthread
    num <integer> | name { tokens }
\stoptexsyntax

\starttexsyntax
\pdfextension endthread
\stoptexsyntax

\starttexsyntax
\pdfextension thread
    num <integer> | name { tokens }
\stoptexsyntax

\starttexsyntax
\pdfextension outline
    [ attr { tokens } ]
    [ useobjnum <integer> ]
    [ count <integer> ]
    { tokens }
\stoptexsyntax

\starttexsyntax
\pdfextension glyphtounicode
    { tokens }
    { tokens }
\stoptexsyntax

\starttexsyntax
\pdfextension catalog
    { tokens }
    [ openaction
      user { tokens } | goto | thread
      [ file { tokens } ]
      [ page <integer> { tokens } | name { tokens } | num <integer> ]
      [ newwindow  | nonewwindow ] ]
\stoptexsyntax

\starttexsyntax
\pdfextension fontattr
    <integer>
    {tokens}
\stoptexsyntax

\starttexsyntax
\pdfextension mapfile
    {tokens}
\stoptexsyntax

\starttexsyntax
\pdfextension mapline
    {tokens}
\stoptexsyntax

\starttexsyntax
\pdfextension includechars
    {tokens}
\stoptexsyntax

\starttexsyntax
\pdfextension info
    {tokens}
\stoptexsyntax

\starttexsyntax
\pdfextension names
    {tokens}
\stoptexsyntax

\starttexsyntax
\pdfextension trailer
    {tokens}
\stoptexsyntax

\stopsubsection

\startsubsection[title={Defaults}]

The engine sets the following defaults.

\starttyping
\pdfcompresslevel         9
\pdfobjcompresslevel      1 % used: (0,9)
\pdfrecompress            0 % mostly for debugging
\pdfdecimaldigits         4 % used: (3,6)
\pdfgamma              1000
\pdfimageresolution      71
\pdfimageapplygamma       0
\pdfimagegamma         2200
\pdfimagehicolor          1
\pdfimageaddfilename      1
\pdfpkresolution         72
\pdfpkfixeddpi            0
\pdfinclusioncopyfonts    0
\pdfinclusionerrorlevel   0
\pdfignoreunknownimages   0
\pdfgentounicode          0
\pdfomitcidset            0
\pdfomitcharset           0
\pdfomitinfodict          0
\pdfomitmediabox          0
\pdfpagebox               0
\pdfminorversion          4
\pdfuniqueresname         0

\pdfhorigin             1in
\pdfvorigin             1in
\pdflinkmargin          0pt
\pdfdestmargin          0pt
\pdfthreadmargin        0pt
\pdfxformmargin         0pt
\stoptyping

\stopsubsection

\startsubsection[title={Backward compatibility}]

If you also want some backward compatibility, you can add:

\starttyping
\let\pdfpagewidth      \pagewidth
\let\pdfpageheight     \pageheight

\let\pdfadjustspacing  \adjustspacing
\let\pdfprotrudechars  \protrudechars
\let\pdfnoligatures    \ignoreligaturesinfont
\let\pdffontexpand     \expandglyphsinfont
\let\pdfcopyfont       \copyfont

\let\pdfxform          \saveboxresource
\let\pdflastxform      \lastsavedboxresourceindex
\let\pdfrefxform       \useboxresource

\let\pdfximage         \saveimageresource
\let\pdflastximage     \lastsavedimageresourceindex
\let\pdflastximagepages\lastsavedimageresourcepages
\let\pdfrefximage      \useimageresource

\let\pdfsavepos        \savepos
\let\pdflastxpos       \lastxpos
\let\pdflastypos       \lastypos

\let\pdfoutput         \outputmode
\let\pdfdraftmode      \draftmode

\let\pdfpxdimen        \pxdimen

\let\pdfinsertht       \insertht

\let\pdfnormaldeviate  \normaldeviate
\let\pdfuniformdeviate \uniformdeviate
\let\pdfsetrandomseed  \setrandomseed
\let\pdfrandomseed     \randomseed

