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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 2013 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{Defining New Arrow Tip Kinds}
+\label{section-arrows}
+
+\subsection{Overview}
+
+In present section we have a look at how you can define new arrow tips for use
+in \pgfname. The low-level commands for selecting which arrow tips are to be
+used have already been described in Section~\ref{section-tips}, the general
+syntax rules for using arrows are detailed in
+Section~\ref{section-tikz-arrows}. Although Section~\ref{section-tikz-arrows}
+describes the use of arrows in \tikzname, in reality, \tikzname\ itself does
+not actually do anything about arrow tips; all of the functionality is
+implemented on the \pgfname\ level in the commands described in
+Section~\ref{section-tikz-arrows}. Indeed, even the |/.tip| key handler
+described in Section~\ref{section-tikz-arrows} is actually implemented on the
+\pgfname\ layer.
+
+What has \emph{not} yet been covered is how you can actually define a complete
+new arrow tip. In \pgfname, arrows are ``meta-arrows'' in the same way that
+fonts in \TeX\ are ``meta-fonts''. When a meta-arrow is resized, it is not
+simply scaled, but a possibly complicated transformation is applied to the
+size.
+
+A meta-font is not one particular font at a specific size with a specific
+stroke width (and with a large number of other parameters being fixed). Rather,
+it is a ``blueprint'' (actually, more like a program) for generating such a
+font at a particular size and width. This allows the designer of a meta-font to
+make sure that, say, the font is somewhat thicker and wider at very small
+sizes. To appreciate the difference: Compare the following texts: ``Berlin''
+and ``\tikz{\node [scale=2,inner sep=0pt,outer sep=0pt]{\tiny Berlin};}''. The
+first is a ``normal'' text, the second is the tiny version scaled by a factor
+of two. Obviously, the first look better. Now, compare ``\tikz{\node
+[scale=.5,inner sep=0pt,outer sep=0pt]{Berlin};}'' and ``{\tiny Berlin}''. This
+time, the normal text was scaled down, while the second text is a ``normal''
+tiny text. The second text is easier to read.
+
+\pgfname's meta-arrows work in a similar fashion: The shape of an arrow tip can
+vary according to a great number of parameters, the line width of the arrow tip
+being one of them. Thus, an arrow tip drawn at a line width of 5pt will
+typically \emph{not} be five times as large as an arrow tip of line width 1pt.
+Instead, the size of the arrow will get bigger only slowly as the line width
+increases.
+
+To appreciate the difference, here are the |Latex| and
+|Classical TikZ Rightarrow| arrows, as drawn by \pgfname\ at four different
+sizes:
+
+\medskip
+\begin{tikzpicture}[1/.tip=Latex, 2/.tip=Classical TikZ Rightarrow]
+ \draw[-1, line width=0.1pt] (0pt,0ex) -- +(3,0) node[thin,right] {line width is 0.1pt};
+ \draw[-1, line width=0.4pt] (0pt,-2em) -- +(3,0) node[thin,right] {line width is 0.4pt};
+ \draw[-1, line width=1.2pt] (0pt,-4em) -- +(3,0) node[thin,right] {line width is 1.2pt};
+ \draw[-1, line width=5pt] (0pt,-6em) -- +(3,0) node[thin,right] {line width is 5pt};
+
+ \draw[-2, line width=0.1pt] (6cm,0ex) -- +(3,0) node[thin,right] {line width is 0.1pt};
+ \draw[-2, line width=0.4pt] (6cm,-2em) -- +(3,0) node[thin,right] {line width is 0.4pt};
+ \draw[-2, line width=1.2pt] (6cm,-4em) -- +(3,0) node[thin,right] {line width is 1.2pt};
+ \draw[-2, line width=5pt] (6cm,-6em) -- +(3,0) node[thin,right] {line width is 5pt};
+\end{tikzpicture}
+
+\medskip
+Here, by comparison, are the same arrows when they are simply ``resized'':
+
+\medskip
+\begin{tikzpicture}[1/.tip=Latex, 2/.tip=Classical TikZ Rightarrow]
+ \draw[-{1[length=1pt]}, line width=0.1pt] (0pt,0ex) -- +(3,0) node[thin,right] {line width is 0.1pt};
+ \draw[-{1[length=4pt]}, line width=0.4pt] (0pt,-2em) -- +(3,0) node[thin,right] {line width is 0.4pt};
+ \draw[-{1[length=12pt]}, line width=1.2pt] (0pt,-4em) -- +(3,0) node[thin,right] {line width is 1.2pt};
+ \draw[-{1[length=32pt]}, line width=5pt] (0pt,-6em) -- +(3,0) node[thin,right] {line width is 5pt};
+
+ \draw[-{2[length=0.455pt]}, line width=0.1pt] (6cm,0ex) -- +(3,0) node[thin,right] {line width is 0.1pt};
+ \draw[-{2[length=1.82pt]}, line width=0.4pt] (6cm,-2em) -- +(3,0) node[thin,right] {line width is 0.4pt};
+ \draw[-{2[length=5.46pt]}, line width=1.2pt] (6cm,-4em) -- +(3,0) node[thin,right] {line width is 1.2pt};
+ \draw[-{2[length=14.56pt]}, line width=5pt] (6cm,-6em) -- +(3,0) node[thin,right] {line width is 5pt};
+\end{tikzpicture}
+
+\bigskip
+As can be seen, simple scaling produces arrow tips that are way too large at
+larger sizes and way too small at smaller sizes.
