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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{Pics: Small Pictures on Paths}
+\label{section-pics}
+
+\subsection{Overview}
+
+A ``pic'' is a ``short picture'' (hence the short name\dots) that can be
+inserted anywhere in \tikzname\ picture where you could also insert a node.
+Similarly to nodes, pics have a ``shape'' (called \emph{type} to avoid
+confusion) that someone has defined. Each time a pic of a specified type is
+used, the type's code is executed, resulting in some drawings to be added to
+the current picture. The syntax for adding nodes and adding pics to a picture
+are also very similar. The core difference is that pics are typically more
+complex than nodes and may consist of a whole bunch of nodes themselves
+together with complex paths joining them.
+
+As a very simple example, suppose we want to define a pic type |seagull| that
+just draw ``two bumps''. The code for this definition is quite easy:
+%
+\begin{codeexample}[code only]
+\tikzset{
+ seagull/.pic={
+ % Code for a "seagull". Do you see it?...
+ \draw (-3mm,0) to [bend left] (0,0) to [bend left] (3mm,0);
+ }
+}
+\end{codeexample}
+
+The first line just tells \TeX\ that you set some \tikzname\ options for the
+current scope (which is the whole document); you could put |seagull/.pic=...|
+anywhere else where \tikzname\ options are allowed (which is just about
+anywhere). We have now defined a |seagull| pic type and can use it as follows:
+%
+\tikzset{
+ seagull/.pic={
+ % Code for a "seagull". Do you see it?...
+ \draw (-3mm,0) to [bend left] (0,0) to [bend left] (3mm,0);
+ }
+}
+\begin{codeexample}[]
+\tikz \fill [fill=blue!20]
+ (1,1)
+ -- (2,2) pic {seagull}
+ -- (3,2) pic {seagull}
+ -- (3,1) pic [rotate=30] {seagull}
+ -- (2,1) pic [red] {seagull};
+\end{codeexample}
+
+As can be see, defining new types of pics is much easier than defining new
+shapes for nodes; but see Section~\ref{section-new-pic-types} for the fine
+details.
+
+Since defining new pics types is easier than defining new node shapes and since
+using pics is as easy as using nodes, why should you use nodes at all? There
+are chiefly two reasons:
+%
+\begin{enumerate}
+ \item Unlike nodes, pics cannot be referenced later on. You \emph{can}
+ reference nodes that are inside a pic, but not ``the pic itself''. In
+ particular, you cannot draw lines between pics the way you can draw
+ them between nodes. In general, whenever it makes sense that some
+ drawing could conceivably be connected to other node-like-things, then
+ a node is better than a pic.
+ \item If pics are used to emulate the full power of a node (which is
+ possible, in principle), they will be slower to construct and take up
+ more memory than a node achieving the same effect.
+\end{enumerate}
+
+Despite these drawbacks, pics are an excellent choice for creating highly
+configurable reusable pieces of drawings that can be inserted into larger
+contexts.
+
+
+\subsection{The Pic Syntax}
+
+\begin{command}{\pic}
+ Inside |{tikzpicture}| this is an abbreviation for |\path pic|.
+\end{command}
+
+The syntax for adding a pic to a picture is very similar to the syntax used for
+nodes (indeed, internally the same parser code is used). The main difference is
+that instead of a node contents you provide the picture's type between the
+braces:
+
+\begin{pathoperation}{pic}{
+ \opt{\meta{foreach statements}}
+ \opt{|[|\meta{options}|]|}
+ \opt{|(|\meta{prefix}|)|}
+ \opt{|at(|\meta{coordinate}|)|}
+ \opt{|:|\meta{animation attribute}|=|\marg{options}}
+ \opt{\marg{pic type}}%
+}
+ Adds a pic to the current \tikzname\ picture of the specified \meta{pic
+ type}. The effect is, basically, that some code associated with the
+ \meta{pic type} is executed (how this works, exactly, is explained later).
+ This code can consist of arbitrary \tikzname\ code. As for nodes, the
+ current path will not be modified by this path command, all drawings
+ produced by the code are ``external'' to the path the same way neither a
+ node nor its border are part of the path on which they are specified.
+
+ Just like the |node| command, this path operation is somewhat complex and
+ we go over it step by step.
+
+ \medskip
+ \textbf{Order of the parts of the specification.}
+ Just like for nodes, everything between ``|pic|'' and the opening brace of
+ the \meta{pic type} is optional and can be given in any order. If there are
+ \meta{foreach statements}, they must come first, directly following
+ ``|pic|''. As for nodes, the ``end'' of the pic specification is normally
+ detected by the presence of the opening brace. You can, however, use the
+ |pic type| option to specify the pic type as an option.