\let\pdfprimitive      \primitive
\let\ifpdfprimitive    \ifprimitive

\let\ifpdfabsnum       \ifabsnum
\let\ifpdfabsdim       \ifabsdim
\stoptyping

And even:

\starttyping
\newdimen\pdfeachlineheight
\newdimen\pdfeachlinedepth
\newdimen\pdflastlinedepth
\newdimen\pdffirstlineheight
\newdimen\pdfignoreddimen
\stoptyping

\stopsubsection

\stopsection

\startsection[title=Directions]

\topicindex {\OMEGA}
\topicindex {\ALEPH}
\topicindex {directions}

\startsubsection[title={Four directions}]

The directional model in \LUATEX\ is inherited from \OMEGA|/|\ALEPH\ but we tried
to improve it a bit. At some point we played with recovery of modes but that was
disabled later on when we found that it interfered with nested directions. That
itself had as side effect that the node list was no longer balanced with respect
to directional nodes which in turn can give side effects when a series of dir
changes happens without grouping.

When extending the \PDF\ backend to support directions some inconsistencies were
found and as a result we decided to support only the four models that make sense
\type {TLT} (latin), \type {TRT} (arabic), \type {RTT} (cjk) and \type {LTL}
(mongolian).

\stopsubsection

\startsubsection[title={How it works}]

The approach is that we again make the list balanced but try to avoid some side
effects. What happens is quite intuitive if we forget about spaces (turned into
glue) but even there what happens makes sense if you look at it in detail.
However that logic makes in|-|group switching kind of useless when no proper
nested grouping is used: switching from right to left several times nested,
results in spacing ending up after each other due to nested mirroring. Of course
a sane macro package will manage this for the user but here we are discussing the
low level dir injection.

This is what happens:

\starttyping
\textdir TRT nur {\textdir TLT run \textdir TRT NUR} nur
\stoptyping

This becomes stepwise:

\startnarrower
\starttyping
injected: [+TRT]nur {[+TLT]run [+TRT]NUR} nur
balanced: [+TRT]nur {[+TLT]run [-TLT][+TRT]NUR[-TRT]} nur[-TRT]
result  : run {RUNrun } run
\stoptyping
\stopnarrower

And this:

\starttyping
\textdir TRT nur {nur \textdir TLT run \textdir TRT NUR} nur
\stoptyping

becomes:

\startnarrower
\starttyping
injected: [+TRT]nur {nur [+TLT]run [+TRT]NUR} nur
balanced: [+TRT]nur {nur [+TLT]run [-TLT][+TRT]NUR[-TRT]} nur[-TRT]
result  : run {run RUNrun } run
\stoptyping
\stopnarrower

Now, in the following examples watch where we put the braces:

\startbuffer
\textdir TRT nur {{\textdir TLT run} {\textdir TRT NUR}} nur
\stopbuffer

\typebuffer

This becomes:

\startnarrower
\getbuffer
\stopnarrower

Compare this to:

\startbuffer
\textdir TRT nur {{\textdir TLT run }{\textdir TRT NUR}} nur
\stopbuffer

\typebuffer

Which renders as:

\startnarrower
\getbuffer
\stopnarrower

So how do we deal with the next?

\startbuffer
\def\ltr{\textdir TLT\relax}
\def\rtl{\textdir TRT\relax}

run {\rtl nur {\ltr run \rtl NUR \ltr run \rtl NUR} nur}
run {\ltr run {\rtl nur \ltr RUN \rtl nur \ltr RUN} run}
\stopbuffer

\typebuffer

It gets typeset as:

\startnarrower
\startlines
\getbuffer
\stoplines
\stopnarrower

We could define the two helpers to look back, pick up a skip, remove it and
inject it after the dir node. But that way we loose the subtype information that
for some applications can be handy to be kept as|-|is. This is why we now have a
variant of \lpr {textdir} which injects the balanced node before the skip.
Instead of the previous definition we can use:

\startbuffer[def]
\def\ltr{\linedir TLT\relax}
\def\rtl{\linedir TRT\relax}
\stopbuffer

\typebuffer[def]

and this time:

\startbuffer[txt]
run {\rtl nur {\ltr run \rtl NUR \ltr run \rtl NUR} nur}
run {\ltr run {\rtl nur \ltr RUN \rtl nur \ltr RUN} run}
\stopbuffer

\typebuffer[txt]

comes out as a properly spaced:

\startnarrower
\startlines
\getbuffer[def,txt]
\stoplines
\stopnarrower

Anything more complex that this, like combination of skips and penalties, or
kerns, should be handled in the input or macro package because there is no way we
can predict the expected behaviour. In fact, the \lpr {linedir} is just a
convenience extra which could also have been implemented using node list parsing.