+
+In addition to the line width, other options may also influence the appearance
+of an arrow tip. In particular, the width of the inner line (the line used to
+create the effect of a double line) influences arrow tips as well as other
+options that are specific to the arrow tip.
+
+
+\subsection{Terminology}
+\label{section-arrow-terminology}
+
+Before we have a look at the exact commands used for defining arrow tips, we
+need to fix some terminology. Consider the following drawing of an arrow tip
+where the arrow tip is drawn transparently so that we can see what is
+``happening behind it'':
+%
+\begin{tikzpicture}
+ \draw [red!50, ,line width=1cm] (0,0) -- (4,0);
+ \path [tips, opacity=.25,line width=1cm, -{Stealth[black,line width=0pt,length=4cm, width=4cm, inset=1cm]}] (0,0) -- (6,0);
+
+ \draw [->,thick] (1,0) -- (8,0) node [right] {$x$-axis};
+ \draw [->,thick] (5,-2.25) -- (5,2.25) node [above] {$y$-axis};
+
+ \foreach \i in {-3,-2,-1,1,2} \draw (\i+5,-1mm) -- (\i+5,1mm) node [above] {\small$\i$};
+ \foreach \i in {-2,-1,1,2} \draw (49mm,\i) -- (51mm,\i) node [right] {\small$\i$};;
+\end{tikzpicture}
+
+I have also added a coordinate system. The code for drawing an arrow tip always
+draws it in the way shown above: Pointing right along the $x$-axis.
+
+We will use the following terminology:
+%
+\begin{itemize}
+ \item The point where tip of the arrow ends is called the \emph{tip end}.
+ It is at $(1,0)$ in our example and we always assume it to lie on the
+ $x$-axis, so we just treat it as a distance, 1 in this case. This is
+ the position where the original path was supposed to end (so if the
+ arrow tip had not been added to the red path, it would have ended
+ here).
+ \item The \emph{back end} of the arrow is where a vertical line just to the
+ left of the arrow intersects the $x$-axis. In our case, this is the
+ point $(-3,0)$ and again we treat it as a distance, $-3$ in this case.
+ \item The \emph{line end} is the position where the path now ends. This
+ should be a position inside the arrow head that gets ``covered'' by the
+ path. Note that a path may have a round or a rect head and should still
+ be covered. Clearly, necessary shortening of the path will be the
+ difference between the tip end and the line end.
+ \item The \emph{visual back end} is the position where the path and the the
+ arrow head ``meet last'' on the path. In our case, because of the
+ inset, the visual back end is not the same as the back end: The arrow
+ ends ``visually'' at $(-2,0)$. The difference between the back end and
+ the visual back end is important when the arrow tip is flexed, see
+ Section~\ref{section-arrow-flex} for an explanation of flexing.
+ \item There is also a \emph{visual tip end}, the counterpart of the visual
+ back end for the front. In our case, the visual tip end and the tip end
+ obviously coincide, but if we were to reverse the arrow tip, the visual
+ tip end would be different form the tip end (while the visual back end
+ would then coincide with the new back end).
+ \item There are four points that make up the \emph{convex hull} of the
+ arrow tip: $(1,0)$, $(-3,2)$, and $(-3,-2)$.
+
+ Normally, \pgfname\ automatically keeps track of a bounding box of
+ everything you draw. However, since arrow tips are drawn so often,
+ \pgfname\ caches the code needing for drawing arrow tips internally and
+ because of this cache it cannot determine the size of the arrow tip
+ just based on the drawing commands used for drawing the tip. Instead, a
+ convex hull of the arrow tip must be explicitly provided in the
+ definition.
+\end{itemize}
+
+When you design a new arrow tip, all of the above parameters must be defined.
+
+
+\subsection{Caching and Rendering of Arrows}
+
+As a last preparation for the description of the commands for declaring arrows,
+it is important to understand the exact process by which \pgfname\ draws
+arrows.
+%
+\begin{enumerate}
+ \item First, you have to define an arrow tip kind using
+ |\pgfdeclarearrow{name=foo,...|. This will tell \pgfname\ that |foo| is %}
+ now the name of an arrow tip. In particular, the parser for arrow tip
+ specifications will now treat |foo| as the name of an arrow tip and
+ will not try to consider |f|, |o|, and |o| as the names of single-char
+ shorthands.
+
+ Other than storing the definitions in the declaration internally, this
+ command has little other effect. In particular, no drawing or other
+ processing takes place.
+ \item Now assume that at some point the arrow tip |foo| is actually used.
+ In this case, certain options may have been set, for instance the user
+ may have requested the arrow tip |foo[length=5pt,open]|. What happens
+ next depends on whether it is the first time the arrow tip |foo| is
+ used with \emph{these exact options} or not.
+ \item Assume that is the first time |foo| is requested at a length of 5pt
+ and in an ``open'' version. \pgfname\ now retrieves the definition of
+ the arrow tip kind that it stored in the first step and executes the
+ so-called \emph{setup} code. When this code is executed, all the
+ options will be in force (for instance, |\pgfarrowlength| will equal
+ |5pt| in our case). The job of the setup code is two-fold: First, it
+ needs to compute all of the parameters listed in
+ Section~\ref{section-arrow-terminology}, that is, it has to compute
+ where the tip end will lie in the arrow tip's coordinate system
+ \emph{at the particular size of 5pt}, where the back end will be, where
+ the convex hull points lie, and so on. Second, the setup code should
+ precompute values that will be important for constructing the path of
+ the arrow. In our example, there is little to do in this regard, but
+ for more complicated arrows, all time-consuming preparations are done
+ now.
