El modelo de dibujo de Cairo

El concepto básico de dibujar con Cairo implica definir caminos «invisibles» y luego tacharlos o rellenarlos para hacerlos visibles.

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.)

El siguiente ejemplo muestra cómo crear un contexto Cairo con un color de frente rojo y un ancho de 2. Cualquier función de dibujo que use este contexto usará esta configuración

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);
myContext->set_line_width(2.0);

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.

Hay varias variables del estado de gráficos que pueden establecerse para un contexto Cairo. Los atributos de contexto más comunes son color (usando set_source_rgb() o set_source_rgba() para colores traslúcidos), anchura de línea (usando set_line_width()), patrón de línea de puntos (usando set_dash), estilo de terminación de línea (usando set_line_cap()), estilo de unión de línea (usando set_line_join()), y estilos de fuente (usando set_font_size(), set_font_face() y otros). Hay muchas otras opciones también, como matrices de transformación, reglas de rellenado, realización de «antialiasing», y otros. Para obtener más información, consulte la documentación de la API de cairomm.

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().

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)
{
    context->save();
    // change graphics state
    // perform drawing operations
    context->restore();
}

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.

El modelo de dibujo de Cairo

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