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-rw-r--r--luaotfload.dtx65
1 files changed, 60 insertions, 5 deletions
diff --git a/luaotfload.dtx b/luaotfload.dtx
index b03154e..e6e9523 100644
--- a/luaotfload.dtx
+++ b/luaotfload.dtx
@@ -755,10 +755,64 @@ and the derived files
%
% \begin{quote}
% \begin{verbatim}
-% \font\test={Latin Modern Roman}:color=FF0000BB
+% \font\test={Latin Modern Roman}:color=FF0000BB
% \end{verbatim}
% \end{quote}
%
+% \item [kernfactor \& letterspace] \hfill \\
+% Define a font with letterspacing (tracking) enabled.
+% In \identifier{luaotfload}, letterspacing is implemented by
+% inserting additional kerning between glyphs.
+%
+% This approach is derived from and still quite similar to the
+% \emphasis{character kerning} (\texmacro{setcharacterkerning} /
+% \texmacro{definecharacterkerning} \& al.) functionality of
+% Context, see the file \fileent{typo-krn.lua} there.
+% The main difference is that \identifier{luaotfload} does not
+% use \LUATEX attributes to assign letterspacing to regions,
+% but defines virtual letterspaced versions of a font.
+%
+% The option \identifier{kernfactor} accepts a numeric value that
+% determines the letterspacing factor to be applied to the font
+% size.
+% E.~g. a kern factor of $0.42$ applied to a $10$ pt font
+% results in $4.2$ pt of additional kerning applied to each
+% pair of glyphs.
+% Ligatures are split into their component glyphs unless
+% explicitly ignored (see below).
+%
+% For compatibility with \XETEX an alternative
+% \identifier{letterspace} option is supplied that interprets the
+% supplied value as a \emphasis{percentage} of the font size but
+% is otherwise identical to \identifier{kernfactor}.
+% Consequently, both definitions in below snippet yield the same
+% letterspacing width:
+%
+% \begin{quote}
+% \begin{verbatim}
+% \font\iwonakernedA="file:Iwona-Regular.otf:kernfactor=0.125"
+% \font\iwonakernedB="file:Iwona-Regular.otf:letterspace=12.5"
+% \end{verbatim}
+% \end{quote}
+%
+% Specific pairs of letters and ligatures may be exempt from
+% letterspacing by defining the \LUA functions
+% \luafunction{keeptogether} and \luafunction{keepligature},
+% respectively, inside the namespace \verb|luaotfload.letterspace|.
+% Both functions are called whenever the letterspacing callback
+% encounters an appropriate node or set of nodes.
+% If they return a true-ish value, no extra kern is inserted at
+% the current position.
+% \luafunction{keeptogether} receives a pair of consecutive
+% glyph nodes in order of their appearance in the node list.
+% \luafunction{keepligature} receives a single node which can be
+% analyzed into components.
+% (For details refer to the \emphasis{glyph nodes} section in the
+% \LUATEX reference manual.)
+% The implementation of both functions is left entirely to the
+% user.
+%
+%
% \item [protrusion \& expansion] \hfill \\
% These keys control microtypographic features of the font,
% namely \emphasis{character protrusion} and \emphasis{font
@@ -771,14 +825,15 @@ and the derived files
% Alternatively and with loss of information, you can dump
% those tables into your terminal by issuing
% \begin{verbatim}
-% \directlua{inspect(fonts.protrusions.setups.default)
-% inspect(fonts.expansions.setups.default)}
+% \directlua{inspect(fonts.protrusions.setups.default)
+% inspect(fonts.expansions.setups.default)}
% \end{verbatim}
% at some point after loading \fileent{luaotfload.sty}.
% }
% For both, only the set \identifier{default} is predefined.
%
-% For example, to enable default protrusion\footnote{%
+% For example, to define a font with the default
+% protrusion vector applied\footnote{%
% You also need to set
% \verb|pdfprotrudechars=2| and
% \verb|pdfadjustspacing=2|
@@ -791,7 +846,7 @@ and the derived files
%
% \begin{quote}
% \begin{verbatim}
-% \font\test=LatinModernRoman:protrusion=default
+% \font\test=LatinModernRoman:protrusion=default
% \end{verbatim}
% \end{quote}
% \end{description}