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+% Copyright 2003 by Till Tantau <tantau@cs.tu-berlin.de>.
+%
+% This program can be redistributed and/or modified under the terms
+% of the LaTeX Project Public License Distributed from CTAN
+% archives in directory macros/latex/base/lppl.txt.
+
+\section{The Soft Path Subsystem}
+
+\label{section-soft-paths}
+
+\makeatletter
+
+
+This section describes a set of commands for creating \emph{soft
+ paths} as opposed to the commands of the previous section, which
+created \emph{hard paths}. A soft path is a path that can still be
+``changed'' or ``molded.'' Once you (or the \pgfname\ system) is
+satisfied with a soft path, it is turned into a hard path, which can
+be inserted into the resulting |.pdf| or |.ps| file.
+
+Note that the commands described in this section are ``high-level'' in
+the sense that they are not implemented in driver files, but rather
+directly by the \pgfname-system layer. For this reason, the commands for
+creating soft paths do not start with |\pgfsys@|, but rather with
+|\pgfsyssoftpath@|. On the other hand, as a user you will never use
+these commands directly, so they are described as part of the
+low-level interface.
+
+
+
+\subsection{Path Creation Process}
+
+When the user writes a command like |\draw (0bp,0bp) -- (10bp,0bp);|
+quite a lot happens behind the scenes:
+\begin{enumerate}
+\item
+ The frontend command is translated by \tikzname\ into commands
+ of the basic layer. In essence, the command is translated to
+ something like
+\begin{codeexample}[code only]
+\pgfpathmoveto{\pgfpoint{0bp}{0bp}}
+\pgfpathlineto{\pgfpoint{10bp}{0bp}}
+\pgfusepath{stroke}
+\end{codeexample}
+\item
+ The |\pgfpathxxxx| command do \emph{not} directly call ``hard''
+ commands like |\pgfsys@xxxx|. Instead, the command |\pgfpathmoveto|
+ invokes a special command called |\pgfsyssoftpath@moveto| and
+ |\pgfpathlineto| invokes |\pgfsyssoftpath@lineto|.
+
+ The |\pgfsyssoftpath@xxxx| commands, which are described below,
+ construct a soft path. Each time such a command is used, special
+ tokens are added to the end of an internal macro that stores the
+ soft path currently being constructed.
+\item
+ When the |\pgfusepath| is encountered, the soft path stored in
+ the internal macro is ``invoked.'' Only now does a special macro
+ iterate over the soft path. For each line-to or move-to
+ operation on this path it calls an appropriate |\pgfsys@moveto| or
+ |\pgfsys@lineto| in order to, finally, create the desired hard path,
+ namely, the string of literals in the |.pdf| or |.ps| file.
+\item
+ After the path has been invoked, |\pgfsys@stroke| is called to
+ insert the literal for stroking the path.
+\end{enumerate}
+
+Why such a complicated process? Why not have |\pgfpathlineto| directly
+call |\pgfsys@lineto| and be done with it? There are two reasons:
+\begin{enumerate}
+\item
+ The \pdf\ specification requires that a path is not interrupted by
+ any non-path-construction commands. Thus, the following code will
+ result in a corrupted |.pdf|:
+\begin{codeexample}[code only]
+\pgfsys@moveto{0}{0}
+\pgfsys@setlinewidth{1}
+\pgfsys@lineto{10}{0}
+\pgfsys@stroke
+\end{codeexample}
+ Such corrupt code is \emph{tolerated} by most viewers, but not
+ always. It is much better to create only (reasonably) legal code.
+\item
+ A soft path can still be changed, while a hard path is fixed. For
+ example, one can still change the starting and end points of a soft
+ path or do optimizations on it. Such transformations are not possible
+ on hard paths.
+\end{enumerate}
+
+
+\subsection{Starting and Ending a Soft Path}
+
+No special action must be taken in order to start the creation of a
+soft path. Rather, each time a command like |\pgfsyssoftpath@lineto|
+is called, a special token is added to the (global) current soft path
+being constructed.
+
+However, you can access and change the current soft path. In this way,
+it is possible to store a soft path, to manipulate it, or to invoke
+it.
+
+\begin{command}{\pgfsyssoftpath@getcurrentpath\marg{macro name}}
+ This command will store the current soft path in \meta{macro name}.
+\end{command}
+
+\begin{command}{\pgfsyssoftpath@setcurrentpath\marg{macro name}}
+ This command will set the current soft path to be the path stored in
+ \meta{macro name}. This macro should store a path that has
+ previously been extracted using the |\pgfsyssoftpath@getcurrentpath|
+ command and has possibly been modified subsequently.
+\end{command}
+
+\begin{command}{\pgfsyssoftpath@invokecurrentpath}
+ This command will turn the current soft path in a ``hard'' path. To
+ do so, it iterates over the soft path and calls an appropriate
+ |\pgfsys@xxxx| command for each element of the path. Note that the
+ current soft path is \emph{not changed} by this command. Thus, in
+ order to start a new soft path after the old one has been invoked
+ and is no longer needed, you need to set the current soft path to be
+ empty. This may seems strange, but it is often useful to immediately
+ use the last soft path again.
