The GDK drawing API described here is now deprecated, in favour of Cairo. See the DrawingArea chapter for information about the Cairo API.

Gdk graphics contexts (Gdk::GC) are a server-side resource. They contain information that describes how drawing is to be done. This provides for fewer arguments to the drawing methods, and less communication between the client and the server. The following example shows how to set up a graphics context with a foreground color of red for drawing.

Gdk::GC some_gc;
some_gc.create(get_window());
Gdk::Color some_color;
Gdk::Colormap some_colormap(Gdk::Colormap::get_system());
some_color.set_red(65535);
some_color.set_green(0);
some_color.set_blue(0);
some_colormap.alloc(some_color);
some_gc.set_foreground(some_color);

The first two lines create the graphics context and assign it to the appropriate widget. The get_window() method is a part of the Gtk::Widget class, so if you put this code into a derived widget's implementation then you can call it just as it is, otherwise you'd use some_widget.get_window().

The next two lines create the Gdk::Color and Gdk::Colormap. After setting the color values you then need to allocate the color. The system figures out what to do in this case. The colormap contains information about how colors can be displayed on your screen, and is able to allocate the requested color. For example, on a display of only 256 colors the exact color requested may not be available, so the closest color to the one requested will be used instead. The final line sets the color as the foreground color.

There are a number of attributes that can be set for a graphics context. There's the foreground and background color. When drawing lines, you can set the thickness of the line with set_line_width(). Whether a solid or dashed line is drawn can be set with set_line_style(). The size and proportions of the dashes are set with set_dashes. How two lines join together, whether round or pointed or beveled off, is set with set_join_style(). Other things that can be set within a graphics context include font style, stippling and tiling for the filling of solid polygons.

Graphics contexts are central to drawing with Gdk, because nearly all Gdk drawing functions and many Pango functions take a Gdk::GC object as an argument. So although Cairo has largely superceded many Gdk drawing functions, you're still likely to run into Gdk::GC objects quite often, so it's important to know what they are and how they're used.