+
+ \begin{key}{/tikz/pic type=\meta{pic type}}
+ This key sets the pic type of the current~|pic|. When this option is
+ used inside an option block of a |pic|, the parsing of the |pic| ends
+ immediately and no pic type in braces is expected. (In other words,
+ this option behaves exactly like the |node contents| option and,
+ indeed, the two are interchangeable.)
+ %
+\begin{codeexample}[]
+\tikz {
+ \path (0,0) pic [pic type = seagull]
+ (1,0) pic {seagull};
+}
+\end{codeexample}
+ \end{key}
+
+ \medskip
+ \textbf{The location of a pic.}
+ Just like nodes, pics are placed at the last position mentioned on the path
+ or, when |at| is used, at a specified position. ``Placing'' a pic somewhere
+ actually means that the coordinate system is translated (shifted) to this
+ last position. This means that inside of the pic type's code any mentioning
+ of the origin refers to the last position used on the path or to the
+ specified |at|.
+ %
+\begin{codeexample}[]
+\tikz { % different ways of placing pics
+ \draw [help lines] (0,0) grid (3,2);
+ \pic at (1,0) {seagull};
+ \path (2,1) pic {seagull};
+ \pic [at={(3,2)}] {seagull};
+}
+\end{codeexample}
+
+ As for nodes, except for the described shifting, the coordinate system of a
+ pic is reset prior to executing the pic type's code. This can be changed
+ using the |transform shape| option, which has the same effect as for nodes:
+ The ``outer'' transformation gets applied to the node:
+ %
+\begin{codeexample}[]
+\tikz [scale=2] {
+ \pic at (0,0) {seagull};
+ \pic at (1,0) [transform shape] {seagull};
+}
+\end{codeexample}
+
+ When the \meta{options} contain transformation commands like |scale| or
+ |rotate|, these transformations always apply to the pic:
+ %
+\begin{codeexample}[]
+\tikz [rotate=30] {
+ \pic at (0,0) {seagull};
+ \pic at (1,0) [rotate=90] {seagull};
+}
+\end{codeexample}
+
+ Just like nodes, pics can also be positioned implicitly and, somewhat
+ unsurprisingly, the same rules concerning positioning and sloping apply:
+ %
+\begin{codeexample}[]
+\tikz \draw
+ (0,0) to [bend left]
+ pic [near start] {seagull}
+ pic {seagull}
+ pic [sloped, near end] {seagull} (4,0);
+\end{codeexample}
+
+ \medskip
+ \textbf{The options of a node.}
+ As always, any given \meta{options} apply only to the pic and have no
+ effect outside. As for nodes, most ``outside'' options also apply to the
+ pics, but not the ``action'' options like |draw| or |fill|. These must be
+ given in the \meta{options} of the pic.
+
+ \medskip
+ \textbf{The code of a pic.}
+ As stated earlier, the main job of a pic is to execute some code in a scope
+ that is shifted according to the last point on the path or to the |at|
+ position specified in the pic. It was also claimed that this code is
+ specified by the \meta{pic type}. However, this specification is somewhat
+ indirect. What really happens is the following: When a |pic| is
+ encountered, the current path is suspended and a new internal scope is
+ started. The \meta{options} are executed and also the \meta{pic type} (as
+ explained in a moment). After all this is done, the code stored in the
+ following key gets executed:
+
+ \begin{key}{/tikz/pics/code=\meta{code}}
+ This key stores the \meta{code} that should be drawn in the current
+ pic. Normally, setting this key is done by the \meta{pic type}, but you
+ can also set it in the \meta{options} and leave the \meta{pic type}
+ empty:
+ %
+\begin{codeexample}[]
+\tikz \pic [pics/code={\draw (-3mm,0) to[bend left] (0,0)
+ to[bend left] (3mm,0);}]
+ {}; % no pic type specified
+\end{codeexample}
+ \end{key}
+
+ Now, how does the \meta{pic type} set |pics/code|? It turns out that the
+ \meta{pic type} is actually just a list of keys that are executed with the
+ prefix |/tikz/pics/|. In the above examples, this ``list of keys'' just
+ consisted of the single key ``|seagull|'' that did not take any arguments,
+ but, in principle, you could provide any arbitrary text understood by
+ |\pgfkeys| here. This means that when we write |pic{seagull}|, \tikzname\
+ will execute the key |/tikz/pics/seagull|. It turns out, see
+ Section~\ref{section-new-pic-types}, that this key is just a style set to
+ |code={\draw(-3mm,0)...;}|. Thus, |pic{seagull}| will cause the |pics/code|
+ key to be set to the text needed to draw the
+ seagull.