+
+ It is \emph{not} the job of the setup to actually draw the arrow tip,
+ only to ``prepare'' this as much as possible.
+
+ The setup code will always be executed only once for each arrow tip
+ kind for a given set of options. Thus, when a user uses
+ |foo[length=5pt,open]| once more later anywhere in the document, the
+ setup code will not be executed again.
+ \item The next thing that happens is that we have a look at the
+ \emph{drawin code} stored in the |code| field of the arrow. In our
+ example, the drawing code would consist of creating a filled path with
+ four straight segments.
+
+ In most cases, what happens now is that the drawing code is executed in
+ a special sandbox in which the low-level driver commands that do the
+ actual drawing are intercepted and stored away in a so-called
+ \emph{cache}. Once such a cache has been created, its contents will be
+ reused whenever |foo[length=5pt,open]| is requested by a user and just
+ like the setup code, the drawing code will not be executed again.
+
+ There are, however, two cases in which the drawing code gets executed
+ each time the arrow is used: First, an arrow tip kind can specify that
+ this should always happen by saying |cachable=false| in its definition.
+ This is necessary if the drawing code contains low-level drawing
+ commands that cannot be intercepted such as a use of |\pgftext| for
+ arrow tips that ``contain text''. Second, when the |bend| option is
+ used, the same arrow tip will look different each time it is used,
+ namely in dependence on the exact curvature of the path to which it is
+ added.
+
+ Because the drawing code may be executed several times, while the setup
+ code may not, we must find a way to ``communicate'' the values computed
+ by the setup code to the drawing code. This is done by explicitly
+ calling |\pgfarrowssave| inside the setup code. Whatever is ``saved''
+ in this way is restored each time before the drawing code is executed.
+\end{enumerate}
+
+As can be seen, the process is a bit involved, but it leads to a reasonably
+fast arrow tip management.
+
+
+\subsection{Declaring an Arrow Tip Kind}
+
+\begin{command}{\pgfdeclarearrow\marg{config}}
+ This command is both used to define a new arrow tip kind and to to declare
+ a so-called shorthand. We have a look at the case that a complete new arrow
+ tip kind is created and then have a look how the command can be used to
+ create shorthands.
+
+
+ \medskip
+ \noindent\textbf{Defining a Complete New Arrow Tip Kind.}
+ The \meta{config} is a key--value list in which different keys are used to
+ setup the to-be defined arrow. The following keys can be given:
+ %
+ \begin{itemize}
+ \item \declare{|name|}|=|\meta{name} or |name=|\meta{start
+ name}|-|\meta{end name}
+
+ This defines the name of the arrow tip. It is legal to define an
+ arrow tip a second time, in this case the previous definition will
+ be overwritten in the current \TeX\ scope. It is customary to use a
+ name with an uppercase fist letter for a ``complete'' arrow tip
+ kind. Short names and lower case names should be used for
+ shorthands that change their meaning inside a document, while arrow
+ tips with uppercase first letters should not be redefined.
+
+ If the name contains a hyphen, the second syntax is assumed and
+ everything before the hyphen will be the name used in start arrow
+ specifications, while the text after the hyphen is the name used in
+ end specifications.
+ \item \declare{|parameters|}|=|\marg{list of macros}
+
+ As explained earlier, an arrow tip typically needs to be redrawn
+ each time an option like |length| or |inset| is changed. However,
+ for some arrow tips, the |inset| has no influence, while for other
+ it is important whether the arrow is reversed or not. (How keys
+ like |length| actually set \TeX\ dimensions like |\pgfarrowlength|
+ is explained in Section~\ref{section-arrow-options}.)
+
+ The job of the |parameters| key is to specify which dependencies
+ the arrow tip has. Everything that will influence any of the
+ parameters computed in the setup code or used in the drawing code
+ should be listed here.
+
+ The \meta{list of macros} will be used inside a
+ |\csname|-|\endcsname| pair and should expand to the current values
+ of the relevant parameters have. For example, if the arrow tip
+ depends on the current value of |\pgfarrowlength| and
+ |\pgfarrowwidth| only, then \meta{list of macros} should be set to
+ |\the\pgfarrowlength,\the\pgfarrowwidth|. (Actually, the comma is
+ optional, the \meta{list of macros} does not really have to be a
+ list, just something that can be expanded unambiguously.)
+
+ Note that the line width (|\pgflinewidth|) and the inner line width
+ (|\pgfinnerlinewidth|) are always parameters and need not be
+ specified in the |parameters|.
+
+ It is important to get this parameter right. Otherwise, arrow tips
+ may look wrong because \pgfname\ thinks that it can reuse some code
+ when, in reality, this code actually depends on a parameter not
+ listed here.
+ \item \declare{|setup code|}|=|\marg{code}
+
+ When an arrow tip is used, the value stored in |parameters| is
+ expanded and it is tested whether the result was encountered
+ before. If not, the \meta{code} gets executed (only this once). The
+ code can now do arbitrarily complicated computations the prepare
+ the later drawing of the arrow tip. Also the \meta{code} must
+ specify the different tip and back ends and the convex hull points.
