Chapter 18. The DrawingArea Widget

Table of Contents

The DrawingArea widget is a blank window that gives you the freedom to create any graphic you desire. Along with that freedom comes the responsibility to draw on the widget. When a widget is first shown, or when it is covered and then uncovered again it needs to redraw itself. Most widgets have code to do this, but the DrawingArea does not, allowing you to write your own draw function to determine how the contents of the widget will be drawn. This is done by setting a draw function with a call to the set_draw_func() member function.

GTK uses the Cairo drawing API. With gtkmm, you may use the cairomm C++ API for cairo.

You can draw very sophisticated shapes using Cairo, but the methods to do so are quite basic. Cairo provides methods for drawing straight lines, curved lines, and arcs (including circles). These basic shapes can be combined to create more complex shapes and paths which can be filled with solid colors, gradients, patterns, and other things. In addition, Cairo can perform complex transformations, do compositing of images, and render antialiased text.

[Note] Cairo and Pango

Although Cairo can render text, it's not meant to be a replacement for Pango. Pango is a better choice if you need to perform more advanced text rendering such as wrapping or ellipsizing text. Drawing text with Cairo should only be done if the text is part of a graphic.

In this section of the tutorial, we'll cover the basic Cairo drawing model, describe each of the basic drawing elements in some detail (with examples), and then present a simple application that uses Cairo to draw a custom clock widget.

The Cairo Drawing Model

The basic concept of drawing in Cairo involves defining 'invisible' paths and then stroking or filling them to make them visible.

To do any drawing in gtkmm with Cairo, you must first get a Cairo::Context object. This class holds all of the graphics state parameters that describe how drawing is to be done. This includes information such as line width, color, the surface to draw to, and many other things. This allows the actual drawing functions to take fewer arguments to simplify the interface. Usually, you use the Cairo::Context that you get as input data to the draw function that you set with the call to set_draw_func(). It's also possible to create a Cairo::Context by calling the Gdk::Surface::create_cairo_context() and Gdk::CairoContext::cairo_create() functions. Since Cairo contexts are reference-counted objects, cairo_create() returns a Cairo::RefPtr<Cairo::Context> object. (Note the difference between Gdk::CairoContext and Cairo::Context.)

The following example shows how to set up a Cairo context with a foreground color of red and a width of 2. Any drawing functions that use this context will use these settings.

Gtk::DrawingArea myArea;
auto gdkCairoContext = myArea.get_surface()->create_cairo_context();
auto myContext = gdkCairoContext->cairo_create();
myContext->set_source_rgb(1.0, 0.0, 0.0);

Each Cairo::Context is associated with a particular Gdk::Surface, so the first line of the above example creates a Gtk::DrawingArea widget and the next two lines use its associated Gdk::Surface to create a Cairo::Context object. The final two lines change the graphics state of the context.

There are a number of graphics state variables that can be set for a Cairo context. The most common context attributes are color (using set_source_rgb() or set_source_rgba() for translucent colors), line width (using set_line_width()), line dash pattern (using set_dash()), line cap style (using set_line_cap()), and line join style (using set_line_join()), and font styles (using set_font_size(), set_font_face() and others). There are many other settings as well, such as transformation matrices, fill rules, whether to perform antialiasing, and others. For further information, see the cairomm API documentation.

The current state of a Cairo::Context can be saved to an internal stack of saved states and later be restored to the state it was in when you saved it. To do this, use the save() method and the restore() method. This can be useful if you need to temporarily change the line width and color (or any other graphics setting) in order to draw something and then return to the previous settings. In this situation, you could call Cairo::Context::save(), change the graphics settings, draw the lines, and then call Cairo::Context::restore() to restore the original graphics state. Multiple calls to save() and restore() can be nested; each call to restore() restores the state from the matching paired save().

[Tip] Tip

It is good practice to put all modifications to the graphics state between save()/restore() function calls. For example, if you have a function that takes a Cairo::Context reference as an argument, you might implement it as follows:

void doSomething(const Cairo::RefPtr<Cairo::Context>& context, int x)
    // change graphics state
    // perform drawing operations

The draw function that you set with a call to set_draw_func() is called with a Cairo context that you shall use for drawing in the Gtk::DrawingArea widget. It is not necessary to save and restore this Cairo context in the draw function.