Graphviz库详解：从布局到插件

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Graphviz指南提供了深入介绍图形可视化工具包的全面教程。该手册主要关注如何将Graphviz作为一个库在软件开发中使用，特别强调了其四个主要的图形布局器：dot、xdot、plain 和 plain-ext，以及GXL和GML格式的支持。章节一介绍了这些工具的基础概念，包括字符串基础布局，着重讲解了如何创建、布局和渲染图谱。在基本图绘制部分（2.1-2.4），用户可以学习如何构建图结构，定义节点和边的属性，以及如何利用Graphviz的API来控制布局过程。例如，2.1.1部分详细阐述了如何设置节点和边的属性，而2.2则讲解了布局算法，如dot、neato、fdp、sfdp和twopi，它们各自的特点和适用场景。节点和边的渲染细节在2.3.1中讨论，包括颜色、形状和样式等。深入到布局器内部（3.1-3.6），章节详细解释了各个布局算法的工作原理，比如dot通过层级和连线长度优化布局，而neato则更侧重于无向图的平面布局。此外，还提到了其他工具如sfdp的分形几何布局和circo的圆圈布局。接下来的章节（4）探讨了Graphviz上下文环境，包括版本特定的数据处理和图形渲染器的使用。5.1部分介绍了GVJ数据结构，这是图形绘制的核心组件，5.2和5.3则分别涉及objstate数据结构和颜色管理，这些都是理解图形输出的关键要素。插件开发是另一个重要的主题（6），包括渲染器插件（6.1）、设备驱动插件（6.2）和图像加载插件（6.3）。这为开发者提供了扩展Graphviz功能的可能性。对于未连接图（unconnected graphs）的处理（7），手册提供了相应的解决方案，以及如何编译链接库和编写示例程序（A、B、C、D章）来实践所学。 Graphviz Guide是一份详尽的手册，涵盖了从基础图形构建到高级使用技巧，以及扩展和自定义选项的全面内容，适合对图可视化有需求的软件工程师和技术人员参考。

Graphviz Library Manual, August 21, 2014 8

The remainder of this manual describes how to build an application using Graphviz as a library in the

usual sense. The next section presents the basic technique for using the Graphviz code. Since the other

approaches are merely ramiﬁcations and extensions of the basic approach, the section also serves as an

overview for all uses. Section 3 breaks each layout algorithm apart into its individual steps. With this

information, the application has the option of eliminating certain of the steps. For example, all of the layout

algorithms can layout edges as splines. If the application intends to draw all edges as line segments, it would

probably wish to avoid the spline computation, especially as it is moderately expensive in terms of time.

Section 2.3 explains how an application can invoke the Graphviz renderers, thereby generating a drawing

of a graph in a concrete graphics format such as png or PostScript. For an application intending to do its

own rendering, Section 5 recommends a technique which allows the Graphviz library to handle all of the

bookkeeping details related to data structures and machine-dependent representations while the application

need only supply a few basic graphics functions. Section 7 discusses an auxiliary library for dealing with

graphs containing multiple connected components.

N.B. Using Graphviz as a library is not thread-safe.

2 Basic graph drawing

Figure 1 gives a template for the basic library use of Graphviz, in this instance using the dot hierarchical

layout. (Appendix B provides the listing of the complete program.) Basically, the program creates a graph

using the cgraph library, setting node and edge attributes to affect how the graph is to be drawn; calls the

layout code; and then uses the position information attached to the nodes and edges to render the graph.

The remainder of this section explores these steps in more detail.

Agraph_t

GVC_t

gvc;

gvc = gvContext(); /

library function

G = createGraph ();

gvLayout (gvc, G, "dot"); /

library function

drawGraph (G);

gvFreeLayout(gvc, g); /

library function

agclose (G); /

library function

gvFreeContext(gvc);

Figure 1: Basic use

Here, we just note the gvc parameter. This is a handle to a Graphviz context, which contains drawing

and rendering information independent of the properties pertaining to a particular graph as well as vari-

ous state information. For the present, we view this an abstract parameter required for various Graphviz

functions. We will discuss it further in Section 4.

2.1 Creating the graph

The ﬁrst step in drawing a graph is to create it. To use the Graphviz layout software, the graph must be

created using the cgraph library.

We can create a graph in one of two main ways, using agread or agopen. The former function takes

a FILE

pointer to a ﬁle open for reading.

Graphviz Library Manual, August 21, 2014 9

FILE

fp;

Agraph_t

G = agread(fp, 0);

It is assumed the ﬁle contains the description of graphs using the DOT language. The agread function

parses one graph at a time, returning a pointer to an attributed graph generated from the input, or NULL if

there are no more graphs or an error occurred.

The Graphviz library provides several specialized variations of agread. If the DOT representation of

the graph is stored in memory at char

cp, then

Agraph_t

G = agmemread(cp);

can be used to parse the representation. By default, the agread function relies on the standard FILE

structure and fgets function of the stdio library. You can supply your own data source dp coupled with

your own discipline disc for reading the data to read a graph using

Agraph_t

G = agread(dp, &disc);

Further details on using agread and disciplines can be found in the cgraph library manual.

The alternative technique is to call agopen.

Agraph_t

G = agopen(name, type, &disc);

The ﬁrst argument is a char

giving the name of the graph; the second argument is an Agdesc t value

describing the type of graph to be created. A graph can be directed or undirected. In addition, a graph can

be strict, i.e., have at most one edge between any pair of nodes, or non-strict, allowing an arbitrary number

of edges between two nodes. If the graph is directed, the pair of nodes is ordered, so the graph can have

edges from node A to node B as well as edges from B to A. These four combinations are speciﬁed by the

values in Table 3. The return value is a new graph, with no nodes or edges. So, to open a graph named

Graph Type Graph

Agundirected Non-strict, undirected graph

Agstrictundirected Strict, undirected graph

Agdirected Non-strict, directed graph

Agstrictdirected Strict, directed graph

Table 3: Graph types

"network" that is directed but not strict, one would use

Agraph_t

G = agopen("network", Agdirected, 0);

The third argument is a pointer to a discipline of functions used for reading, memory, etc. If the value of 0

or NULL is used, the library uses a default discipline.

Nodes and edges are created by the functions agnode and agedge, respectively.

Agnode_t

agnode(Agraph_t

, char

, int);

Agedge_t

agedge(Agraph_t

, Agnode_t

, char

, int);

The ﬁrst argument is the graph containing the node or edge. Note that if this is a subgraph, the node or edge

will also belong to all containing graphs. The second argument to agnode is the node’s name. This is a key

for the node within the graph. If agnode is called twice with the same name, the second invocation will

not create a new node but simply return a pointer to the previously created node with the given name. The

Graphviz Library Manual, August 21, 2014 10

third argument speciﬁes whether or not a node of the given name should be created if it does not already

exist.

Edges are created using agedge by passing in the edge’s two nodes. If the graph is not strict, additional

calls to agedge with the same arguments will create additional edges between the two nodes. The string

argument allows you to supply a further name to distinguish between edges with the same head and tail.

If the graph is strict, extra calls will simply return the already existing edge. For directed graphs, the ﬁrst

and second node arguments are taken to be the tail and head nodes, respectively. For undirected graph, they

still play this role for the functions agfstout and agfstin, but when checking if an edge exists with

agedge or agfindedge, the order is irrelevant. As with agnode, the ﬁnal argument speciﬁes whether

or not the edge should be created if it does not yet exist.

As suggested above, a graph can also contain subgraphs. These are created using agsubg:

Agraph_t

agsubg(Agraph_t

, char

, int);

The ﬁrst argument is the immediate parent graph; the second argument is the name of the subgraph; the ﬁnal

argument indicates if the subgraph should be created.

Subgraphs play three roles in Graphviz. First, a subgraph can be used to represent graph structure,

indicating that certain nodes and edges should be grouped together. This is the usual role for subgraphs

and typically speciﬁes semantic information about the graph components. In this generality, the drawing

software makes no use of subgraphs, but maintains the structure for use elsewhere within an application.

In the second role, a subgraph can provide a context for setting attributes. In Graphviz, these are often

attributes used by the layout and rendering functions. For example, the application could specify that blue

is the default color for nodes. Then, every node within the subgraph will have color blue. In the context of

graph drawing, a more interesting example is:

subgraph {

rank = same; A; B; C;

}

This (anonymous) subgraph speciﬁes that the nodes A, B and C should all be placed on the same rank if

drawn using dot.

The third role for subgraphs combines the previous two. If the name of the subgraph begins with

"cluster", Graphviz identiﬁes the subgraph as a special cluster subgraph. The drawing software

will

do the layout of the graph so that the nodes belonging to the cluster are drawn together, with the entire

drawing of the cluster contained within a bounding rectangle.

We note here some important ﬁelds used in nodes, edges and graphs. If np, ep and gp are pointers to a

node, edge and graph, respectively, agnameof(np) and agraphof(np) give the name of the node and

the root graph containing it, agtail(ep) and aghead(ep) give the tail and head nodes of the edge,

and agroot(gp) gives the root graph containing the subgraph. For the root graph, this ﬁeld will point to

itself.

2.1.1 Attributes

In addition to the abstract graph structure provided by nodes, edges and subgraphs, the Graphviz libraries

also support graph attributes. These are simply string-valued name/value pairs. Attributes are used to specify

any additional information which cannot be encoded in the abstract graph. In particular, the attributes are

heavily used by the drawing software to tailor the various geometric and visual aspects of the drawing.

if supported

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