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author | Norbert Preining <norbert@preining.info> | 2019-09-02 13:46:59 +0900 |
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committer | Norbert Preining <norbert@preining.info> | 2019-09-02 13:46:59 +0900 |
commit | e0c6872cf40896c7be36b11dcc744620f10adf1d (patch) | |
tree | 60335e10d2f4354b0674ec22d7b53f0f8abee672 /graphics/pgf/base/doc/text-en/pgfmanual-en-pgfsys-paths.tex |
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diff --git a/graphics/pgf/base/doc/text-en/pgfmanual-en-pgfsys-paths.tex b/graphics/pgf/base/doc/text-en/pgfmanual-en-pgfsys-paths.tex new file mode 100644 index 0000000000..e432823121 --- /dev/null +++ b/graphics/pgf/base/doc/text-en/pgfmanual-en-pgfsys-paths.tex @@ -0,0 +1,217 @@ +% 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} |