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+% Copyright 2010-2011 by Renée Ahrens
+% Copyright 2010-2011 by Olof Frahm
+% Copyright 2010-2011 by Jens Kluttig
+% Copyright 2010-2011 by Matthias Schulz
+% Copyright 2010-2011 by Stephan Schuster
+% Copyright 2011 by Jannis Pohlmann
+% Copyright 2011 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 graph drawing
+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}[]
+\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
+|graph| 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={->,>=spaced stealth'},
+ 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 -- cloes 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 graph
+ drawing 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 syntatic 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 ``auxilliary'' 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}
+ \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 B\'ezier coordintes), and
+ |--cycle|.
+
+ 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 goverened 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 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 behaviour of the layout keys in \pgfname\ (and
+only there, other display layers need to implement their own
+behaviour) 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}[]
+\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}[]
+\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}[]
+\tikz [tree layout]
+ \node {root}
+ child { node {left child} }
+ child { node {right child} };
+\end{codeexample}
+ Or even more succinctly:
+\begin{codeexample}[]
+\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 alogrithmic 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 availble 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}
+
+
+
+
+