+
+ Indeed, you can also use the \meta{pic type} simply to set the |code| of
+ the pic. This is useful for cases when you have some code that you ``just
+ want to execute, but do not want to define a new pic type''. Here is a
+ typical example where we use pics to add some markings to a path:
+ %
+\begin{codeexample}[]
+\tikz \draw (0,0) .. controls(1,0) and (2,1) .. (3,1)
+ foreach \t in {0, 0.1, ..., 1} {
+ pic [pos=\t] {code={\draw circle [radius=2pt];}}
+ };
+\end{codeexample}
+
+ In our seagull example, we always explicitly used |\draw| to draw the
+ seagull. This implies that when a user writes something
+ |pic[fill]{seagull}| in the hope of having a ``filled'' seagull, nothing
+ special will happen: The |\draw| inside the pic explicitly states that the
+ path should be drawn, not filled, and the fact that in the surrounding
+ scope the |fill| option is set has no effect. The following key can be used
+ to change this:
+ %
+ \begin{key}{/tikz/pic actions}
+ This key is a style that can be used (only) inside the code of a pic.
+ There, it will set the ``action'' keys set inside the \meta{options} of
+ the pic (``actions'' are drawing, filling, shading, and clipping or any
+ combination thereof).
+
+ To see how this key works, let us define the following pic:
+ %
+\begin{codeexample}[code only]
+\tikzset{
+ my pic/.pic = {
+ \path [pic actions] (0,0) circle[radius=3mm];
+ \draw (-3mm,-3mm) rectangle (3mm,3mm);
+ }
+}
+\end{codeexample}
+ %
+ In the code, whether or not the circle gets drawn/filled/shaded
+ depends on which options where given to the |pic| command when it
+ is used. In contrast, the rectangle will always (just) be drawn.
+ %
+\tikzset{
+ my pic/.pic = {
+ \path [pic actions] (0,0) circle[radius=3mm];
+ \draw (-3mm,-3mm) rectangle (3mm,3mm);
+ }
+}
+\begin{codeexample}[width=6cm]
+\tikz \pic {my pic}; \space
+\tikz \pic [red] {my pic}; \space
+\tikz \pic [draw] {my pic}; \space
+\tikz \pic [draw=red] {my pic}; \space
+\tikz \pic [draw, shading=ball] {my pic}; \space
+\tikz \pic [fill=red!50] {my pic};
+\end{codeexample}
+ \end{key}
+
+ \medskip
+ \textbf{Code executed behind or in front of the path.}
+ As for nodes, a pic can be ``behind'' the current path or ``in front of
+ it'' and, just as for nodes, the two options |behind path| and
+ |in front of path| are used to specify which is meant. In detail, if |node|
+ and |pic| are both used repeatedly on a path, in the resulting picture we
+ first see all nodes and pics with the |behind path| option set in the order
+ they appear on the path (nodes and pics are interchangeable in this
+ regard), then comes the path, and then come all nodes and pics that are in
+ front of the path in the order they appeared.
+ %
+\begin{codeexample}[]
+\tikz \fill [fill=blue!20]
+ (1,1)
+ -- (2,2) pic [behind path] {seagull}
+ -- (3,2) pic {seagull}
+ -- (3,1) pic [rotate=30] {seagull}
+ -- (2,1) pic [red, behind path] {seagull};
+\end{codeexample}
+ %
+ In contrast to nodes, a pic need not only be completely behind the path or
+ in front of the path as specified by the user. Instead, a pic type may also
+ specify that a certain part of the drawing should always be behind the path
+ and it may specify that a certain other part should always be before the
+ path. For this, the values of the following keys are relevant:
+
+ \begin{key}{/tikz/pics/foreground code=\meta{code}}
+ This key stores \meta{code} that will always be drawn in front of the
+ current path, even when |behind path| is used. If |behind path| is not
+ used and |code| is (also) set, the code of |code| is drawn first,
+ following by the foreground \meta{code}.
+ \end{key}
+
+ \begin{key}{/tikz/pics/background code=\meta{code}}
+ Like |foreground code|, only that the \meta{code} is always put behind
+ the path, except when the |behind path| option is applied to the pic,
+ then the background code is drawn in front of the ``behind path'' code.
