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+% Copyright 2010-2019 by Renée Ahrens
+% Copyright 2010-2019 by Olof Frahm
+% Copyright 2010-2019 by Jens Kluttig
+% Copyright 2010-2019 by Matthias Schulz
+% Copyright 2010-2019 by Stephan Schuster
+% Copyright 2019 by Jannis Pohlmann
+% Copyright 2019 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{Using Graph Drawing in PGF}
+\label{section-gd-pgf}
+
+{\noindent {\emph{by Till Tantau}}}
+
+\begin{purepgflibrary}{graphdrawing}
+ This package provides the core support for graph drawing inside \pgfname.
+ It does so by providing \pgfname\ macros for controlling the graph drawing
+ system, but also implements the binding to the graph drawing system (see
+ Section~\ref{section-gd-binding-layer} for details on bindings).
+\end{purepgflibrary}
+
+\ifluatex
+\else
+ This section of the manual can only be typeset using Lua\TeX.
+ \expandafter\endinput
+\fi
+
+
+\subsection{Overview}
+
+Just like everywhere else in \pgfname, \tikzname\ is ``just a convenient
+syntax'' in the context of graph drawing. The ``hard work'' of binding the
+internal representations of nodes and edges with the graph drawing system
+written in Lua is not done by \tikzname, but rather by a set of macros that are
+part of the basic \pgfname\ layer.
+
+The documentation of the \pgfname\ part of the graph drawing system that is
+presented in the following includes only those macros that other \TeX\ packages
+could conceivably call in order to use the graph drawing system without using
+\tikzname; for instance, for efficiency reasons. (The internal callback
+functions defined in the |graphdrawing| library that are part of the binding
+between \pgfname\ and the graph drawing system are not documented, should not
+be called, and may change in the future.)
+
+
+\subsection{How Graph Drawing in PGF Works}
+
+The core idea behind graph drawing in \pgfname\ is that inside special
+\emph{graph drawing scopes} whenever \pgfname\ creates a node, we intercept
+this node creation and \emph{do not} immediately place the node. Rather, we
+pass it down to Lua part of the graph drawing system via calls to appropriate
+methods of the (Lua) class |InterfaceToDisplay|. The effect will be that the
+nodes are ``tucked away'' in some internal tables. For edges, we introduce a
+special command called |\pgfgdedge| that tells the graph drawing system that
+there is an edge between two tucked-away nodes. Then, at the end of the graph
+drawing scope, a graph drawing algorithm written in Lua starts to work on the
+graph by computing new positions for the nodes. Once the algorithm has
+finished, the graph drawing system starts sending back the nodes and edges to
+\pgfname\ via the methods of the class |BindingToPGF|. These methods reinsert
+some code into the \TeX\ output stream that finally places the nodes at their
+final positions. Note that graph drawing algorithms are perfectly oblivious to
+all of this; indeed, the graph drawing algorithms can even be used
+independently of \TeX.
+
+Let us have a look at a simple example to see what happens when a graph is
+specified:
+%
+\begin{codeexample}[preamble={\usetikzlibrary{graphs,graphdrawing}
+\usegdlibrary{trees}}]
+\tikz[tree layout]
+ \graph {root [as=Hello] -> World[fill=blue!20]};
+\end{codeexample}
+
+The key |tree layout| internally calls the key |request scope and layout|,
+which in turn calls the macro |\pgfgdbeginscope|, which starts a graph drawing
+scope inside the graph drawing system. Once this macro has been called, until
+the next call of |\pgfgdendscope|, all nodes that are created actually get
+passed down to the graph drawing engine. This is implemented on the lowest
+layer, namely by directly intercepting nodes freshly created using |\pgfnode|.
+In our example, this happens in two places: For the |root| node and for the
+|World| node. The |graphs| library and \tikzname\ internally call the
+|\pgfnode| macro for these two nodes (after a large number of internal syntax
+translations, but the graph drawing system does not care about them).
+
+Note that the node boxes will have been fully created before they are passed
+down to the graph drawing engine -- only their final position is not yet fixed.
+It is not possible to modify the size of nodes inside the graph drawing engine,
+but you can create new nodes in certain situations.