+ This is done by calling the following macros inside the
+ \meta{code}:
+ %
+ \begin{command}{\pgfarrowssettipend\marg{dimension}}
+ When this command is called inside the setup code of an arrow
+ tip, it specifies that the tip of the drawn arrow will end
+ exactly at \meta{dimension}. For example, for our earlier
+ example of the large arrow tip, where the tip end was at 1cm,
+ we would call
+ %
+\begin{codeexample}[code only]
+\pgfarrowssettipend{1cm}
+\end{codeexample}
+ %
+ Note that for efficiency reasons, the \meta{dimension} is not
+ passed through |\pgfmathsetlength|; rather what happens is that
+ |\pgf@x=|\meta{dimension} gets executed. In particular, you can
+ pack further computations into the \meta{dimension} by simply
+ starting it with a number and then appending some code that
+ modifies |\pgf@x|. Here is an example where instead of 1cm we
+ use $1\mathrm{cm} - \frac12\mathrm{linewidth}$ as the tip end:
+ %
+\begin{codeexample}[code only]
+\pgfarrowssettipend{1cm\advance\pgf@x by-.5\pgflinewidth}
+\end{codeexample}
+ %
+ If the command is not called at all inside the setup code, the
+ tip end is set to |0pt|.
+ \end{command}
+
+ \begin{command}{\pgfarrowssetbackend\marg{dimension}}
+ Works like the command for the tip end, only it sets the back
+ end. In our example we would call
+ %
+\begin{codeexample}[code only]
+\pgfarrowssettipend{-3cm}
+\end{codeexample}
+ %
+ Defaults to |0pt|.
+ \end{command}
+
+ \begin{command}{\pgfarrowssetlineend\marg{dimension}}
+ Sets the line end, so in the example we have
+ |\pgfarrowssettipend{-1cm}|. Default to |0pt|.
+ \end{command}
+
+ \begin{command}{\pgfarrowssetvisualbackend\marg{dimension}}
+ Sets the visual back end, |\pgfarrowssetvisualbackend{-2cm}| in
+ our example. Default to the value of the normal back end.
+ \end{command}
+
+ \begin{command}{\pgfarrowssetvisualtipend\marg{dimension}}
+ Sets the visual tip end. Default to the value of the normal tip
+ end and, thus, we need not set it in our example.
+ \end{command}
+
+ \begin{command}{\pgfarrowshullpoint\marg{x dimension}\marg{y dimension}}
+ Adds a point to the convex hull of the arrow tip. As for the
+ previous commands, no math parsing is done; instead \pgfname\
+ says |\pgf@x=|\meta{x dimension} and then |\pgf@y=|\meta{y
+ dimension}. Thus, both ``dimensions'' can contain code for
+ advancing and thus modifying |\pgf@x| and |\pgf@y|.
+
+ In our example we would write
+ %
+\begin{codeexample}[code only]
+\pgfarrowshullpoint{1cm}{0pt}
+\pgfarrowshullpoint{-3cm}{2cm}
+\pgfarrowshullpoint{-3cm}{-2cm}
+\end{codeexample}
+ \end{command}
+
+ \begin{command}{\pgfarrowsupperhullpoint\marg{x dimension}\marg{y dimension}}
+ This command works like the previous command, only it normally
+ adds \emph{two} points to the convex hull: First, the point
+ $(\meta{x dimension},\meta{y dimension})$ and, secondly, the
+ point $(\meta{x dimension},-\meta{y dimension})$. However, the
+ second point is only added if the arrow is not a harpoon.
+
+ Thus, in our example we could simplify the convex hull to
+ %
+\begin{codeexample}[code only]
+\pgfarrowshullpoint{1cm}{0pt}
+\pgfarrowsupperhullpoint{-3cm}{2cm}
+\end{codeexample}
+ %
+ If the \meta{y dimension} is zero or less, only one point,
+ namely $(\meta{x dimension},\meta{y dimension})$, is added to
+ the hull. Thus, we could also have used the upper convex hull
+ command in the first of the two of the above commands.
+ \end{command}
+
+ \begin{command}{\pgfarrowssave\marg{macro}}
+ As explained earlier, the setup code needs to ``communicate''
+ with the drawing code via ``saved values''. This command get
+ the name of a macro and will store the value this macro had
+ internally. Then, each time drawing code is executed, the value
+ of this macro will be restored.
+ \end{command}
+
+ \begin{command}{\pgfarrowssavethe\marg{register}}
+ Works like |\pgfarrowssave|, only the parameter must be a
+ register and |\the|\meta{register} will be saved. Typically,
+ you will write something like
+ %
+\begin{codeexample}[code only]
+\pgfarrowssavethe{\pgfarrowlength}
+\pgfarrowssavethe{\pgfarrowwidth}
+\end{codeexample}
+ %
+ To ensure that inside the drawing code the the dimension
+ registers |\pgfarrowlength| and |\pgfarrowwidth| are setup with
+ the values they had during the setup.
+ \end{command}
+ \item \declare{|drawing code|}|=|\marg{code}
+
+ This code will be executed at least once for each setting of the
+ parameters when the time arrow tip is actually drawn. Usually, this
+ one execution will be all and the low-level commands generated
+ inside the \meta{code} will we stored in a special cache; but in
+ some cases the \meta{code} gets executed each time the arrow tip is
+ used, so do not assume anything about it. Inside the \meta{code},
+ you have access to all values that were saved in the setup code as
+ well as to the line width.
