From 6f9e1680085e7bb4d258f6f8116369d122e196e1 Mon Sep 17 00:00:00 2001 From: Norbert Preining Date: Mon, 16 Jan 2023 03:03:27 +0000 Subject: CTAN sync 202301160303 --- graphics/pgf/base/doc/pgfmanual-en-gd-overview.tex | 310 +++++++++++++++++++++ 1 file changed, 310 insertions(+) create mode 100644 graphics/pgf/base/doc/pgfmanual-en-gd-overview.tex (limited to 'graphics/pgf/base/doc/pgfmanual-en-gd-overview.tex') diff --git a/graphics/pgf/base/doc/pgfmanual-en-gd-overview.tex b/graphics/pgf/base/doc/pgfmanual-en-gd-overview.tex new file mode 100644 index 0000000000..190451b149 --- /dev/null +++ b/graphics/pgf/base/doc/pgfmanual-en-gd-overview.tex @@ -0,0 +1,310 @@ +% Copyright 2019 by Renée Ahrens, Olof Frahm, Jens Kluttig, Matthias Schulz, Stephan Schuster +% Copyright 2019 by Till Tantau +% Copyright 2019 by Jannis Pohlmann +% +% 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{Introduction to Algorithmic Graph Drawing} +\label{section-intro-gd} + +\emph{by Till Tantau} + +\ifluatex +\else + This section of the manual can only be typeset using Lua\TeX. + \expandafter\endinput +\fi + + +\subsection{What Is Algorithmic Graph Drawing?} + +\emph{Algorithmic graph drawing} (or just \emph{graph drawing} in the +following) is the process of computing algorithmically where the nodes of a +graph are positioned on a page so that the graph ``looks nice''. The idea is +that you, as human (or you, as a machine, if you happen to be a machine and +happen to be reading this document) just specify which nodes are present in a +graph and which edges are present. Additionally, you may add some ``hints'' +like ``this node should be near the center'' or ``this edge is pretty +important''. You do \emph{not} specify where, exactly, the nodes and edges +should be. This is something you leave to a \emph{graph drawing algorithm}. The +algorithm gets your description of the graph as an input and then decides where +the nodes should go on the page. +% +\begin{codeexample}[preamble={\usetikzlibrary{graphs,graphdrawing} +\usegdlibrary{trees}}] +\tikz \graph [binary tree layout, level distance=5mm] { + 4 -- { + 3 -- 0 -- 1[second], + 10 -- { + 8 -- { + 6 -- {5,7}, + 9 + } } } +}; +\end{codeexample} + +\begin{codeexample}[preamble={\usetikzlibrary{graphs,graphdrawing,quotes} +\usegdlibrary{force}}] +\tikz \graph [spring layout, + edge quotes mid, + edges={nodes={font=\scriptsize, fill=white, sloped, inner sep=1pt}}] +{ + 1 ->["Das"] 2 ->["ist"] 3 ->["das"] 4 ->["Haus"] + 2 ->["vom" near start] 5 ->["Ni"] 4 ->["ko" near start] + 1 ->["laus", orient=right] 5; +}; +\end{codeexample} + +Naturally, graph drawing is a bit of a (black?) art. There is no ``perfect'' +way of drawing a graph, rather, depending on the circumstances there are +several different ways of drawing the same graph and often it will just depend +on the aesthetic sense of the reader which layout he or she would prefer. For +this reason, there are a huge number of graph drawing algorithms ``out there'' +and there are scientific conference devoted to such algorithms, where each year +dozens of new algorithms are proposed. + +Unlike the rest of \pgfname\ and \tikzname, which is implemented purely in +\TeX, the graph drawing algorithms are simply too complex to be implemented +directly in \TeX. Instead, the programming language Lua is used by the +|graphdrawing| library -- a programming language that has been integrated into +recent versions of \TeX. This means that (a) as a user of the graph drawing +engine you run \TeX\ on your documents in the usual way, no external programs +are called since Lua is already integrated into \TeX, and (b) it is pretty easy +to implement new graph drawing algorithms for \tikzname\ since Lua can be used +and no \TeX\ programming knowledge is needed. + + +\subsection{Using the Graph Drawing System} + +``Users'' of the graph drawing engine can invoke the graph drawing algorithms +often by just adding a single option to their picture. Here is a typical +example, where the |layered layout| option tells \tikzname\ that the graph +should be drawn (``should be laid out'') using a so-called ``layered graph +drawing algorithm'' (what these are will be explained later): +% +\begin{codeexample}[preamble={\usetikzlibrary{arrows.meta,graphs,graphdrawing} +\usegdlibrary{layered}}] +\tikz [>={Stealth[round,sep]}] + \graph [layered layout, components go right top aligned, nodes=draw, edges=rounded corners] + { + first root -> {1 -> {2, 3, 7} -> {4, 5}, 6 }, 4 -- 5; + second root -> x -> {a -> {u,v}, b, c -> d -> {w,z} }; + third root -> child -> grandchild -> youngster -> third root; + }; +\end{codeexample} +% +Here is another example, where a different layout method is used that is more +appropriate for trees: +% +\begin{codeexample}[preamble={\usetikzlibrary{graphdrawing} +\usegdlibrary{trees}}] +\tikz [grow'=up, binary tree