+
+In contrast, the single edge of the graph that is created by the |->| command
+is not fully created before it is passed down to the graph drawing system. This
+would not really make sense since before the final positions of the nodes are
+fixed, we cannot even begin to compute the length of this edge, let alone where
+it should start or end. For this reason, on the upper \tikzname\ layer, the
+normal edge creation that would be caused by |->| via |new ->| is suppressed.
+Instead, the command |\pgfgdedge| is called. Similarly, inside a graph drawing
+scope, \tikzname\ will suppress both the |edge| and the |edge from parent|
+command and cause |\pgfgdedge| to be called instead.
+
+An overview of what happens is illustrated by the following call graph:
+
+\bigskip
+
+\begin{tikzpicture}[
+ class name/.style={draw,minimum size=20pt, fill=blue!20},
+ object node/.style={draw,minimum size=15pt, fill=yellow!20},
+ p/.style={->,>={Stealth[round,sep]}},
+ livespan/.style={very thick},
+ xscale=0.8,
+]
+ % class names above
+ \node (tikz) at (0,4) [class name] {\tikzname\ layer (\TeX)};
+ \node (tex) at (6,4) [class name] {\pgfname\ layer (\TeX)};
+ \node (interface) at (13,4) [class name] {Display layer (Lua)};
+ % lines from the class names to the bottom of the picture
+ \draw[livespan] (tikz) -- (0,-6.5);
+ \draw[livespan] (tex) -- (6,-6.5);
+ \draw[livespan] (interface) -- (13,-6.5);
+ % first command: \graph{ -- generates new graph in lua interface
+ \node (tikz-begin-graph) at (0,3) [object node] {|\graph[... layout]{|}; %}
+ \node (tex-begin-graph) at (6,3) [object node] {|\pgfgdbeginscope|};
+ \node (interface-new-graph) at (13,3) [object node] {|beginGraphDrawingScope(|...|)|};
+ \draw [p] (tikz-begin-graph.east) -- (tex-begin-graph.west);
+ \draw [p] (tex-begin-graph.east) -- (interface-new-graph.west);
+ % second command: a -> b -- generates two nodes in lua
+ % and one edge
+ \node (tikz-node) at (0,2) [object node] {|a -> b;|};
+ \node (tex-node) at (6,2) [object node, double copy shadow] {|\pgfnode|};
+ \draw[p] (tikz-node.east) -- (tex-node.west);
+
+ \node (interface-add-node) at (13,2) [object node, double copy shadow] {|createVertex(|...|)|};
+ \draw[p] (tex-node.east) -- (interface-add-node.west);
+
+ \node (tex-add-edge) at (6,1) [object node, double copy shadow] {|\pgfgdedge|};
+ \node (interface-add-edge) at (13,1) [object node, double copy shadow] {|createEdge(|...|)|};
+ \draw[p] (tikz-node.east) -- (1.5,2) -- (1.5,1) -- (tex-add-edge.west);
+ \draw[p] (tex-add-edge.east) -- (interface-add-edge.west);
+
+ % scope ends -- closes graph, layouts it and draws it
+ \node (tikz-end) at (0,0) [object node] {|};|};
+ \node (tex-end) at (6,0) [object node] {|\pgfgdendscope|};
+ \node (interface-draw-graph) at (13,0) [object node] {|runGraphDrawingAlgorithm()|};
+ \node (interface-finish-graph) at (13,-2) [object node] {|endGraphDrawingScope()|};
+
+ \node (invoke-algorithm) at (15.5,-1) [object node] {invoke algorithm};
+ \draw[p] (tikz-end.east) -- (tex-end.west);
+ \draw[p] (tex-end.east) -- (interface-draw-graph.west);
+ \draw[p] (interface-draw-graph.east) -| (invoke-algorithm.20);
+ \draw[p] (tex-end.east) -- (9.5,0) -- (9.5,-2) -- (interface-finish-graph.west);
+
+ % begin shipout
+ \node (tex-begin-shipout) at (6,-3) [object node] {|\pgfgdcallbackbeginshipout|};
+
+ \node (tex-puttexbox) at (6,-4) [object node, double copy shadow] {|\pgfgdcallbackrendernode|};
+ \node (tex-putedge) at (6,-5) [object node, double copy shadow] {|\pgfgddefaultedgecallback|};
+
+ \node (tex-end-shipout) at (6,-6) [object node] {|\pgfgdcallbackendshipout|};
+
+ \draw [p] (interface-finish-graph.-170) |- (tex-begin-shipout.east);