Directions are complicated by the fact that they often need to work over groups
so a separate grouping related stack is used. A side effect is that there can be
paragraphs with only a local par node followed by direction synchronization
nodes. Paragraphs like that are seen as empty paragraphs and therefore ignored.
Because \type {\noindent} doesn't inject anything but a \type {\indent} injects
an box, paragraphs with only an indent and directions are handles ad paragraphs
with content.

\stopsubsection

\startsubsection[title={Controlling glue with \lpr {breakafterdirmode}}]

Glue after a dir node is ignored in the linebreak decision but you can bypass that
by setting \lpr {breakafterdirmode} to~\type {1}. The following table shows the
difference. Watch your spaces.

\def\ShowSome#1{%
    \BC \type{#1}
    \NC \breakafterdirmode\zerocount\hsize\zeropoint#1
    \NC
    \NC \breakafterdirmode\plusone\hsize\zeropoint#1
    \NC
    \NC \NR
}

\starttabulate[|l|Tp(1pt)|w(5em)|Tp(1pt)|w(5em)|]
    \DB
    \BC \type{0}
    \NC
    \BC \type{1}
    \NC
    \NC \NR
    \TB
    \ShowSome{pre {\textdir TLT xxx} post}
    \ShowSome{pre {\textdir TLT xxx }post}
    \ShowSome{pre{ \textdir TLT xxx} post}
    \ShowSome{pre{ \textdir TLT xxx }post}
    \ShowSome{pre { \textdir TLT xxx } post}
    \ShowSome{pre {\textdir TLT\relax\space xxx} post}
    \LL
\stoptabulate

\stopsubsection

\startsubsection[title={Controling parshapes with \lpr {shapemode}}]

Another adaptation to the \ALEPH\ directional model is control over shapes driven
by \prm {hangindent} and \prm {parshape}. This is controlled by a new parameter
\lpr {shapemode}:

\starttabulate[|c|l|l|]
\DB value    \BC \prm {hangindent} \BC \prm {parshape} \NC \NR
\TB
\BC \type{0} \NC  normal             \NC normal            \NC \NR
\BC \type{1} \NC  mirrored           \NC normal            \NC \NR
\BC \type{2} \NC  normal             \NC mirrored          \NC \NR
\BC \type{3} \NC  mirrored           \NC mirrored          \NC \NR
\LL
\stoptabulate

The value is reset to zero (like \prm {hangindent} and \prm {parshape})
after the paragraph is done with. You can use negative values to prevent
this. In \in {figure} [fig:shapemode] a few examples are given.