+ \end{key}
+
+ \medskip
+ \textbf{The foreach statement for pics.}
+ As for nodes, a pic specification may start with |foreach|. The effect and
+ semantics are the same as for nodes.
+ %
+\begin{codeexample}[]
+\tikz \pic foreach \x in {1,2,3} at (\x,0) {seagull};
+\end{codeexample}
+
+ \medskip
+ \textbf{Styles for pics.}
+ The following styles influence how nodes are rendered:
+ %
+ \begin{stylekey}{/tikz/every pic (initially \normalfont empty)}
+ This style is installed at the beginning of every pic.
+ %
+\begin{codeexample}[]
+\begin{tikzpicture}[every node/.style={draw}]
+ \draw (0,0) node {A} -- (1,1) node {B};
+\end{tikzpicture}
+\end{codeexample}
+ \end{stylekey}
+
+ \medskip
+ \textbf{Name scopes.}
+ You can specify a \meta{name} for a pic using the key |name=|\meta{name} or
+ by giving the name in parenthesis inside the pic's specification. The
+ effect of this is, for once, quite different from what happens for nodes:
+ All that happens is that |name prefix| is set to \meta{name} at the
+ beginning of the pic.
+
+ The |name prefix| key was already introduced in the description of the
+ |node| command: It allows you to set some text that is prefixed to all
+ nodes in a scope. For pics this makes particular sense: All nodes defined
+ by a pic's code can be referenced from outside the pic with the prefix
+ provided.
+
+ To see how this works, let us add some nodes to the code of the seagull:
+ %
+\begin{codeexample}[code only]
+\tikzset{
+ seagull/.pic={
+ % Code for a "seagull". Do you see it?...
+ \coordinate (-left wing) at (-3mm,0);
+ \coordinate (-head) at (0,0);
+ \coordinate (-right wing) at (3mm,0);
+
+ \draw (-left wing) to [bend left] (0,0) (-head) to [bend left] (-right wing);
+ }
+}
+\end{codeexample}
+ %
+\tikzset{
+ seagull/.pic={
+ % Code for a "seagull". Do you see it?...
+ \coordinate (-left wing) at (-3mm,0);
+ \coordinate (-head) at (0,0);
+ \coordinate (-right wing) at (3mm,0);
+
+ \draw (-left wing) to [bend left] (0,0) (-head) to [bend left] (-right wing);
+ }
+}
+
+ Now, we can use it as follows:
+ %
+\begin{codeexample}[code only]
+\tikz {
+ \pic (Emma) {seagull};
+ \pic (Alexandra) at (0,1) {seagull};
+
+ \draw (Emma-left wing) -- (Alexandra-right wing);
+}
+\end{codeexample}
+
+ Sometimes, you may also wish your pic to access nodes outside the pic
+ (typically, because they are given as parameters). In this case, the name
+ prefix gets in the way since the nodes outside the picture do not have this
+ prefix. The trick is to locally reset the name prefix to the value it had
+ outside the picture, which is achieved using the following style:
+
+ \begin{key}{/tikz/name prefix ..}
+ This key is available only inside the code of a pic. There, it
+ (locally) changes the name prefix to the value it had outside the pic.
+ This allows you to access nodes outside the current pic.
+ \end{key}
+
+ \medskip
+ \textbf{Animations for pics.}
+ Just as for nodes, you can use the attribute--colon syntax to add an
+ animation to a pic:
+ %
+\begin{codeexample}[animation list={0.5,1,1.5,2}]
+\tikz {
+ \pic :rotate={0s="0", 20s="90"} {seagull};
+ \pic at (1.5,1.5) {seagull};
+}
+\end{codeexample}
+ %
+ Naturally, you can also use animations in the code of a picture:
+ %
+\begin{codeexample}[animation list={0.5,1,1.5,2},width=3cm]
+\begin{tikzpicture} [flapping seagull/.pic={
+ \draw (0,0) :path={
+ 0s= {"{(180:3mm) to [bend left] (0,0) to [bend left] (0:3mm)}"=base},
+ 1s= "{(160:3mm) to [bend left] (0,0) to [bend left] (20:3mm)}",
+ 2s= "{(180:3mm) to [bend left] (0,0) to [bend left] (0:3mm)}",
+ repeats };
+ }]
+ \pic :rotate={0s="0", 20s="90"} {flapping seagull};
+ \pic at (1.5,1.5) {flapping seagull};
+\end{tikzpicture}
+\end{codeexample}
+ %
+\end{pathoperation}
+
+There are two general purpose keys that pics may find useful:
+
+\begin{key}{/tikz/pic text=\meta{text}}
+ This macro stores the \meta{text} in the macro |\tikzpictext|, which is
+ |\let| to |\relax| by default. Setting the |pic text| to some value is the
+ ``preferred'' way of communicating a (single) piece of text that should
+ become part of a pic (typically of a node). In particular, the |quotes|
+ library maps quoted parameters to this key.