+
+ The \meta{code} should draw the arrow tip ``going right along the
+ $x$-axis''. \pgfname\ will take care of setting up a canvas
+ transformation beforehand to a rotation such that when the drawing
+ is rendered, the arrow tip that is actually drawn points in the
+ direction of the line. Alternatively, when bending is switched on,
+ even more complicated low-level transformations will be done
+ automatically.
+
+ The are some special considerations concerning the \meta{code}:
+ %
+ \begin{itemize}
+ \item In the \meta{code} you may \emph{not} use |\pgfusepath|
+ since this would try to add arrow tips to the arrow tip and
+ lead to a recursion. Use the ``quick'' versions
+ |\pgfusepathqstroke| and so on instead, which never try to
+ add arrow tips.
+ \item If you stroke the path that you construct, you should
+ first set the dashing to solid and set up fixed joins and
+ caps, as needed. This will ensure that the arrow tip will
+ always look the same.
+ \item When the arrow tip code is executed, it is automatically
+ put inside a low-level scope, so nothing will ``leak out''
+ from the scope.
+ \item The high-level coordinate transformation matrix will be
+ set to the identity matrix when the code is executed for
+ the first time.
+ \end{itemize}
+ \item \declare{|cache|}|=|\meta{true or false}
+
+ When set to |true|, which is the default, the \meta{code} will be
+ executed only once for a particular value of parameters and the
+ low-level commands created by the drawing code (using the system
+ layer protocol subsystem, see Section~\ref{section-protocols}) will
+ be cached and reused later on. However, when the drawing code
+ contains ``uncachable'' code like a call to |\pgftext|, caching
+ must be switched off by saying |cache=false|.
+ \item \declare{|bending mode|}|=|\meta{mode}
+
+ This key is important only when the |bend| option is used with an
+ arrow, see Section~\ref{section-arrow-flex} for an introduction to
+ this option. The |bend| option asks us to, well, bend the arrow
+ head. For some arrow head this is not possible or leads to very
+ strange drawings (for instance, when the |\pgftext| command is
+ used) and then it is better to switch bending off for the arrow
+ head (|flex| will then be used instead). To achieve this, set
+ \meta{mode} to |none|.
+
+ For most arrow tips it does, however, make sense to bend them.
+ There are (at least) two different mathematical ways of doing so,
+ see Section~\ref{section-library-curvilinear} for details. Which of
+ these ways is use can be configured by setting \meta{mode} to
+ either |orthogonal| or to |polar|. It is best to try simply try out
+ both when designing an arrow tip to see which works better. Since
+ |orthogonal| is quicker and often gives good oder even better
+ results, it is the default. Some arrow tips, however, profit from
+ saying |bending mode=polar|.
+ \item \declare{|defaults|}|=|\meta{arrow keys}
+
+ The \meta{arrow keys} allow you to configure the default values for
+ the parameters on which an arrow tip depends. The \meta{arrow keys}
+ will be executed first before any other arrow tip options are
+ executed, see Section~\ref{section-arrow-scopes} for the exact
+ sequence. Also see Section~\ref{section-arrow-options} below for
+ more details on arrow options.
+ \end{itemize}
+
+ This concludes the description of the keys you provide for the declaration
+ of an arrow. Let us now have a look at a simple example that uses these
+ features: We want to define an arrow tip kind |foo| that produces the arrow
+ tip we used as our running example. However, to make things a bit more
+ interesting, let us make it ``configurable'' insofar as the length of the
+ arrow tip can be configured using the |length| option, which sets the
+ |\pgfarrowlength|. By default, this length should be the gigantic 4cm we
+ say in the example, but uses should be able to set it to anything they
+ like. We will not worry about the arrow width or insets, of arrow line
+ width, or harpoons, or anything else in this example to keep it simple.
+
+ Here is the code:
+ %
+\begin{codeexample}[code only]
+\pgfdeclarearrow{
+ name = foo,
+ parameters = { \the\pgfarrowlength },
+ setup code = {
+ % The different end values:
+ \pgfarrowssettipend{.25\pgfarrowlength}
+ \pgfarrowssetlineend{-.25\pgfarrowlength}
+ \pgfarrowssetvisualbackend{-.5\pgfarrowlength}
+ \pgfarrowssetbackend{-.75\pgfarrowlength}
+ % The hull
+ \pgfarrowshullpoint{.25\pgfarrowlength}{0pt}
+ \pgfarrowshullpoint{-.75\pgfarrowlength}{.5\pgfarrowlength}
+ \pgfarrowshullpoint{-.75\pgfarrowlength}{-.5\pgfarrowlength}
+ % Saves: Only the length:
+ \pgfarrowssavethe\pgfarrowlength
+ },
+ drawing code = {
+ \pgfpathmoveto{\pgfqpoint{.25\pgfarrowlength}{0pt}}
+ \pgfpathlineto{\pgfqpoint{-.75\pgfarrowlength}{.5\pgfarrowlength}}
+ \pgfpathlineto{\pgfqpoint{-.5\pgfarrowlength}{0pt}}
+ \pgfpathlineto{\pgfqpoint{-.75\pgfarrowlength}{-.5\pgfarrowlength}}
+ \pgfpathclose
+ \pgfusepathqfill
+ },
+ defaults = { length = 4cm }
+}
+\end{codeexample}
