Writing Layout Plugins

How to write a custom layout engine.

To create a new layout plugin called xxx, you first need to provide two functions: xxx_layout and xxx_cleanup. The semantics of these are described below.

Layout

void xxx_layout(Agraph_t * g)

Initialize the graph.

  • If the algorithm will use the common edge routing code, it should call setEdgeType (g, ...);.

  • For each node, call common_init_node and gv_nodesize.

  • If the algorithm will use spline_edges() to route the edges, the node coordinates need to be stored in ND_pos, so this should be allocated here. This, and the two calls mentioned above, are all handled by a call to neato_init_node().

  • For each edge, call common_init_edge.

  • The algorithm should allocate whatever other data structures it needs. This can involve fields in the A*info_t fields. In addition, each of these fields contains a void* alg; subfield that the algorithm can use the store additional data. Once we move to cgraph, this will all be replaced with algorithm specific records.

  • Layout the graph. When finished, each node should have its coordinates stored in points in ND_coord_i(n), each edge should have its layout described in ED_spl(e). (N.B. As of version 2.21, ND_coord_i has been replaced by ND_coord, which are now floating point coordinates.)

To add edges, there are 3 functions available:

  1. spline_edges1 (Agraph_t*, int edgeType) Assumes the node coordinates are stored in ND_coord_i, and that GD_bb is set. For each edge, this function constructs the appropriate data and stores it in ED_spl.
  2. spline_edges0 (Agraph_t*) Assumes the node coordinates are stored in ND_pos, and that GD_bb is set. This function uses the ratio attribute if set, copies the values in ND_pos to ND_coord_i (converting from inches to points); and calls spline_edges1 using the edge type specified by setEdgeType().
  3. spline_edges (Agraph_t*) Assumes the node coordinates are stored in ND_pos. This function calculates the bounding box of g and stores it in GD_bb, then calls spline_edges0().

If the algorithm only works with connected components, the code can use the pack library to get components, lay them out individually, and pack them together based on user specifications. A typical schema is given below. One can look at the code for twopi, circo, neato or fdp for more detailed examples.

Agraph_t **ccs;
Agraph_t *sg;
Agnode_t *c = NULL;
int ncc;
int i;

ccs = ccomps(g, &ncc, 0);
if (ncc == 1) {
    /* layout nodes of g */
    adjustNodes(g);  /* if you need to remove overlaps */
    spline_edges(g); /* generic edge routing code */

} else {
    pack_info pinfo;
    pack_mode pmode = getPackMode(g, l_node);

    for (i = 0; i < ncc; i++) {
        sg = ccs[i];
        /* layout sg */
        adjustNodes(sg);  /* if you need to remove overlaps */
    }
    spline_edges(g);  /* generic edge routing */

    /* initialize packing info, e.g. */
    pinfo.margin = getPack(g, CL_OFFSET, CL_OFFSET);
    pinfo.doSplines = 1;
    pinfo.mode = pmode;
    pinfo.fixed = 0;
    packSubgraphs(ncc, ccs, g, &pinfo);
}
for (i = 0; i < ncc; i++) {
    agdelete(g, ccs[i]);
}

free(ccs);

Be careful in laying of subgraphs if you rely on attributes that have only been set in the root graph. With connected components, edges can be added with each component, before packing (as above) or after the components have been packed (see circo).

It is good to check for trivial cases where the graph has 0 or 1 nodes, or no edges.

At the end of xxx_layout, call

dotneato_postprocess(g);

The following template will work in most cases, ignoring the problems of handling disconnected graphs and removing node overlaps:

static void
xxx_init_node(node_t * n)
{
  neato_init_node(n);
  /* add algorithm-specific data, if desired */
}

static void
xxx_init_edge(edge_t * e)
{
  common_init_edge(e);
  /* add algorithm-specific data, if desired */
}

static void
xxx_init_node_edge(graph_t * g)
{
  node_t *n;
  edge_t *e;

  for (n = agfstnode(g); n; n = agnxtnode(g, n)) {
      xxx_init_node(n);
  }
  for (n = agfstnode(g); n; n = agnxtnode(g, n)) {
      for (e = agfstout(g, n); e; e = agnxtout(g, e)){          
          xxx_init_edge(e);
      }
  }
}

void
xxx_layout (Agraph_t* g)
{
  xxx_init_node_edge(g);
  /* Set ND_pos(n) for each node n */
  spline_edges(g);
  dotneato_postprocess(g);
}

Cleanup

void xxx_cleanup(Agraph_t * g)

Free up any resources allocated in the layout.

