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Gtk+/Gnome Application Development
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Using the Canvas

GnomeCanvas is easy to use; this is its virtue compared to GtkDrawingArea or some other low-level approach. This section describes how to create a canvas, and work with canvas items. It ends with a programming example.

Preparing the GnomeCanvas Widget

The first decision you have to make is whether to use the canvas in GDK mode or antialiased mode. When you create a canvas widget, you must specify the mode you want; there is no way to change it later. gnome_canvas_new() creates a GDK canvas. gnome_canvas_new_aa() creates an antialiased canvas. These are shown in Figure 5.

Sometimes it matters which visual and colormap the canvas will use. In particular:

  • In GDK mode, if you want to use the GnomeCanvasImage item to display images, you must use Imlib's visual and colormap. GnomeCanvasImage uses Imlib to render images.

  • In antialiased mode, GDK's RGB buffer rendering facilities (see the section called RGB Buffers in the chapter called GDK Basics) are used to copy the RGB buffer to the screen. You must use the visual and colormap from the GDK RGB module.

To create a widget with a non-default visual and colormap, gtk_widget_push_visual() and gtk_widget_push_colormap() are used. Here is the code to create a GDK canvas that supports the image item:


  GtkWidget* canvas;
  gtk_widget_push_visual(gdk_imlib_get_visual());
  gtk_widget_push_colormap(gdk_imlib_get_colormap());
  canvas = gnome_canvas_new();
  gtk_widget_pop_visual();
  gtk_widget_pop_colormap();

      

To create an antialiased canvas, do this:


  GtkWidget* canvas;
  gtk_widget_push_visual(gdk_rgb_get_visual());
  gtk_widget_push_colormap(gdk_rgb_get_cmap());
  canvas = gnome_canvas_new_aa();
  gtk_widget_pop_colormap();
  gtk_widget_pop_visual();

      
       #include <libgnomeui/gnome-canvas.h>
      

GtkWidget* gnome_canvas_new(void);

GtkWidget* gnome_canvas_new_aa(void);

Figure 5. Canvas Constructors

Gtk+/Gnome Application Development
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