Using graphviz API directly to render graphs

Dev
1

Hi.
I’m trying to use the gvrender_engine_t API to get the layout information performed using the dot layout algorithm. I have followed the official guide to write my own rendering plugin. Unfortunately I only noticed the deployment approach (i.e. compiling as a shared object, copying to a specific folder, … as outlined in Chapter 6) at the end. I cloned the repo to see if there is a way to “hack in” my gvplugin_library_t at runtime and noticed the gvAddLibrary function. I tried the following:

gvrender_engine_t engine {
    .begin_job = &my_begin_job,
    .end_job = &my_end_job,
    ...
};
gvrender_features_t features {
    .flags = ...
    ...
};
gvplugin_installed_t render_plugins[] = {
    {0,    // idx
     "qt", // name
     9001, // "quality"
     &engine,
     &features},
    {0, nullptr, 0, nullptr, nullptr} // sentinel
};
gvplugin_api_t apis[] = {
    {API_render, render_plugins}, {static_cast<api_t>(0), nullptr} // sentinel
};
gvplugin_library_t gvplugin_qt_LTX_library{const_cast<char*>("myplug"), apis};

I can verify that my plugin is loaded:

GVC_t* gvc;
gvc = gvContext();
int size = 0;
char** plugin_before = gvPluginList(gvc, "render", &size);
spdlog::info("plugins before: {} (size: {})",
                fmt::join(plugin_before, plugin_before + size, ", "),
                size);
gvAddLibrary(gvc, &gvplugin_qt_LTX_library);
size = 0;
char** plugin_after = gvPluginList(gvc, "render", &size);
spdlog::info("plugins after: {} (size: {})", fmt::join(plugin_after, plugin_after + size, ", "), size);

which shows “qt” as a valid rendering plugin, however, I cannot use it with gvRender and it also seems to interfere with the render call directly after it:

auto* cgraph = createMyGraph();
gvLayout(gvc, cgraph, "dot");
gvRender(gvc, cgraph, "qt", nullptr); // Error: Error: Format: "qt" not recognized. Use one of: canon cmap cmapx cmapx_np dot dot_json ...
gvRenderFilename(gvc, cgraph, "png", "/tmp/out.png"); // Error: renderer for qt is unavailable
gvRenderFilename(gvc, cgraph, "png", "/tmp/out.png"); // 2nd call works!

I’m gonna look deeper into the code to figure out where the error is coming from, but I would appreciate any tips regarding this approach. Knowing that it’s a dead-end would e.g. help me go for the alternative, which is reading out the predefined attributes attached to my cgraph, as mentioned in the docs (Chapter 2.3).