+ %
+ We can now use it:
+ %
+\pgfdeclarearrow{
+ name = foo,
+ parameters = { \the\pgfarrowlength },
+ setup code = {
+ % The different end values:
+ \pgfarrowssettipend{.25\pgfarrowlength}
+ \pgfarrowssetlineend{-.25\pgfarrowlength}
+ \pgfarrowssetvisualbackend{-.5\pgfarrowlength}
+ \pgfarrowssetbackend{-.75\pgfarrowlength}
+ % The hull
+ \pgfarrowshullpoint{.25\pgfarrowlength}{0pt}
+ \pgfarrowshullpoint{-.75\pgfarrowlength}{.5\pgfarrowlength}
+ \pgfarrowshullpoint{-.75\pgfarrowlength}{-.5\pgfarrowlength}
+ % Saves: Only the length:
+ \pgfarrowssavethe\pgfarrowlength
+ },
+ drawing code = {
+ \pgfpathmoveto{\pgfqpoint{.25\pgfarrowlength}{0pt}}
+ \pgfpathlineto{\pgfqpoint{-.75\pgfarrowlength}{.5\pgfarrowlength}}
+ \pgfpathlineto{\pgfqpoint{-.5\pgfarrowlength}{0pt}}
+ \pgfpathlineto{\pgfqpoint{-.75\pgfarrowlength}{-.5\pgfarrowlength}}
+ \pgfpathclose
+ \pgfusepathqfill
+ },
+ defaults = { length = 4cm }
+}
+\begin{codeexample}[]
+\tikz \draw [-foo] (0,0) -- (8,0);
+\end{codeexample}
+\begin{codeexample}[]
+\tikz \draw [-{foo[length=2cm,bend]}] (0,0) to [bend left] (3,0);
+\end{codeexample}
+
+
+ \medskip
+ \noindent\textbf{Defining a Shorthand.}
+ The |\pgfdeclarearrow| command can also used to define
+ \emph{shorthands}. This works as follows:
+ \begin{itemize}
+ \item First, you must provide a |name| just in the same way as when you
+ define a full-flung new arrow tip kind.
+ \item Second, instead of all of the other options listed above, you
+ just use one more option:
+
+ \smallskip
+ \declare{|means|}|=|\meta{end arrow specification}
+
+ This sets up things so that whenever \meta{name} is now used in an
+ arrow specification, it will be replaced by the \meta{end arrow
+ specification} (the problems resulting form the \meta{name} begin
+ used in a start arrow specification are taken care of
+ automatically). See also Section~\ref{section-arrow-tip-macro} for
+ details on the order in which options get executed in such cases.
+
+ Note that the \meta{end arrow specification} will be executed
+ immediately to build the so-called arrow option caches, a concept
+ explored in more detail in
+ Section~\ref{section-arrow-option-cache}. In practice, this has
+ mainly two effects: First, all arrow tips referred to in the
+ specification must already exist (at least as ``dummy'' versions).
+ Second, all dimensions mentioned in options of the \meta{end arrow
+ specification} will be evaluated immediately. For instance, when
+ you write
+ %
+\begin{codeexample}[code only]
+\pgfdeclarearrow{ name=foo, means = bar[length=2cm+\mydimen] }
+\end{codeexample}
+ %
+ The value |2cm+\mydimen| is evaluated immediately. When |foo| is
+ used later on and |\mydimen| has changed, this has no effect.
+ \end{itemize}
+\end{command}
+
+
+\subsection{Handling Arrow Options}
+\label{section-arrow-options}
+
+When you declare an arrow tip, your drawing code should take into account the
+different arrow keys set for it (like the arrow tip length, width, or
+harpooning). The different arrow keys that are available have been described in
+detail in Section~\ref{section-arrow-config}; but how do we access the values
+set by an option like |length| or |harpoon| or |bend| in the drawing code? In
+the present section we have a look at how this works.
+
+
+\subsubsection{Dimension Options}
+
+Most arrow keys, like |length| or |width'|, simple set a \TeX\ dimension
+register to a certain value. For example, |length| sets the value of the \TeX\
+dimension register |\pgfarrowlength|. Note that |length| takes several values
+as input with a complicated semantics as explained for the |length| key on
+page~\pageref{length-arrow-key}. All of these settings are not important for
+the setup code: When it gets executed, the code behind the |length| key will
+have computed a simple number that is stored in |\pgfarrowlength|. Indeed,
+inside the setup code you do not have access to the exact value given to the
+|length| key; just to the final computed value.
+
+The following \TeX\ dimensions are available to the setup code:
+%
+\begin{itemize}
+ \item |\pgfarrowslength|. It gets set by the arrow keys |length| and
+ |angle|.
+ \item |\pgfarrowswidth|. It gets set by |width|, |width'|, and |angle|.
+ \item |\pgfarrowsinset|. It gets set by |inset| and |inset'|.
+ \item |\pgfarrowslinewidth|. It gets set by |line width| and |line width'|.
+\end{itemize}
+
+If your setup code depends on any of them, add them to the |parameters| key of
+the arrow tip.
+
+
+\subsubsection{True--False Options}
+
+A number of arrow keys just do a yes/no switch, like |reversed|. All of them
+setup a \TeX-if that you can access in the setup code:
+%
+\begin{itemize}
+ \item |\ifpgfarrowreversed| is setup by |reversed|.
+ \item |\ifpgfarrowswap| is setup by |swap| and also |right|.
+ \item |\ifpgfarrowharpoon| is setup by |harpoon| and also |left| and
+ |right|.
+ \item |\ifpgfarrowroundcap| is set to true by |line cap=round| and set to
+ false by |line cap=butt|. It also gets (re)set by |round| and |sharp|.