Finish with calls to gv_cleanup_node and gv_cleanup_edge for each node and edge. This cleans up splines labels, ND_pos, shapes and 0's out the A*info_t, so these have to occur last, but could be part of explicit xxx_cleanup_node and xxx_cleanup_edge, if desired.

At the end, you should do:

if (g != g->root) memset(&(g->u), 0, sizeof(Agraphinfo_t));

This is necessary for the graph to be laid out again, as the layout code assumes this structure is clean.

libgvc does a final cleanup to the root graph, freeing any drawing, freeing its label, and zeroing out Agraphinfo_t of the root graph.

The following template will work in most cases:

static void xxx_cleanup_graph(Agraph_t * g)
{
  /* Free any algorithm-specific data attached to the graph */
  if (g != g->root) memset(&(g->u), 0, sizeof(Agraphinfo_t));
}

static void xxx_cleanup_edge (Agedge_t* e)
{
  /* Free any algorithm-specific data attached to the edge */
  gv_cleanup_edge(e);
}

static void xxx_cleanup_node (Agnode_t* n)
{
  /* Free any algorithm-specific data attached to the node */
  gv_cleanup_node(e);
}

void xxx_cleanup(Agraph_t * g)
{
  Agnode_t *n;
  Agedge_t *e;

  for (n = agfstnode(g); n; n = agnxtnode(g, n)) {
      for (e = agfstout(g, n); e; e = agnxtout(g, e)) {
          xxx_cleanup_edge(e);
      }
      xxx_cleanup_node(n);
  }
  xxx_cleanup_graph(g);
}

Most layouts use auxiliary routines similar to neato, so the entry points can be added in plugin/neato_layout.

Add to gvlayout_neato_layout.c:

gvlayout_engine_t xxxgen_engine = {
    xxx_layout,
    xxx_cleanup,
};

and the line

{LAYOUT_XXX, "xxx", 0, &xxxgen_engine, &neatogen_features},

to gvlayout_neato_types and a new emum LAYOUT_XXX to layout_type in that file.

The above allows the new layout to piggyback on top of the neato plugin, but requires rebuilding the plugin. In general, a user can (and probably should) build a layout plugin totally separately.

To do this, after writing xxx_layout and xxx_cleanup, it is necessary to:

  1. Add the types and data structures:

    typedef enum { LAYOUT_XXX } layout_type;

static gvlayout_features_t xxxgen_features = {
    0
};
gvlayout_engine_t xxxgen_engine = {
    xxx_layout,
    xxx_cleanup,
};
static gvplugin_installed_t gvlayout_xxx_types[] = {
    {LAYOUT_XXX, "xxx", 0, &xxxgen_engine, &xxxgen_features},
    {0, NULL, 0, NULL, NULL}
};
static gvplugin_api_t apis[] = {
    {API_layout, &gvlayout_xxx_types},
    {(api_t)0, 0},
};
gvplugin_library_t gvplugin_xxx_layout_LTX_library = { "xxx_layout", apis };

  2. Combine all of this into a dynamic library whose name contains the string gvplugin_ and install the library in the same directory as the other Graphviz plugins. For example, on Linux systems, the dot layout plugin is in the library libgvplugin_dot_layout.so.

  3. Run dot -c to regenerate the config file.

NOTES:

  • Additional layouts can be added as extra lines in gvlayout_xxx_types.
  • Obviously, most of the names and strings can be arbitrary. One constraint is that external identifier for the gvplugin_library_t type must end in _LTX_library. In addition, the string xxx in each entry of gvlayout_xxx_types is the name used to identify the layout algorithm, so needs to be distinct from any other layout name.
  • The features of a layout algorithm are currently limited to a flag of bits, and the only flag supported is LAYOUT_USES_RANKDIR, which enables the layout to the rankdir attribute.

Changes need to be made to any applications that statically know about layout algorithms.

Automake Configuration

If you want to integrate your code into the Graphviz software and use its build system, follow the instructions below. You can certainly build and install your plugin using your own build software.

  1. Put your software in lib/xxxgen, and added the hooks describe above into gvlayout_neato_layout.c
  2. In lib/xxxgen, provide a Makefile.am (based on a simple example like lib/fdpgen/Makefile.am)
  3. In lib/Makefile.am, add xxxgen to SUBDIRS
  4. In configure.ac, add lib/xxxgen/Makefile to AC_CONFIG_FILES.
  5. In lib/plugin/neato_layout/Makefile.am, insert $(top_builddir)/lib/xxxgen/libxxxgen_C.la in libgvplugin_neato_layout_C_la_LIBADD.
  6. Remember to run autogen.sh because on its own configure can guess wrong.

This also assumes you have a good version of the various automake tools on your system.

Last modified December 6, 2022: Fix typo (de77e3d)