layout, nodes={circle,draw}] + \node {1} + child { node {2} + child { node {3} } + child { node {4} + child { node {5} } + child { node {6} } + } + } + child { node {7} + child { node {8} + child[missing] + child { node {9} } + } + }; +\end{codeexample} +% +A final example, this time using a ``spring electrical layout'' (whatever that +might be\dots): +% +\begin{codeexample}[ + preamble={\usetikzlibrary{decorations.pathmorphing,graphdrawing} +\usegdlibrary{force}}] +\tikz [spring electrical layout, node distance=1.3cm, + every edge/.style={ + decoration={coil, aspect=-.5, post length=1mm, + segment length=1mm, pre length=2mm}, + decorate, draw}] +{ + \foreach \i in {1,...,6} + \node (node \i) [fill=blue!50, text=white, circle] {\i}; + + \draw (node 1) edge (node 2) + (node 2) edge (node 3) + edge (node 4) + (node 3) edge (node 4) + edge (node 5) + edge (node 6); +} +\end{codeexample} +% +In all of the example, the positions of the nodes have only been computed +\emph{after} all nodes have been created and the edges have been specified. For +instance, in the last example, without the option |spring electrical layout|, +all of the nodes would have been placed on top of each other. + + +\subsection{Extending the Graph Drawing System} + +The graph drawing engine is also intended to make is (relatively) easy to +implement new graph drawing algorithms. These algorithms can either be +implemented in the Lua programming language (which is \emph{much} easier to +program than \TeX\ itself) or in C/C++ (but at a great cost regarding +portability). The Lua code for a graph drawing algorithm gets an +object-oriented model of the input graph as an input and must just compute the +desired new positions of the nodes. The complete handling of passing options +and configurations back-and-forth between the different \tikzname\ and +\pgfname\ layers is handled by the graph drawing engine. + +As a caveat, the graph drawing engine comes with a library of functions and +methods that simplify the writing of new graph drawing algorithms. As a typical +example, when you implement a graph drawing algorithm for trees, you typically +require that your input is a tree; but you can bet that users will feed all +sorts of graphs to your algorithm, including disjoint unions of cliques. The +graph drawing engine offers you to say that a precondition to running your +algorithm is that the graph is a |tree| and instead of the original graph your +algorithm will be provided with a spanning tree of the graph on which it can +work. There are numerous further automatic pre- and postprocessing steps that +include orienting, anchoring, and packing of components, to name a few. + +The bottom line is that the graph drawing engine makes it easy +to try out new graph drawing algorithms for medium sized graphs (up +to a few hundred nodes) in Lua. For larger graphs, C/C++ code must be +used. + + +\subsection{The Layers of the Graph Drawing System} +\label{section-gd-layers} + +Even though the graph drawing system presented in the following sections was +developed as part of \pgfname, it can be used independently of \pgfname\ and +\tikzname: It was (re)designed so that it can be used by arbitrary programs as +long as they are able to run Lua. To achieve this, the graph drawing system +consists of three layers: +% +\begin{enumerate} + \item At the ``bottom'' we have the \emph{algorithmic layer}. This layer, + written in Lua, contains all graph drawing algorithms. Interestingly, + options must also be declared on this layer, so an algorithm together + with all options it uses can and must be specified entirely on this + layer. If you intend to implement a new graph drawing algorithm, you + will only be interested in the functionality of this layer. + + Algorithm ``communicate'' with the graph drawing system through a + well-defined interface, encapsulated in the class + |InterfaceToAlgorithms|. + \item At the ``top'' we have the \emph{display layer}. This layer is not + actually part of the graph drawing system. Rather, it is a piece of + software that ``displays'' graphs and \tikzname\ is just one example of + such a software. Another example might be a graph editor that uses the + graph drawing system to lay out the graph it displays. Yet another + example might be a command line tool for drawing graphs described in a + file. Finally, you may also wish to use the graph drawing system as a + simple subroutine for rendering graphs produced in a larger program. + + Since the different possible instantiations of the display layer are + quite heterogeneous, all display layers must communicate with the graph + drawing system through a special interface, encapsulated in the class + |InterfaceToDisplay|. + + The main job of this class is to provide a set of methods for + specifying that a graph has certain nodes and edges and that certain + options have been