+ \item |\ifpgfarrowroundjoin| is set to true by |line join=round| and set to
+ false by |line join=miter|. It also gets (re)set by |round| and
+ |sharp|.
+ \item |\ifpgfarrowopen| is set to true by |fill=none| and by |open| (which
+ is a shorthand for |fill=none|) and set to false by |color| and all
+ other |fill=|\meta{color}.
+\end{itemize}
+
+If you code depends on any of these, you must add them to the |parameters| in
+such a way that the parameters are different when the \TeX-if is set from when
+it is not set. An easy way to achieve this is to write something like
+%
+\begin{codeexample}[code only]
+ parameters = { \the\pgfarrowlength,...,
+ \ifpgfarrowharpoon h\fi\
+ \ifpgfarrowroundjoin j\fi}
+\end{codeexample}
+%
+In other words, for each set parameter on which the arrow tip depends, a
+specific letter is added to the parameters, making them unique.
+
+The first two of the above keys are a bit special: Reversing and swapping an
+arrow tip can be done just by fiddling with the transformation matrix: a
+reverse is a ``flip'' along the $y$-axis and a swap is a flip along the
+$x$-axis. This is done automatically by \pgfname.
+
+Nevertheless, you may wish to modify you code in dependence especially of the
+|reverse| key: When |\ifpgfarrowreverse| is true, \pgfname\ will flip the
+coordinate system along the $y$-axis, will negate all end values (like line
+end, tip end, and so on) and will exchange the meaning of back end and tip end
+as well as of visual back end and visual back end. Usually, this is exactly
+what one need; \emph{except} that the line end may no longer be appropriate.
+After all, the line end should be chosen so that it is completely covered by
+the arrow. Now, when the arrow tip is open, a reversed arrow should no longer
+have the line end near the old visual back end, but near to the old visual tip
+end.
+
+For these reasons, you may need to make the computation of the line end
+dependent on whether the arrow is reversed or not. Note that when you specify a
+different line end for a reversed arrow tip, the transformation and inverting
+of the coordinate system will still be done, meaning that if |reverse| is true,
+you need to specify a line end in the ``old'' coordinate system that is at the
+position where, after everything is inverted, it will be at the correct
+position. Usually that means that if the |reverse| option is set, you need to
+\emph{increase} the line end.
+
+
+\subsubsection{Inaccessible Options}
+
+There are some options that influence the way an arrow tip looks, but that you
+cannot access inside the setup code. Handling these options lies entirely with
+\pgfname. If you wish your setup code to handle these options, you have to
+setup your own ``parallel'' options.
+%
+\begin{itemize}
+ \item |quick|, |flex|, |flex'|, and |bend| are all handled automatically.
+ You can, however, set the |bending mode| to avoid bending of your arrow
+ tip.
+ \item The colors set by |color| and |fill|. You can, however, access them
+ indirectly, namely through the current stroke and fill colors.
+ \item |sep|
+\end{itemize}
+
+
+\subsubsection{Defining New Arrow Keys}
+\label{section-arrow-option-cache}
+
+The set of predefined options is already quite long and most arrow tips will
+not need more than the predefined options. However, sometimes an arrow tip may
+need to introduce a new special-purpose option. For instance, suppose we wish
+to introduce a new fictive arrow key |depth|. In such cases, you must do two
+things:
+%
+\begin{enumerate}
+ \item Introduce a new dimension register or macro that will hold the
+ configuration value and which will be accessed by the setup code. The
+ could be achieved by saying
+ %
+\begin{codeexample}[code only]
+\newdimen\pgfarrowdepth
+\end{codeexample}
+ %
+ \item Introduce a new arrow key option |/pgf/arrow keys/depth| that allows
+ users to configure the new macro or register.
+\end{enumerate}
+
+When an arrow is selected via for instance |foo[depth=5pt]|, the key--value
+pairs between the square brackets are executed with the path prefix
+|/pgf/arrow keys|. Thus, in the example, our depth key would get executed.
+Thus, it is tempting to write something like
+%
+\begin{codeexample}[code only]
+\pgfkeys{/pgf/arrow keys/depth/.code = \pgfmathsetlength{\pgfarrowdepth}{#1}}
+\end{codeexample}
+
+Sadly, this will not work. The reason is that there is yet another level of
+caching involved when \pgfname\ processes arrow tips: The option cache! The
+problem is each time an arrow tip is used, even when the drawing code of the
+arrow tip is nicely cached, we still need to process the options in
+|foo[length=5pt]| to find out which version in the cache we would like to
+access. To make matters worse, |foo| might be a shorthand that calls other
+arrow tips, which add more options, and so on. Unfortunately, executing keys is
+quite an expensive operation (\pgfname's key--value parser is powerful, but
+that power comes at a price). So, whenever possible, we do \emph{not} want the
+key--value parser to be started.
+
+For these reasons, when something like |foo[|\meta{options}|]| is encountered
+inside a shorthand, the \meta{options} are executed only once. They should now
+setup the \emph{arrow option cache}, which is some code that, when executed,
+should setup the values that the \meta{options} configure. In our example, the
+|depth| key should add something to the arrow option cache that sets
+|\pgfarrowdepth| to the given value.
