Displaying RGB Image in GTK+-2.2

I'm writing a class that can take my own RGB images and display them to windows using GTK+-2.2. My Image class stores the images as packed 24-bit RGB bytes, so the conversion should be trivial. I'm using the gdk_draw_rgb(...) method to draw to my window, but nothing gets drawn at all - the window just shows up gray.

I did get this to work using Cairo, unfortunately Cairo can only represent images in 32bpp format, and doing that conversion was just too slow.

  class ImageDisplay
  {
    public:
      ImageDisplay();
      ~ImageDisplay();

      void showImage(Image img, std::string label="");

    private:
      std::thread _gtkThread;

      std::map<std::string, GtkWidget*> _windows;
  };

// ######################################################################
void gtkThreadMethod()
{
  g_thread_init(NULL);
  gdk_threads_init();
  gdk_threads_enter();

  int argc=1;
  char **argv = new char*;
  argv[0] = new char[8];
  sprintf(argv[0], 开发者_如何学运维"display");
  gtk_init(&argc, &argv);

  gdk_rgb_set_verbose(TRUE);

  gtk_main();

  gdk_threads_leave();
}

// ######################################################################
ImageDisplay::ImageDisplay()
{ 
  // Start gtk in its own thread
  _gtkThread = std::thread(gtkThreadMethod);
}

// ######################################################################
ImageDisplay::~ImageDisplay()
{  
  // Tell GTK that it's time to quit
  gdk_threads_enter();
  gtk_main_quit();
  gdk_threads_leave();

  // Wait for the thread to die
  _gtkThread.join();
}

// ######################################################################
void ImageDisplay::showImage(Image img, std::string label)
{
  gdk_threads_enter();

  // Create a new window if one doesn't yet exist
  if(_windows.find(label) == _windows.end())
  {
    GtkWidget* window = gtk_window_new(GTK_WINDOW_TOPLEVEL);
    gtk_window_set_title(GTK_WINDOW(window), label.c_str());
    gtk_window_set_default_size(GTK_WINDOW(window), img.dims().w(), img.dims().h());
    gtk_widget_set_app_paintable(window, TRUE);
    gtk_window_set_resizable(GTK_WINDOW(window), true);
    GdkGeometry size_hints;
    size_hints.min_aspect = 1;
    size_hints.max_aspect = 1;
    gtk_window_set_geometry_hints(GTK_WINDOW(window), window,
        &size_hints, GDK_HINT_ASPECT);
    gtk_widget_show_all(window);
    _windows[label] = window;
  }

  GtkWidget* window = _windows[label];

  GdkGC *gc = gdk_gc_new(gtk_widget_get_root_window(window));
  gdk_draw_rgb_image(
      gtk_widget_get_root_window(window),
      gc,
      0, 0,
      img.dims().w(), img.dims().h(),
      GDK_RGB_DITHER_NORMAL, 
      (const unsigned char*)img.const_begin(), 
      img.dims().w()*3);

  gdk_threads_leave();
}

gdk_draw_rgb_image(
  gtk_widget_get_root_window(window),

There's your problem. In X terminology (which GTK+ borrows from heavily), the "root window" refers to the desktop background. You want gtk_widget_get_window which will get you the GdkDrawable which represents your window.

However... I haven't walked very far up the stack from the line above, and I am not sure what the caller of this code looks like, but you generally want to draw in an "expose event" handler, rather than immediately after calling gtk_window_new. The last time I was writing this kind of code (it's been a while, I'll admit), what I would do is create a GdkPixmap to draw in and then copy its contents to the user-visible GdkWindow on the expose event. The GtkDrawingArea widget is helpful here, so I would search for examples using that.

To display a (client-side) image, I think you should look into using the GtkImage widget, rather than "overloading" a random widget to do custom painting.

This will in turn expose a GdkPixbuf holding the pixels.