+ \draw [p] (interface-finish-graph.-170) |- (tex-puttexbox.east);
+ \draw [p] (interface-finish-graph.-170) |- (tex-putedge.east);
+ \draw [p] (interface-finish-graph.-170) |- (tex-end-shipout.east);
+ %(interface-finish-graph.east) -- (12.5,-2) -- (12.5,-4) -- (sys-puttexbox.west);
+
+ % put edge
+ %(interface-finish-graph.east) -- (12.5,-2) -- (12.5,-5) -- (sys-put-edge.west);
+ % end shipout
+ %(interface-finish-graph.east) -- (12.5,-2) -- (12.5,-6) -- (sys-end-shipout.west);
+\end{tikzpicture}
+\medskip
+
+The above diagram glosses over the fact that the display layer does not
+actually call any of the macros of \TeX\ directly, but uses a so called
+\emph{binding} (see the class |BindingToPGF|). However, this will not be
+important for the present section since you cannot access the binding directly.
+
+
+\subsubsection{Graph Drawing Scopes}
+\label{section-gd-scopes}
+
+When the graph drawing system is active, some pretty basic things inside
+\pgfname\ change -- such as the fact that nodes are no longer created in the
+normal manner. For this reason, the graph drawing system must be switched on
+and of explicitly through opening and closing a so called \emph{graph drawing
+scope}. These scopes can, in principle, be nested, namely a graph contains a
+node that contains some text that in turn contains a subpicture that contains a
+drawing of a graph. However, this is \emph{not} the same as subgraphs nodes and
+sublayouts, which are all part of the same graph drawing scope. Normally, graph
+drawing scopes are not nested.
+
+Graph drawing scopes are created using the following commands:
+
+\begin{command}{\pgfgdbeginscope}
+ This macro starts a \TeX\ scope inside which the following things happen:
+ %
+ \begin{enumerate}
+ \item The display layer method |beginGraphDrawingScope| is called,
+ which created a new graph drawing scope inside the graph drawing
+ system and places it on top of an internal stack. From now on, all
+ subsequent interface calls will refer to this scope until
+ |\pgfgdendscope| is called, which will pop the scope once more.
+ \item Inside the \TeX\ scope, nodes are not placed immediately. Rather,
+ |\pgfpositionnodelater|, see
+ Section~\ref{section-shapes-deferred-node-positioning}, is used to
+ call |InterfaceToDisplay.createVertex| for all nodes created inside
+ the scope. This will cause them to be put inside some internal
+ table.
+ \item Some additional \meta{code} is executed, which has been set using
+ the following command:
+ %
+ \begin{command}{\pgfgdaddspecificationhook\marg{code}}
+ This command adds the \meta{code} to the code that is executed
+ whenever a graph drawing scope starts. For instance, the
+ \tikzname\ library |graphdrawing| uses this macro to add some
+ \meta{code} that will redirect the |edge| and
+ |edge from parent| path commands to |\pgfgdedge|.
+ \end{command}
+ \item |\pgftransformreset| is called.
+ \item The following \TeX-if is set to true:
+ {
+ \let\ifpgfgdgraphdrawingscopeactive=\relax
+ \begin{textoken}{\ifpgfgdgraphdrawingscopeactive}
+ Will be true inside a graph drawing scope.
+ \end{textoken}
+ }
+ \end{enumerate}
+ %
+ The above has a number of consequences for what can happen inside a graph
+ drawing scope:
+ %
+ \begin{itemize}
+ \item Since nodes are not actually created before the end of the scope,
+ you cannot reference these nodes. Thus, you cannot write
+ %
+\begin{codeexample}[code only]
+\tikz [spring layout] {
+ \node (a) {a};
+ \node (b) {b};
+ \draw (a) -- (b);
+}
+\end{codeexample}
+ %
+ The problem is that we cannot connect |(a)| and |(b)| via a
+ straight line since these nodes do not exist at that point (they
+ are available only deeply inside the Lua).
+ \item In order to create edges between nodes inside a graph drawing
+ scope, you need to call the |\pgfgdedge| command, described below.
+ \end{itemize}
+
+ Additionally, when \tikzname\ is used, the following things also happen:
+ %
+ \begin{itemize}
+ \item If the |graphs| library has been loaded, the default positioning
+ mechanisms of this library are switched off, leaving the
+ positioning to the graph drawing engine. Also, when an edge is
+ created by the |graphs| library, this is signalled to the
+ |graphdrawing| library. (To be more precise: The keys |new ->| and
+ so on are redefined so that they call |\pgfgdedge| instead of
+ creating an edge.
+ \item The |edge| path command is modified so that it also calls
+ |\pgfgdedge| instead of immediately creating any edges.
+ \item The |edge from parent| path command is modified so that is also
+ calls |\pgfgdedge|.
+ \item The keys |append after command| and |prefix after command| keys
+ are modified so that they are executed only via |late options| when
+ the node has ``reached its final parking position''.
+ \end{itemize}
+
+ Note that inside a graph drawing scope you first have to open a (main)
+ layout scope (using the |\pgfgdbeginlayout| command described later on)
+ before you can add nodes and edges to the scope.
+\end{command}
+
+\begin{command}{\pgfgdendscope}
+ This macro is used to end a graph drawing scope. It must be given on the
+ same \TeX\ grouping level as the corresponding |\pgfgdbeginscope|. When the
+ macro is called, it triggers a lot of new calls:
+ %
+ \begin{enumerate}
+ \item The special treatment of newly created boxes is ended. Nodes are
+ once more created normally.
+ \item The effects of the \meta{code} that was inserted via the
+ specification hook command also ends (provided it had no global
+ effects).
+ \item We call |InterfaceToDisplay.runGraphDrawingAlgorithm|. This will
+ cause the algorithm(s) for the graph to be executed (since a graph
+ can have sublayouts, several algorithms may be run). See
+ Section~\ref{section-gd-layout-scopes} below.
+ \item Next, we call |InterfaceToDisplay.endGraphDrawingScope|. This
+ causes all nodes that were intercepted during the graph drawing
+ scope to be reinserted into the output stream at the positions that
+ were computed for them. Also, for each edge that was requested via
+ |\pgfgdedge|, the callback macro is called (see below).
+ \end{enumerate}
+\end{command}
+
+Inside a graph drawing scope, nodes are automatically passed down to the graph
+drawing engine, while for edges a command has to be called explicitly:
+
+\begin{command}{\pgfgdedge\marg{first node}\marg{second node}\marg{edge direction}\marg{edge options}\marg{edge nodes}}
+ This command is used to tell the graph drawing engine that there is an edge
+ between \meta{first node} and \meta{second node} in your graph. The
+ ``kind'' of connection is indicated by \meta{direction}, which may be one
+ of the following:
+ %
+ \begin{itemize}
+ \item |->| indicates a directed edge (also known as an arc) from
+ \meta{first node} to \meta{second node}.
+ \item |--| indicates an undirected edge between \meta{first node} and
+ \meta{second node},
+ \item |<-| indicates a directed edge from \meta{second node} to
+ \meta{first node}, but with the ``additional hint'' that this is a
+ ``backward'' edge. A graph drawing algorithm may or may not take
+ this hint into account.
+ \item |<->| indicates a bi-directed edge between \meta{first node} and
+ \meta{second node}.
+ \item |-!-| indicates that the edge from \meta{first node} to
+ \meta{second node} is ``missing''.
+ \end{itemize}
+ %
+ Note that in all cases, the syntactic digraph will contain an arc from
+ \meta{first node} to \meta{second node}, regardless of the value of
+ \meta{direction}. The \meta{direction} is ``just'' a ``semantic
+ annotation''.
+
+ The parameters \meta{edge options} and \meta{edge nodes} are a bit more
+ tricky. When an edge between two vertices of a graph is created via
+ |\pgfgdedge|, nothing is actually done immediately. After all, without
+ knowing the final positions of the nodes \meta{first node} and \meta{second
+ node}, there is no way of creating the actual drawing commands for the
+ edge. Thus, the actual drawing of the edge is done only when the graph
+ drawing algorithm is done (namely in the macro |\pgfgdedgecallback|, see
+ later).
+
+ Because of this ``delayed'' drawing of edges, options that influence the
+ edge must be retained until the moment when the edge is actually drawn.
+ Parameters \meta{edge options} and \meta{edge nodes} store such options.
+
+ Let us start with \meta{edge options}. This parameter should be set to a
+ list of key--value pairs like
+ %
+\begin{codeexample}[code only]
+/tikz/.cd, color=red, very thick, orient=down
+\end{codeexample}
+ %
+ Some of these options may be of interest to the graph drawing algorithm
+ (like the last option) while others will only be important during the
+ drawing of edge (like the first option). The options that are important for
+ the graph drawing algorithm must be pushed onto the graph drawing system's
+ option stack.
+
+ The tricky part is that options that are of interest to the graph drawing
+ algorithm must be executed \emph{before} the algorithm starts, but the
+ options as a whole are usually only executed during the drawing of the
+ edges, which is \emph{after} the algorithm has finished. To overcome this
+ problem, the following happens:
+
+ The options in \meta{edge options} are executed ``tentatively'' inside
+ |\pgfgdedge|. However, this execution is done in a ``heavily guarded
+ sandbox'' where all effects of the options (like changing the color or the
+ line width) do not propagate beyond the sandbox. Only the changes of the
+ graph drawing edge parameters leave the sandbox. These parameters are then
+ passed down to the graph drawing system.
+
+ Later, when the edge is drawn using |\pgfgdedgecallback|, the options
+ \meta{edge options} are available once more and then they are executed
+ normally.
+
+ Note that when the options in \meta{edge options} are executed, no path is
+ preset. Thus, you typically need to start it with, say, |/tikz/.cd|. Also
+ note that the sandbox is not perfect and changing global values will have
+ an effect outside the sandbox. Indeed, ``putting things in a sandbox'' just
+ means that the options are executed inside a \TeX\ scope inside an
+ interrupted path inside a \TeX\ box that is thrown away immediately.
+
+ The text in \meta{edge nodes} is some ``auxiliary'' text that is simply
+ stored away and later directed to |\pgfgdedgecallback|. This is used for
+ instance by \tikzname\ to store its node labels.
+\end{command}
+
+\begin{command}{\pgfgdsetedgecallback\marg{macro}}
+ This command allows you to change the \meta{macro} that gets called form
+ inside the graph drawing system at the end of the creation of a graph, when
+ the nodes have been positioned. The \meta{macro} will be called once for
+ each edge with the following parameters:
+ %
+ \begin{quote}
+ \meta{macro}\marg{first node}\marg{second node}\marg{direction}\marg{edge options}\marg{edge nodes}\\
+ \marg{algorithm-generated options}\marg{bend information}\marg{animations}
+ \end{quote}
+
+ The first five parameters are the original values that were passed down to
+ the |\pgfgdedge| command.
+
+ The \meta{algorithm-generated options} have been ``computed by the
+ algorithm''. For instance, an algorithm might have determined, say, flow
+ capacities for edges and it might now wish to communicate this information
+ back to the upper layers. These options should be executed with the path
+ |/graph drawing|.
+
+ The parameter \meta{bend information} contains algorithmically-computed
+ information concerning how the edge should bend. This will be a text like
+ |(10pt,20pt)--(30pt,40pt)| in \tikzname-syntax and may include the path
+ commands |--|, |..| (followed by Bézier coordinates), and |--cycle|.
+
+ The parameter \meta{animations} contains algorithmically-generated
+ animation commands (calls to |\pgfanimateattribute|. The |whom| will be set
+ to |pgf@gd|.
+
+ The default \meta{macro} simply draws a line between the nodes. When the
+ |graphdrawing| library of the \tikzname\ layer is loaded, a more fancy
+ \meta{macro} is used that takes all of the parameters into account.
+\end{command}
+
+
+\subsection{Layout Scopes}
+\label{section-gd-layout-scopes}
+
+As described in Section~\ref{section-gd-sublayouts}, the graph drawing engine
+does not always apply only a single algorithm. Rather, several different
+algorithm may be applied to different parts of the graph. How this happens,
+exactly, is governed by a hierarchy of layouts, which are setup using the
+commands |\pgfgdbeginlayout| and |\pgfgdendlayout|.
+
+\begin{command}{\pgfgdbeginlayout}
+ This command first starts a new \TeX\ scope and then informs the display
+ layer that a new (sub)layout should be started. For each graph there may be
+ a hierarchy of layouts, each of which contains a certain number of vertices
+ and edges. This hierarchy is created through calls to this macros and the
+ corresponding calls of |\pgfgdendlayout|. For each graph drawing scope
+ there has to be exactly one main layout that encompasses all nodes and
+ edges and also all sublayouts. Thus, after a graph drawing scope has been
+ opened, a layout scope also needs to be opened almost immediately.
+
+ For each layout created via this macro, a graph drawing algorithm will be
+ run later on the subgraph of all nodes that make up the layout. Which
+ algorithm is run for the layout is dictated by which layout key (one of the
+ |... layout| keys) is ``in force'' when the macro is called. Thus, using a
+ layout key for selecting an algorithm must always be done \emph{before} the
+ layout is started. (However, see the discussion of layout keys in the next
+ subsection for more details on what really happens.)
+
+ A vertex can be part of several layouts, either because they are nested or
+ because they overlap (this happens when a node is later on added to another
+ layout by calling |\pgfgdsetlatenodeoption|). This means that it is not
+ immediately obvious how conflicts arising from the different ways different
+ algorithms ``would like to place nodes'' should be resolved. The method for
+ this resolving is detailed in Section~\ref{section-gd-layout-resolve}.
+\end{command}
+
+\begin{command}{\pgfgdendlayout}
+ This command ends the \TeX\ scope of the current layout. Once closed, no
+ nodes or edges can be added to a layout.
+\end{command}
+
+\begin{command}{\pgfgdsetlatenodeoption\marg{node name}}
+ This command can only be called when the node named \meta{node name} has
+ already been created inside the current graph drawing scope. The effect of
+ calling this macro will be that all options currently on the graph drawing
+ system's option stack will be added to the node's option, possibly
+ overwriting the original option settings. Furthermore, the node will become
+ part of all layouts currently on the option stack. This means that you can
+ use this command to add a node to several layouts that are not included in
+ one another.
+\end{command}
+
+
+\subsection{Layout Keys}
+
+\emph{Layout keys} are keys like |tree layout| or |layered layout| that are
+used to select a specific graph drawing algorithm. From the graph drawing
+system's point of view, these keys ``just'' select an algorithm and when
+several layout keys are used in a row, the last one would ``win''; just as when
+you say |orient=90| directly followed by |orient=0|, the result is that the
+|orient| key is set to |0| because the last key ``wins''.
+
+Unfortunately, if keys like |tree layout| were ``just'' to select an algorithm,
+we would still need a key or some special syntax to actually start a
+(sub)layout. In early versions of the system this was exactly what people had
+to do and this was somewhat awkward. Because of this problem, the behavior of
+the layout keys in \pgfname\ (and only there, other display layers need to
+implement their own behavior) is now a bit more involved. When you use a key
+like |tree layout| (more precisely, any key that was declared as an algorithm
+key on the algorithm layer of the graph drawing system) in any scope in
+\pgfname, the following happens:
+%
+\begin{enumerate}
+ \item The graph drawing system is told that a specific algorithm has been
+ selected (the Reingold--Tilford-algorithm in this case; this
+ information was communicated to the graph drawing system during the
+ declaration of the algorithm). Being ``told'' about this means that a
+ special entry is pushed onto the current options stack of the graph
+ drawing system.
+ \item An internal ``request'' for a ``scope and a layout'' is made. This
+ has several effects:
+ \item We first test whether we are already inside a layout scope. If not,
+ we use |\pgfgdbeginscope| to open a graph drawing scope. This scope
+ will be closed appropriately (see |\pgfgdsetrequestcallback| for
+ details).