+
+Adding something to the arrow option cache is done using the following command:
+
+\begin{command}{\pgfarrowsaddtooptions\marg{code}}
+ This command should be called by keys with the prefix |/pgf/arrow keys| to
+ add code to the arrow option cache. For our |depth| key example, we could
+ use this key as follows:
+ %
+\begin{codeexample}[code only]
+\pgfkeys{/pgf/arrow keys/depth/.code=
+ \pgfarrowsaddtooptions{\pgfmathsetlength{\pgfarrowdepth}{#1}}
+\end{codeexample}
+ %
+ Actually, this is still not optimal since the expensive |\pgfmathsetlength|
+ command is now called each time an arrow tip is used with the |depth|
+ option set. The trick is to do the expensive operation only once and then
+ store only very quick code in the arrow option cache:
+ %
+\begin{codeexample}[code only]
+\pgfkeys{/pgf/arrow keys/depth/.code=
+ \pgfmathsetlength{\somedimen}{#1}
+ \pgfarrowsaddtooptions{\pgfarrowdepth=\somedimen} % buggy
+\end{codeexample}
+ %
+ The above code will not (yet) work since |\somedimen| will surely have a
+ different value when the cache is executed. The trick is to use some
+ |\expandafter|s:
+ %
+\begin{codeexample}[code only]
+\pgfkeys{/pgf/arrow keys/depth/.code=
+ \pgfmathsetlength{\somedimen}{#1}
+ \expandafter\pgfarrowsaddtooptions\expandafter{\expandafter\pgfarrowdepth\expandafter=\the\somedimen}
+\end{codeexample}
+ %
+\end{command}
+
+\begin{command}{\pgfarrowsaddtolateoptions\marg{code}}
+ This command works like |\pgfarrowsaddtooptions|, only the \meta{code} will
+ be executed ``later'' than the code added by the normal version of the
+ command. This is useful for keys that depend on the length of an arrow:
+ Keys like |width'| want to define the arrow width as a multiple of the
+ arrow length, but when the |width'| key is given, the length may not yet
+ have been specified. By making the computation of the width a ``late''
+ option, we ensure that |\pgfarrowlength| will have been setup correctly.
+\end{command}
+
+If you define a new option that sets a dimensions and if that dimension should
+change in accordance to the setting of either |scale length| or |scale width|,
+you need to make \pgfname\ ``aware'' of this using the following key:
+
+\begin{command}{\pgfarrowsaddtolengthscalelist\marg{dimension register}}
+ Each time an arrow tip is used, the given \meta{dimension register} will be
+ multiplied by the |scale length| factor prior to the actual drawing. You
+ call this command only once in the preamble somewhere.
+\end{command}
+
+\begin{command}{\pgfarrowsaddtowidthscalelist\marg{dimension register}}
+ Works like |\pgfarrowsaddtolengthscalelist|, only for width parameters.
+\end{command}
+
+
+\begin{command}{\pgfarrowsthreeparameters\marg{line-width dependent size specification}}
+ This command is useful for parsing the values given to keys like |length|
+ or |width| the expect a dimension followed optionally for some numbers.
+ This command converts the \meta{line-width dependent size specification},
+ which may consist of one, two, or three numbers, into a triple of three
+ numbers in curly braces, which gets stored in the macro
+ |\pgfarrowstheparameters|. Here is an example, where |\showvalueofmacro| is
+ used in this example to show the value stored in a macro:
+ %
+ \makeatletter
+ \def\showvalueofmacro#1{%
+ \texttt{\expandafter\expandafter\expandafter\expandafter\expandafter\expandafter\expandafter\pgfutil@gobble\expandafter\expandafter\expandafter\string\expandafter\csname#1\endcsname}
+ }
+\begin{codeexample}[]
+\pgfarrowsthreeparameters{2pt 1}
+\showvalueofmacro\pgfarrowstheparameters
+\end{codeexample}
+ %
+\end{command}
+
+\begin{command}{\pgfarrowslinewidthdependent\marg{dimension}\marg{line width factor}\marg{outer factor}}
+ This command take three parameters and does the ``line width dependent
+ computation'' described on page~\pageref{length-arrow-key} for the |length|
+ key. The result is returned in |\pgf@x|.
+
+ The idea is that you can setup line-width dependent keys like |length| or
+ |width| using code like the following:
+ %
+\begin{codeexample}[code only]
+\pgfkeys{/pgf/arrow keys/depth/.code={%
+ \pgfarrowsthreeparameters{#1}%
+ \expandafter\pgfarrowsaddtolateoptions\expandafter{%
+ \expandafter\pgfarrowslinewidthdependent\pgfarrowstheparameters% compute...
+ \pgfarrowdepth\pgf@x% ... and store.
+ }%
+}
+\end{codeexample}
+ %
+\end{command}
+
+\begin{command}{\pgfarrowslengthdependent\marg{dimension}\marg{length factor}\marg{dummy}}
+ This command take three parameters, of which the last one is ignored, and
+ does the ``length dependent computation'' described for the |width'| and
+ |inset'| keys. The result is returned in |\pgf@x|.
+
+ You can setup length dependent keys using code like the following:
+ %
+\begin{codeexample}[code only]
+\pgfkeys{/pgf/arrow keys/depth'/.code={%
+ \pgfarrowsthreeparameters{#1}%
+ \expandafter\pgfarrowsaddtolateoptions\expandafter{%
+ \expandafter\pgfarrowslengthdependent\pgfarrowstheparameters% compute...
+ \pgfarrowdepth\pgf@x% ... and store.
+ }%
+}
+\end{codeexample}
+ %
+\end{command}
+
+
+%%% Local Variables:
+%%% mode: latex
+%%% TeX-master: "pgfmanual"
+%%% End: