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authorNorbert Preining <norbert@preining.info>2019-09-02 13:46:59 +0900
committerNorbert Preining <norbert@preining.info>2019-09-02 13:46:59 +0900
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+% Copyright 2006 by Till Tantau
+%
+% This file may be distributed and/or modified
+%
+% 1. under the LaTeX Project Public License and/or
+% 2. under the GNU Free Documentation License.
+%
+% See the file doc/generic/pgf/licenses/LICENSE for more details.
+
+
+\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, 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| commands 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 seem 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}