set for them. However, this interface also allows you + to query all options that have been declared by algorithms, including + their documentation. This way, an editor or a command line tool can + display a list of all graph drawing algorithms and how they can be + configured. + \item The algorithm layer and the display layer are ``bound together'' + through the \emph{binding layer}. Most of the bookkeeping concerning + the to-be-drawn graphs is done by the graph drawing system + independently of which algorithm is used and also independently of + which display layer is used, but some things are still specific to each + display layer. For instance, some algorithms may create new nodes and + the algorithms may then need to know how large these nodes will be. For + this, the display layer must be ``queried'' during a run of the + algorithm -- and it is the job of the binding layer to achieve this + callback. + + As a rule, the binding layer implements the ``backward'' communication + from the graph drawing system back to the display layer, while the + display layer's interface class provides only functions that are called + from the display layer but which will not ``talk back''. +\end{enumerate} + +All of the files concerned with graph drawing reside in the |graphdrawing| +subdirectory of |generic/pgf|. + + +\subsection{Organisation of the Graph Drawing Documentation} + +The documentation of the graph drawing engine is structured as follows: +% +\begin{enumerate} + \item Following this overview section, the next section documents the graph + drawing engine from ``the \tikzname\ user's point of view''. No + knowledge of Lua or algorithmic graph drawing is needed for this + section, everyone who intends to use algorithmic graph drawing in + \tikzname\ may be interested in reading it. + \item You will normally only use \tikzname's keys and commands in order to + use the graph drawing system, but, internally, these keys call more + basic \pgfname\ commands that do the ``hard work'' of binding the world + of \TeX\ boxes and macros to the object-oriented world of Lua. + Section~\ref{section-gd-pgf} explains how this works and which commands + are available for authors of packages that directly need to use the + graph drawing system inside \pgfname, avoiding the overhead incurred by + \tikzname. + + Most readers can safely skip this section. + \item The next sections detail which graph drawing algorithms are currently + implemented as part of the \tikzname\ distribution, see + Sections~\ref{section-first-graphdrawing-library-in-manual} + to~\ref{section-last-graphdrawing-library-in-manual}. + \item Section~\ref{section-gd-algorithm-layer} is addressed at readers who + wish to implement their own graph drawing algorithms. For this, + \emph{no knowledge at all} of \TeX\ programming is needed. The section + explains the graph model used in Lua, the available libraries, the + graph drawing pipeline, and everything else that is part of the Lua + side of the engine. + \item Section~\ref{section-gd-display-layer} details the display layer of + the graph drawing system. You should read this section if you wish to + implement a new display system (that is, a non-\TeX-based program) that + intends to use the graph drawing system. + \item Section~\ref{section-gd-binding-layer} explains how binding layers + can be implemented. This section, too, is of interest only to readers + who wish to write new display systems. +\end{enumerate} + + +\subsection{Acknowledgements} + +Graph drawing in \tikzname\ began as a student's project under my supervision. +Ren\'ee Ahrens, Olof-Joachim Frahm, Jens Kluttig, Matthias Schulz, and Stephan +Schuster wrote the first prototype of a graph drawing system inside \tikzname\ +that uses Lua\TeX\ for the implementation of graph drawing algorithms. + +This first, early version was greatly extended on the algorithmic side by +Jannis Pohlmann who wrote his Diploma thesis on graph drawing under my +supervision. He implemented, in particular, the Sugiyama method +(|layered layout|) and force based algorithms. Also, he rewrote some of the +code of the prototype. + +At some point it became apparent that the first implementation had a number of +deficiencies, both concerning the structure, the interfaces, and (in +particular) the performance. Because of this, I rewrote the code of the graph +drawing system, both on the \TeX\ side and on the Lua side in its current form. +However, I would like to stress that without the work of the people mentioned +above graph drawing in \tikzname\ would not exist. + +The documentation was written almost entirely by myself, though I did copy some +paragraphs from Jannis's Diploma thesis, which I can highly recommend everyone +to read. + +In the future, I hope that other people will contribute algorithms, which will +be available as libraries. -- cgit v1.2.3