+\end{key}
+
+\begin{key}{/tikz/pic text options=\meta{options}}
+ This macro stores the \meta{options} in the macro |\tikzpictextoptions|,
+ which is |\let| to the empty string by default. The |quotes| library maps
+ options for quoted parameters to this key.
+\end{key}
+
+
+\subsubsection{The Quotes Syntax}
+\label{section-pic-quotes}
+
+When you load the |quotes| library, you can use the ``quotes syntax'' inside
+the options of a pic. Recall that for nodes this syntax is used to add a label
+to a node. For pics, the quotes syntax is used to set the |pic text| key.
+Whether or not the pic type's code takes this key into consideration is,
+however, up to the key.
+
+In detail, when the |quotes| library is loaded, each time a key--value pair in
+a list of options passed to an |pic| starts with |"|, the key--value pair must
+actually be a string of the following form:
+%
+\begin{quote}
+ |"|\meta{text}|"|\opt{|'|}\opt{\meta{options}}
+\end{quote}
+%
+This string is transformed into the following:
+%
+\begin{quote}
+ |every pic quotes/.try,pic text=|\meta{text}|, pic text options={|\meta{options}|}|
+\end{quote}
+
+As example of a pic type that takes these values into account is the |angle|
+pic type:
+%
+\begin{codeexample}[]
+\tikz \draw (3,0) coordinate (A)
+ -- (0,1) coordinate (B)
+ -- (1,2) coordinate (C)
+ pic [draw, "$\alpha$"] {angle};
+\end{codeexample}
+
+As described in Section~\ref{section-label-quotes}, the apostrophe becomes part
+of the \meta{options}, when present. As can be seen above, the following style
+is executed:
+%
+\begin{stylekey}{/tikz/every pic quotes (initially \normalfont empty)}
+\end{stylekey}
+
+
+\subsection{Defining New Pic Types}
+\label{section-new-pic-types}
+
+As explained in the description of the |pic| command, in order to define a new
+pic type you need to
+%
+\begin{enumerate}
+ \item define a key with the path prefix |/tikz/pics| that
+ \item sets the key |/tikz/pics/code| to the code of the pic.
+\end{enumerate}
+
+It turns out that this is easy enough to achieve using styles:
+
+\begin{codeexample}[code only]
+\tikzset{
+ pics/seagull/.style ={
+ % Ok, this is the key that should, when
+ % executed, set the code key:
+ code = { %
+ \draw (...) ... ;
+ }
+ }
+}
+\end{codeexample}
+
+Even though the above pattern is easy enough, there is a special so-called key
+handler that allows us to write even simpler code, namely:
+%
+\begin{codeexample}[code only]
+\tikzset{
+ seagull/.pic = {
+ \draw (...) ... ;
+ }
+}
+\end{codeexample}
+
+\begin{handler}{{.pic}|=|\meta{some code}}
+ This handler can only be used with a key with the prefix |/tikz/|, so just
+ should normally use it only as an option to a \tikzname\ command or to the
+ |\tikzset| command. It takes the \meta{key}'s path and, inside that path,
+ it replaces |/tikz/| by |/tikz/pics/| (so, basically, it adds the
+ ``missing'' |pics| part of the path). Then, it sets up things so that the
+ resulting name to key is a style that executes |code=some code|.
+\end{handler}
+
+In almost all cases, the |.pic| key handler will suffice to setup keys.
+However, there are cases where you really need to use the first version using
+|.style| and |code=|:
+%
+\begin{itemize}
+ \item Whenever your pic type needs to set the foreground or the background
+ code.
+ \item In case of complicated arguments given to the keys.
+\end{itemize}
+
+As an example, let us define a simple pic that draws a filled circle behind the
+path. Furthermore, we make the circle's radius a parameter of the pic:
+%
+\begin{codeexample}[]
+\tikzset{
+ pics/my circle/.style = {
+ background code = { \fill circle [radius=#1]; }
+ }
+}
+\tikz [fill=blue!30]
+ \draw (0,0) pic {my circle=2mm} -- (1,1) pic {my circle=5mm};
+\end{codeexample}