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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{The Binding Layer}
+\label{section-gd-binding-layer}
+
+\ifluatex
+\else
+ This section of the manual can only be typeset using Lua\TeX.
+ \expandafter\endinput
+\fi
+
+
+\subsection{Overview}
+
+This section explains how the \emph{binding} of the graph drawing system to a
+particular display layer works. Let me stress that all of this is important
+only for readers who
+%
+\begin{itemize}
+ \item either wish to write new display system (see
+ Section~\ref{section-gd-display-layer})
+ \item or wish to know more about how the graph drawing system works on the
+ pure \pgfname\ layer (this is were the binding occurs).
+\end{itemize}
+
+\emph{Bindings} are used to encapsulate the details of the communication
+between the graph drawing system and a display system (see
+Section~\ref{section-gd-display-layer} for an introduction to display systems).
+
+Consider a display system that communicates with the graph drawing system. At
+some point, the display system would like to run an algorithm to lay out a
+graph. To achieve this, it will call different functions from the class
+|InterfaceToDisplay| and the effect of this is that a representation of the
+to-be-drawn graph is constructed internally and that the appropriate algorithms
+are run. All of this is in some sense independent of the actual display system,
+the class |InterfaceToDisplay| offers the same standard interface to all
+display systems.
+
+At some point, however, the graph drawing system may need to ``talk back'' to
+the display system. For instance, once the graph has been laid out, to trigger
+the actual rendering of the graph, the graph drawing system must ``tell'' the
+display layer where the vertices lie. For some display systems this is easy: if
+the display system itself is written in Lua, it could just access the syntactic
+digraph directly. However, for systems like \tikzname\ or systems written in
+another language, the graph drawing system needs a set of functions that it can
+call that will tell the display system what is going on. This is were bindings
+come in.
+
+The class |Binding| is an interface that defines numerous methods that will be
+called by the graph drawing system in different situations (see the
+documentation below for details). For instance, there is a function
+|renderVertex| that is called by the graph drawing system whenever a vertex
+should be rendered by the display system. The class is really just an interface
+in the sense of object-oriented programming. For each display system you need
+to create a subclass of |Binding| like |BindingToPGF| or |BindingToASCII| that
+implement the methods declared by |Binding|. The number of methods that need to
+be implemented depends on the display system.
+
+In the following, you will find the documentation of the |Binding| class in
+Section~\ref{section-gd-binding-doc}. Following this, we first have a quick
+look at how the |BindingToPGF| works and then go over a simple example of a
+binding to a more or less imaginary display system. This example should help
+readers interested in implementing their own bindings.
+
+
+\subsection{The Binding Class and the Interface Core}
+\label{section-gd-binding-doc}
+
+\includeluadocumentationof{pgf.gd.bindings.Binding}
+\includeluadocumentationof{pgf.gd.interface.InterfaceCore}
+
+
+\subsection{The Binding To PGF}
+
+\includeluadocumentationof{pgf.gd.bindings.BindingToPGF}
+
+
+\subsection{An Example Binding Class}
+
+\label{section-gd-binding-layer-example}
+
+In the present section a complete binding is presented to an imaginary
+``\textsc{ascii} art display system'' is presented. The idea is that this
+display system will depict graphs using just normal letters and spaces so that,
+when the text is typeset in a monospace font, a visualization of the graph
+results. For instance:
+
+\bigskip
+\noindent
+\begin{minipage}[t]{.5\textwidth}
+\emph{Graph rendered by |BindingToPGF|:}
+\medskip
+
+\tikz [anchor=base]\graph [layered layout,level distance=2.35cm,sibling
+distance=1.2cm,edges={rounded corners,>={Stealth[round,sep]}}] {
+ Alice;
+ Bob;
+ Charly;
+ Dave;
+ Eve;
+ Fritz;
+ George;
+ Alice -> Bob;
+ Alice -> Charly;
+ Charly -> Dave;
+ Bob -> Dave;
+ Dave -> Eve;
+ Eve -> Fritz;
+ Fritz -> Alice;
+ George -> Eve;
+ George -> Fritz;
+ Alice -> George;
+};
+\end{minipage}%
+\begin{minipage}[t]{.49\textwidth}
+\emph{Graph rendered by |BindingToASCII|:}
+
+\begin{verbatim}
+ Alice
+ .......
+ .. . . .
+ ... . . .
+ ... .. . ..
+ .. . . .
+ Charly Bob . .
+ .. . . .
+ . . . .
+ . . . .
+ .. . . .
+ .. . .
+ Dave George .
+ .. . ... .
+ . . .. .
+ . . ...