\startplacefigure[reference=fig:shapemode,title={The effect of \type {shapemode}.}]
    \startcombination[2*3]
        {\ruledvbox \bgroup \setuptolerance[verytolerant]
            \hsize .45\textwidth \switchtobodyfont[6pt]
                \pardir TLT \textdir TLT
                \hangindent 40pt \hangafter -3
                \leftskip10pt \input tufte \par
         \egroup} {TLT: hangindent}
        {\ruledvbox \bgroup \setuptolerance[verytolerant]
            \hsize .45\textwidth \switchtobodyfont[6pt]
            \pardir TLT \textdir TLT
            \parshape 4 0pt .8\hsize 10pt .8\hsize 20pt .8\hsize 0pt \hsize
            \input tufte \par
         \egroup} {TLT: parshape}
        {\ruledvbox \bgroup \setuptolerance[verytolerant]
            \hsize .45\textwidth \switchtobodyfont[6pt]
            \pardir TRT \textdir TRT
            \hangindent 40pt \hangafter -3
            \leftskip10pt \input tufte \par
         \egroup} {TRT: hangindent mode 0}
        {\ruledvbox \bgroup \setuptolerance[verytolerant]
            \hsize .45\textwidth \switchtobodyfont[6pt]
            \pardir TRT \textdir TRT
            \parshape 4 0pt .8\hsize 10pt .8\hsize 20pt .8\hsize 0pt \hsize
            \input tufte \par
         \egroup} {TRT: parshape mode 0}
        {\ruledvbox \bgroup \setuptolerance[verytolerant]
            \hsize .45\textwidth \switchtobodyfont[6pt]
            \shapemode=3
            \pardir TRT \textdir TRT
            \hangindent 40pt \hangafter -3
            \leftskip10pt \input tufte \par
         \egroup} {TRT: hangindent mode 1 & 3}
        {\ruledvbox \bgroup \setuptolerance[verytolerant]
            \hsize .45\textwidth \switchtobodyfont[6pt]
            \shapemode=3
            \pardir TRT \textdir TRT
            \parshape 4 0pt .8\hsize 10pt .8\hsize 20pt .8\hsize 0pt \hsize
            \input tufte \par
         \egroup} {TRT: parshape mode 2 & 3}
    \stopcombination
\stopplacefigure

\stopsubsection

\startsubsection[title={Symbols or numbers}]

Internally the implementation is different from \ALEPH. First of all we use no
whatsits but dedicated nodes, but also we have only 4 directions that are mapped
onto 4 numbers. A text direction node can mark the start or end of a sequence of
nodes, and therefore has two states. At the \TEX\ end we don't see these states
because \TEX\ itself will add proper end state nodes if needed.

The symbolic names \type {TLT}, \type {TRT}, etc.\ originate in \OMEGA. In
\LUATEX\ we also have a number based model which sometimes makes more sense.

\starttabulate[|c|l|l|]
\DB value     \BC equivalent \NC \NR
\TB
\BC \type {0} \NC TLT \NC \NR
\BC \type {1} \NC TRT \NC \NR
\BC \type {2} \NC LTL \NC \NR
\BC \type {3} \NC RTT \NC \NR
\LL
\stoptabulate

We support the \OMEGA\ primitives \orm {textdir}, \orm {pardir}, \orm {pagedir},
\orm {pardir} and \orm {mathdir}. These accept three character keywords. The
primitives that set the direction by number are: \lpr {textdirection}, \lpr
{pardirection}, \lpr {pagedirection} and \lpr {bodydirection} and \lpr
{mathdirection}. When specifying a direction for a box you can use \type {bdir}
instead of \type {dir}.

\stopsubsection

\stopsection

\startsection[title=Implementation notes]

\startsubsection[title=Memory allocation]

\topicindex {memory}

The single internal memory heap that traditional \TEX\ used for tokens and nodes
is split into two separate arrays. Each of these will grow dynamically when
needed.

The \type {texmf.cnf} settings related to main memory are no longer used (these
are: \type {main_memory}, \type {mem_bot}, \type {extra_mem_top} and \type
{extra_mem_bot}). \quote {Out of main memory} errors can still occur, but the
limiting factor is now the amount of RAM in your system, not a predefined limit.

Also, the memory (de)allocation routines for nodes are completely rewritten. The
relevant code now lives in the C file \type {texnode.c}, and basically uses a
dozen or so \quote {avail} lists instead of a doubly|-|linked model. An extra
function layer is added so that the code can ask for nodes by type instead of
directly requisitioning a certain amount of memory words.

Because of the split into two arrays and the resulting differences in the data
structures, some of the macros have been duplicated. For instance, there are now
\type {vlink} and \type {vinfo} as well as \type {token_link} and \type
{token_info}. All access to the variable memory array is now hidden behind a
macro called \type {vmem}. We mention this because using the \TEX book as
reference is still quite valid but not for memory related details. Another
significant detail is that we have double linked node lists and that most nodes
carry more data.

The input line buffer and pool size are now also reallocated when needed, and the
\type {texmf.cnf} settings \type {buf_size} and \type {pool_size} are silently
ignored.