+\end{command}
+
+\begin{command}{\pgfsyssoftpath@flushcurrentpath}
+ This command will invoke the current soft path and then set it to be
+ empty.
+\end{command}
+
+
+
+\subsection{Soft Path Creation Commands}
+
+\begin{command}{\pgfsyssoftpath@moveto\marg{x}\marg{y}}
+ This command appends a ``move-to'' segment to the current soft
+ path. The coordinates \meta{x} and \meta{y} are given as normal
+ \TeX\ dimensions.
+
+ \example One way to draw a line:
+\begin{codeexample}[code only]
+\pgfsyssoftpath@moveto{0pt}{0pt}
+\pgfsyssoftpath@lineto{10pt}{10pt}
+\pgfsyssoftpath@flushcurrentpath
+\pgfsys@stroke
+\end{codeexample}
+\end{command}
+
+\begin{command}{\pgfsyssoftpath@lineto\marg{x}\marg{y}}
+ Appends a ``line-to'' segment to the current soft path.
+\end{command}
+
+\begin{command}{\pgfsyssoftpath@curveto\marg{a}\marg{b}\marg{c}\marg{d}\marg{x}\marg{y}}
+ Appends a ``curve-to'' segment to the current soft path with controls
+ $(a,b)$ and $(c,d)$.
+\end{command}
+
+\begin{command}{\pgfsyssoftpath@rect\marg{lower left x}\marg{lower left y}\marg{width}\marg{height}}
+ Appends a rectangle segment to the current soft path.
+\end{command}
+
+\begin{command}{\pgfsyssoftpath@closepath}
+ Appends a ``close-path'' segment to the current soft path.
+\end{command}
+
+
+
+
+\subsection{The Soft Path Data Structure}
+
+A soft path is stored in a standardized way, which makes it possible to
+modify it before it becomes ``hard.'' Basically, a soft path is a long
+sequence of triples. Each triple starts with a \emph{token} that
+identifies what is going on. This token is followed by two dimensions in
+braces. For example, the following is a soft path that means ``the
+path starts at $(0\mathrm{bp}, 0\mathrm{bp})$ and then
+continues in a straight line to $(10\mathrm{bp},
+0\mathrm{bp})$.''
+
+\begin{codeexample}[code only]
+\pgfsyssoftpath@movetotoken{0bp}{0bp}\pgfsyssoftpath@linetotoken{10bp}{0bp}
+\end{codeexample}
+
+A curve-to is hard to express in this way since we need six numbers to
+express it, not two. For this reasons, a curve-to is expressed using
+three triples as follows: The command
+\begin{codeexample}[code only]
+\pgfsyssoftpath@curveto{1bp}{2bp}{3bp}{4bp}{5bp}{6bp}
+\end{codeexample}
+\noindent
+results in the following three triples:
+\begin{codeexample}[code only]
+\pgfsyssoftpath@curvetosupportatoken{1bp}{2bp}
+\pgfsyssoftpath@curvetosupportbtoken{3bp}{4bp}
+\pgfsyssoftpath@curvetotoken{5bp}{6bp}
+\end{codeexample}
+
+These three triples must always ``remain together.'' Thus, a lonely
+|supportbtoken| is forbidden.
+
+In details, the following tokens exist:
+\begin{itemize}
+\item
+ \declare{|\pgfsyssoftpath@movetotoken|} indicates a move-to
+ operation. The two following numbers indicate the position to which
+ the current point should be moved.
+\item
+ \declare{|\pgfsyssoftpath@linetotoken|} indicates a line-to
+ operation.
+\item
+ \declare{|\pgfsyssoftpath@curvetosupportatoken|} indicates the first
+ control point of a curve-to operation. The triple must be followed
+ by a |\pgfsyssoftpath@curvetosupportbtoken|.
+\item
+ \declare{|\pgfsyssoftpath@curvetosupportbtoken|} indicates the second
+ control point of a curve-to operation. The triple must be followed
+ by a |\pgfsyssoftpath@curvetotoken|.
+\item
+ \declare{|\pgfsyssoftpath@curvetotoken|} indicates the target
+ of a curve-to operation.
+\item
+ \declare{|\pgfsyssoftpath@rectcornertoken|} indicates the corner of
+ a rectangle on the soft path. The triple must be followed
+ by a |\pgfsyssoftpath@rectsizetoken|.
+\item
+ \declare{|\pgfsyssoftpath@rectsizetoken|} indicates the size of
+ a rectangle on the soft path.
+\item
+ \declare{|\pgfsyssoftpath@closepath|} indicates that the subpath
+ begun with the last move-to operation should be closed. The parameter
+ numbers are currently not important, but if set to anything
+ different from |{0pt}{0pt}|, they should be set to the coordinate of
+ the original move-to operation to which the path ``returns'' now.
+\end{itemize}
+
+
+
+