+ \item Next, a layout scope is opened using |\pgfgdbeginlayout|. It will
+ also be closed appropriately.
+\end{enumerate}
+
+The net effect of the above is that the first use of a layout key in a picture
+starts both a graph drawing scope and also a main layout, while subsequent uses
+of layout keys inside a picture will only open sublayouts.
+
+\begin{command}{\pgfgdsetrequestcallback\marg{macro}}
+ This command sets up \meta{macro} as the macro that is called whenever a
+ layout key ``requests'' that a layout and, possibly, a graph drawing scope
+ is opened. When \meta{macro} is called, it gets two parameters, the
+ \meta{begin code} and the \meta{end code}. In addition to whatever setup
+ the \meta{macro} would like to do, it should execute the \meta{begin code}
+ at the beginning of a \TeX\ scope (the code will open graph drawing and
+ layout scopes) and the \meta{end code} at the end of the same \TeX\ scope.
+
+ The need for this slightly strange macro arises from the fact that in
+ \tikzname\ we often write things like |[spring layout,node sep=2cm]|. The
+ point is that when the |spring layout| key is executed, we do \emph{not}
+ wish to open a layout scope immediately. Rather, this should happen only
+ after the option |nodes sep=2cm| has been executed. For this reason,
+ \tikzname\ sets up a special \meta{macro} that ``delays'' the execution of
+ the \meta{begin code} until the end of the opening of the next scope.
+
+ Because of this, in \tikzname\ layout keys can only be used as an option
+ when a \tikzname\ scope is started. Thus, you can pass them to |\tikz|, to
+ |{tikzpicture}|, to |\scoped|, to |{scope}|, to |graph|, and to |{graph}|.
+ For instance, the |tree layout| option can be used in the following ways:
+ %
+\begin{codeexample}[preamble={\usetikzlibrary{graphs,graphdrawing}
+\usegdlibrary{trees}}]
+\tikz [tree layout] \graph {1 -> {b,c}};
+\tikz \graph [tree layout] {2 -> {b,c}};
+\tikz \path graph [tree layout] {3 -> {b,c}};
+
+\begin{tikzpicture}[tree layout]
+ \graph {4 -> {b,c}};
+\end{tikzpicture}
+
+\begin{tikzpicture}
+ \scoped [tree layout] \graph {5 -> {b,c}};
+
+ \begin{scope}[tree layout, xshift=1.5cm]
+ \graph {6 -> {b,c}};
+ \end{scope}
+\end{tikzpicture}
+\end{codeexample}
+
+ You can \emph{not} use layout keys with a single node or on a path. In
+ particular, to typeset a tree given in the |child| syntax somewhere inside
+ a |{tikzpicture}|, you must prefix it with the |\scoped| command:
+ %
+\begin{codeexample}[preamble={\usetikzlibrary{graphdrawing}
+\usegdlibrary{trees}}]
+\begin{tikzpicture}
+ \scoped [tree layout]
+ \node {root}
+ child { node {left child} }
+ child { node {right child} };
+\end{tikzpicture}
+\end{codeexample}
+ %
+ Naturally, the above could have been written more succinctly as
+ %
+\begin{codeexample}[preamble={\usetikzlibrary{graphdrawing}
+\usegdlibrary{trees}}]
+\tikz [tree layout]
+ \node {root}
+ child { node {left child} }
+ child { node {right child} };
+\end{codeexample}
+ %
+ Or even more succinctly:
+ %
+\begin{codeexample}[preamble={\usetikzlibrary{graphs,graphdrawing}
+\usegdlibrary{trees}}]
+\tikz \graph [tree layout] { root -- {left child, right child} };
+\end{codeexample}
+ %
+\end{command}
+
+
+\subsection{Parameters}
+\label{section-gd-parameters}
+
+When a graph drawing algorithm starts working, a set of options, called ``graph
+drawing parameters'' or just ``parameters'' can influence the way the algorithm
+works. For instance, a graph drawing parameter might be the average distance
+between vertices which the algorithm should take into account. Another example
+might be the fact the certain nodes are special nodes and that a certain edge
+should have a large label.