\stopsubsection

\startsubsection[title=Sparse arrays]

The \prm {mathcode}, \prm {delcode}, \prm {catcode}, \prm {sfcode}, \prm {lccode}
and \prm {uccode} (and the new \lpr {hjcode}) tables are now sparse arrays that
are implemented in~\CCODE. They are no longer part of the \TEX\ \quote
{equivalence table} and because each had 1.1 million entries with a few memory
words each, this makes a major difference in memory usage. Performance is not
really hurt by this.

The \prm {catcode}, \prm {sfcode}, \prm {lccode}, \prm {uccode} and \lpr {hjcode}
assignments don't show up when using the \ETEX\ tracing routines \prm
{tracingassigns} and \prm {tracingrestores} but we don't see that as a real
limitation.

A side|-|effect of the current implementation is that \prm {global} is now more
expensive in terms of processing than non|-|global assignments but not many users
will notice that.

The glyph ids within a font are also managed by means of a sparse array as glyph
ids can go up to index $2^{21}-1$ but these are never accessed directly so again
users will not notice this.

\stopsubsection

\startsubsection[title=Simple single|-|character csnames]

\topicindex {csnames}

Single|-|character commands are no longer treated specially in the internals,
they are stored in the hash just like the multiletter csnames.

The code that displays control sequences explicitly checks if the length is one
when it has to decide whether or not to add a trailing space.

Active characters are internally implemented as a special type of multi|-|letter
control sequences that uses a prefix that is otherwise impossible to obtain.

\stopsubsection

\startsubsection[title=The compressed format file]

\topicindex {format}

The format is passed through \type {zlib}, allowing it to shrink to roughly half
of the size it would have had in uncompressed form. This takes a bit more \CPU\
cycles but much less disk \IO, so it should still be faster. We use a level~3
compression which we found to be the optimal trade|-|off between filesize and
decompression speed.

\stopsubsection

\startsubsection[title=Binary file reading]

\topicindex {files+binary}

All of the internal code is changed in such a way that if one of the \type
{read_xxx_file} callbacks is not set, then the file is read by a \CCODE\ function
using basically the same convention as the callback: a single read into a buffer
big enough to hold the entire file contents. While this uses more memory than the
previous code (that mostly used \type {getc} calls), it can be quite a bit faster
(depending on your \IO\ subsystem).

\stopsubsection

\startsubsection[title=Tabs and spaces]

\topicindex {space}
\topicindex {newline}

We conform to the way other \TEX\ engines handle trailing tabs and spaces. For
decades trailing tabs and spaces (before a newline) were removed from the input
but this behaviour was changed in September 2017 to only handle spaces. We are
aware that this can introduce compatibility issues in existing workflows but
because we don't want too many differences with upstream \TEXLIVE\ we just follow
up on that patch (which is a functional one and not really a fix). It is up to
macro packages maintainers to deal with possible compatibility issues and in
\LUATEX\ they can do so via the callbacks that deal with reading from files.

The previous behaviour was a known side effect and (as that kind of input
normally comes from generated sources) it was normally dealt with by adding a
comment token to the line in case the spaces and|/|or tabs were intentional and
to be kept. We are aware of the fact that this contradicts some of our other
choices but consistency with other engines and the fact that in \KPSE\ mode a
common file \IO\ layer is used can have a side effect of breaking compatibility.
We still stick to our view that at the log level we can (and might be) more
incompatible. We already expose some more details.

\stopsubsection

\startsubsection[title=Hyperlinks]

\topicindex {hyperlinks}

There is an experimental feature that makes multi|-|line hyper links behave a
little better, fixing some side effects that showed up in r2l typesetting but
also can surface in l2r. Because this got unnoticed till 2023, and because it
depends bit on how macro packages deal with hyper links, the fix is currently
under parameter control:

\starttyping
\pdfvariable linking = 1
\stoptyping

That way (we hope) legacy documents come out as expected, whatever those
expectations are. One of the aspects dealt with concerns (unusual) left and right
skips.

\stopsubsection

\stopsection

\stopchapter

\stopcomponent
