Trying to render GTK+ window to image

Question

GTK uses cairo for drawing. So I'm trying to create a hello world app that writes to an image (svg, png, ...) instead of X11. I'm facing 2 problems: - The image is empty - When starting without X11 running (which is the actual goal) I get the error "** (a.out:9021): WARNING **: Could not open X display"

The code is draft!

#include <string>
#include <iostream>
#include <thread>
#include <chrono>
#include <cmath>

#include <cairo.h>
#include <cairommconfig.h>
#include <cairomm/context.h>
#include <cairomm/surface.h>
#include <gtk/gtk.h>
#include <gdk-pixbuf/gdk-pixbuf.h>

int main(int argc, char *argv[])
{

  gtk_init(&argc, &argv);
  GtkWidget *window;
  GtkWidget *button;
   //   GtkWidget *main_window = gtk_initialize();

  window = gtk_offscreen_window_new();

  button = gtk_button_new_with_label ("Hello World");
  gtk_container_add (GTK_CONTAINER (window), button);
  gtk_widget_show  (window);
  GdkWindow *gdk_window = gtk_widget_get_window(GTK_WIDGET(window));
  std::cout << "gdk window: " << gdk_window << std::endl;
  cairo_surface_t * surfp =  gdk_offscreen_window_get_surface(gdk_window);
  std::cout << "Created Window will now draw to png" << std::endl;

  std::string filename = "image.svg";
  double width = 600;
  double height = 400;

  Cairo::SvgSurface srfobj(surfp);

  Cairo::RefPtr<Cairo::SvgSurface> refptr(&srfobj);

  Cairo::RefPtr<Cairo::Context> cr = Cairo::Context::create(refptr);

  cr->save(); // save the state of the context
  cr->show_page();

  std::cout << "Wrote SVG file \"" << filename << "\"" << std::endl;
  std::chrono::milliseconds dura( 200 );
  std::this_thread::sleep_for(dura);

  return 0;
}

• Why is this code not working?
• Can I run a gtk app without X11 running, or should I just ignore the warning?

Answer 1

Here is my solution based on your example code - keep in mind it is a dirty solution and may not work using newer versions of GTK3. It works to save the UI of a window (only tested with the one button), but still requires (somewhere) a running X-server. It also ignores / don't use your settings for the picture size - you'll have to resize it at your own. I don't know if (and how) it is possible to cut this string (X-Server // X-Framebuffer) too (DirectFB seems not to be really supported anymore), but...

Have fun!

// Default
#include <string>
#include <iostream>
#include <thread>
#include <chrono>

// cairo / cairomm / gtk
#include <cairo.h>
#include <cairomm/context.h> //libcairomm-1.0-dev
#include <gtk/gtk.h>

int main(int argc, char *argv[]) {
    // Init
    gtk_init(&argc, &argv);

    // Create window with a button
    GtkWidget *window;
    GtkWidget *button;

    window = gtk_offscreen_window_new();
    button = gtk_button_new_with_label("Hello World");
    gtk_container_add(GTK_CONTAINER(window), button);
    gtk_widget_show_all(window);

    // Make a gdk window out of it, prepare cairo and draw it to it
    GdkWindow* gdk_window = gtk_widget_get_window(GTK_WIDGET(window));
    cairo_surface_t* surfp = gdk_offscreen_window_get_surface(gdk_window);
    cairo_t* context = cairo_create(surfp);
    gtk_widget_draw(GTK_WIDGET(window), context);

    // Yay - begin the dump!
    Cairo::SvgSurface srfobj(surfp);
    std::string filename = "image.png";
    srfobj.write_to_png(filename);
    std::cout << "Done." << std::endl;

    // Aaand a little sleep...
    std::chrono::milliseconds dura(1000);
    std::this_thread::sleep_for(dura);

    return 0;
}

Answer 2

Try to use gtk_widget_draw (widget_ptr, cairo_ctx_ptr); to draw a widget (or a hierarchy of widgets) to a cario context?

Answer 3

The answer to both your questions is that you cannot run GTK+ applications without some sort of output. You're using gtk-x11 which requires an XServer. You might have some luck with the DirectFB backend, but I wouldn't hold your breath as I don't know if it's even maintained anymore.

Because Gtk doesn't run without an XServer the resulting image is empty.