+
+These graph drawing parameters are different from ``usual'' \pgfname\ options:
+An algorithmic parameter influences the way the algorithm works, while usual
+options influence the way the result looks like. For instance, the fact that a
+node is red is not a graph drawing parameter, while the shape of a node might
+be an graph drawing parameter.
+
+The possible graph parameters are declared by the algorithmic layer through the
+|declare| method; you cannot declare parameters on the \pgfname\ layer since
+this would not be compatible across different display systems.
+
+Users use a graph parameter in the same way as a normal key. The difference is
+that each time a key representing a graph drawing parameter is used, a special
+function of the graph drawing system's interface is called to ``push'' the
+parameter onto an internal option stack (and elements are popped from this
+stack whenever the \TeX\ scope closes in which the key was used).
+
+The net effect of all of this is that the graph drawing system keeps track of a
+stack of option in parallel to \TeX. You cannot, however, access the current
+values of graph drawing parameters from \TeX\ since they are tucked away deep
+inside the graph drawing system.
+
+
+\subsection{Events}
+
+\emph{Events} are used to pass information from the parser about the syntactic
+structure of a graph to graph drawing algorithms. Consider, for instance, a
+graph that is actually a tree in which some node ``misses'' its first child. In
+this case, the information that the child is missing is neither part of any
+node (because the node is missing, after all) nor is it an option of the whole
+graph. However, events are created by the parser the allow an algorithm to
+reconstruct the fact that the child is missing. Naturally, graph drawing
+algorithms may choose to ignore events and most will.
+
+Most of the creation and handling of events is done automatically. The only
+reason you might wish to use the following commands is when you write a
+``parser extension'' together with a new graph drawing algorithm. For instance,
+you might come up with new options that, when used, trigger events.
+
+\begin{command}{\pgfgdevent\marg{kind}\marg{parameter}}
+ Calls |createEvent| of the graph drawing system's interface class. This
+ creates a new |Event| object on the Lua layer whose |kind| field is set to
+ \meta{kind} and the |parameters| field to \meta{parameter}. You must be
+ inside a graph drawing scope to use this command.
+\end{command}
+
+\begin{command}{\pgfgdbegineventgroup\marg{parameter}}
+ Starts an event group. This just means that an |Event| of kind |begin| is
+ created with the given \meta{parameter}.
+\end{command}
+
+\begin{command}{\pgfgdendeventgroup}
+ Ends an event group. This is done by adding an event of kind |end| without
+ any parameters to the event string.
+\end{command}
+
+\begin{command}{\pgfgdeventgroup\marg{parameters}}
+ Starts an event group just like |\pgfgdbegineventgroup|, but adds a
+ corresponding closing |end| event at the end of the current \TeX\ group
+ (using |\aftergroup|).
+\end{command}
+
+
+\subsection{Subgraph Nodes}
+
+\begin{command}{\pgfgdsubgraphnode\marg{name}\marg{node options}\marg{node text}}
+ A subgraph node is a node that ``surrounds'' the nodes of a subgraph. The
+ special property of a subgraph node opposed to a normal node is that it is
+ created only after the subgraph has been laid out. However, the difference
+ to a collection like |hyper| is that the node is available immediately as a
+ normal node in the sense that you can connect edges to it.
+
+ What happens internally is that subgraph nodes get ``registered''
+ immediately both on the \pgfname\ level and on the Lua level, but the
+ actual node is only created inside the layout pipeline using a callback.
+ The actual node creation happens when the innermost layout in which the
+ subgraph node is declared has finished.
+
+ When you create a subgraph node using this macro, you also start a
+ collection (of an internal kind) that stores the subgraph. All following
+ nodes in the current \TeX\ scope will become part of this collection.
+
+ The \meta{name} is the node name by which you can refer to this node in the
+ following. The \meta{node options} are normal \pgfname\ options (like |red|
+ or |draw| or |circle|) that will influence the appearance when it is
+ created later on. The \meta{node text} is the text that will be passed to
+ |\pgfnode| upon creation of the node.
+
+ See |InterfaceToDisplay.pushSubgraphVertex| for more details.
+\end{command}