Cairo-ritmodellen

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.)

Följande exempel visar hur du konfigurerar en Cairo-kontext med förgrundsfärgen röd och bredden 2. Alla ritfunktioner som använder denna kontext kommer använda dessa inställningar.

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.

Det finns ett antal grafiktillståndsvariabler som kan ställas in för en Cairo-kontext. De vanligaste kontextattributen är färg (med set_source_rgb() eller set_source_rgba() för genomskinliga färger), linjebredd (med set_line_width()), linjestreckningsmönster (med set_dash()), stil på linjeände (med set_line_cap()), linjesammankopplingsstil (med set_line_join()) och typsnittsstilar (med set_font_size(), set_font_face() med flera). Det finns även många andra inställningar, såsom transformationsmatriser, fyllnadsregler, huruvida kantutjämning ska utföras med mera. För vidare information kan du se API-dokumentationen för 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();
}

Ritfunktionen som du ställer in med ett anrop till set_draw_func() anropas med en Cairo-kontext som du ska använda för att rita i Gtk::DrawingArea-komponenten. Det är inte nödvändigt att spara och återställa denna Cairo-kontext i ritfunktionen.