+ .. . . ...
+ .. . ..
+ Eve . ..
+ .. . ..
+ . . .
+ . . .
+ .. . ..
+ ...
+ Fritz
+\end{verbatim}
+\end{minipage}
+\bigskip
+
+The binding will reside in a file |BindingToASCII.lua|, whose contents is
+detailed below, and which is used by calling the |bind| function of
+|InterfaceToDisplay|, see its documentation for details.
+
+The binding's code starts with some initializations:
+%
+\begin{codeexample}[code only]
+-- File BindingToASCII.lua
+
+-- Imports
+local lib = require "pgf.gd.lib"
+
+-- Subclass the Binding class:
+local BindingToASCII = lib.class { base_class = require "pgf.gd.bindings.Binding" }
+\end{codeexample}
+
+The interesting code is the code for ``rendering'' a graph. The graph drawing
+system will invoke the binding's methods |renderStart| and |renderStop| to
+signal that the graph drawing algorithms have finished and that the vertices
+and edges can now be drawn.
+
+In our \textsc{ascii} renderer, we use a two-dimensional field holding
+characters that severs as the ``drawing canvas''. At the beginning of the
+rendering, we initialize it with blanks:
+%
+\begin{codeexample}[code only]
+local canvas
+
+function BindingToASCII:renderStart()
+ canvas = {}
+ -- Clear the canvas
+ for x=-30,30 do
+ canvas [x] = {}
+ for y=-30,30 do
+ canvas[x][y] = ' '
+ end
+ end
+end
+\end{codeexample}
+
+In order to ``render'' a vertex, the graph drawing system will call the
+|renderVertex| method. The binding of \tikzname\ does a lot of complicated
+things in this method to retrieve the underlying node's box from internal table
+and to somehow reinstall the box in \TeX's output stream; for our
+\textsc{ascii} binding things are much simpler: We simply put the vertex's name
+at the canvas position corresponding to the vertex's |pos| coordinate. Note
+that this simple version of an \textsc{ascii} renderer does not try to scale
+things; thus, array out of bounds might occur here.
+%
+\begin{codeexample}[code only]
+function BindingToASCII:renderVertex(v)
+ canvas [math.floor(v.pos.x)][math.floor(v.pos.y)] = v.name
+end
+\end{codeexample}
+
+The rendering of edges is a more complicated process. Given two vertices, we
+put dots at the canvas positions between them; provided there are no vertices
+(so edges are behind the nodes). Here is the essential part of the code (for
+the complete code, have a look at |pgf/gd/examples/BindingToASCII.lua|):
+%
+\begin{codeexample}[code only]
+function BindingToASCII:renderEdge(e)
+ local function connect (p,q)
+ -- Connect the points p and q
+ local x1, y1, x2, y2 = math.floor(p.x+0.5), math.floor(p.y+0.5), math.floor(q.x+0.5), math.floor(q.y+0.5)
+ ...
+ local delta_x = x2-x1
+ local delta_y = y2-y1
+ ...
+ local slope = delta_y/delta_x
+ for i=x1,x2 do
+ local x,y = i, math.floor(y1 + (i-x1)*slope + 0.5)
+
+ if canvas[x][y] == " " then
+ canvas[x][y] = '.'
+ end
+ end
+ ...
+ end
+
+ -- Iterate over all points on the path from tail to head:
+ local p = e.tail.pos
+ for i=1,#e.path do
+ connect(p, e.tail.pos + e.path[i])
+ p = e.tail.pos + e.path[i]
+ end
+ connect(p, e.head.pos)
+end
+\end{codeexample}
+
+The methods |renderVertex| and |renderEdge| will be called once for each vertex
+and edge of the to-be-rendered graph. At the end, the |renderStop| method is
+called. In our case, this method will output the canvas using |print|. A slight
+complication arises when node names are longer than just one character. In this
+case, the following code ``centers'' them on their coordinate and makes sure
+that they do not get overwritten by the dots forming edges:
+%
+\begin{codeexample}[code only]
+function BindingToASCII:renderStop()
+ for y=10,-30,-1 do
+ local t = {}
+ for x=-30,30 do
+ local s = canvas[x][y]
+ for i=1,#s do
+ pos = x+30+i-math.floor(#s/2)
+ if not t[pos] or t[pos] == " " or t[pos] == "." then
+ t[pos] = string.sub(s,i,i)
+ end
+ end
+ end
+ print(table.concat(t))
+ end
+end
+\end{codeexample}
+
+At the end, we need to return the created object:
+%
+\begin{codeexample}[code only]
+return BindingToASCII
+\end{codeexample}