Re: How to maintain a reference to Cairo::Context or Cairo::Surface?

["Milosz Derezynski" <internalerror gmail com>]

2008/12/4 BC Zhu <zhubicen gmail com>

Cairo::RefPtr<Cairo::Context> cr = window->create_cairo_context();//
 The above sentence indicate me that cr is an smart pointer , so i can keep it as a class memeber or pass it as an argument to anthoer function, if this i can used it anywhere, and doesn't need to call " get_window()->create_cairo_context()" again and again. May be " get_window()->create_cairo_context()" is a cheap action, but according to the rule "DONOT REPEAT YOURSELF", i think its better to creat it one time, and use it any where.

Hey BC,

Generally this rule is right, but I do think that there are implications with widget window double buffering.

From the GTK+ FAQ at library.gnome.org (http://library.gnome.org/devel/gtk/stable/gtk-question-index.html):

"If you create a cairo context outside the expose handler, it is backed by the GDK window itself, not the double-buffering pixmap."

Now this isn't an exact response to what you are asking, but what this implies to me is that the specific drawable you create the cairo context for
in the expose handler is not guaranteed to be the same thing that you need to draw in the next call of expose, and so forth.

Also, this example for Cairo in Mono (yes, it is Mono, but the relevant point is not about Mono, but the way Cairo/Gtk+ interaction works)
clearly states that reusing the same Cairo Context (in the example it's of class type Cairo.Graphics) over multiple calls of expose will not work:
"Don't try to keep Graphics across multiple expose events, this will not work due to double-buffering."


M.
 

2008/12/3 Milosz Derezynski <internalerror gmail com>

It does not seem neccessary to create 2 separate Cairo Contexts:

class MyArea : public Gtk::DrawingArea
{
public:
    virtual ~MyArea();
protected:
//Override default signal handler:
    virtual bool on_expose_event(

GdkEventExpose* event);

};
MyArea::MyArea()
{
}
MyArea::~MyArea()
{
}
bool MyArea::on_expose_event(GdkEventExpose* event)
{
// This is where we draw on the window

    Cairo::RefPtr<Cairo::Context> cr = window->create_cairo_context();

    cr->set_line_width(10.0);
    cr->move_to(0, 0);
    cr->line_to(100, 100);
    cr->stroke();   
    return true;
}

This will work equally well. Is there any specific reason why you want to keep the surface member in the class and use it the way you wrote it in the preceding mail?

M.

2008/12/3 BC Zhu <zhubicen gmail com>

Now  i am using Gtkmm. I want to maintain a reference to Cairo::Context or Cairo::Surface, so i can draw something on it anywhere.
I am trying to do this , but failed.
With following code , i am trying to keep a reference to Cairo:::Surface，but when on_expose_event was called secondly , it will fail.
And i have tried to maintain a reference to Cairo::Context too, but failed either.
Could some body give me an advice?
Thanks in advance!


class MyArea : public Gtk::DrawingArea
{
public:
    virtual ~MyArea();
protected:
//Override default signal handler:
    virtual bool on_expose_event(GdkEventExpose* event);
private:
    Cairo::RefPtr<Cairo::Surface> surf_;
};
MyArea::MyArea()
{
}
MyArea::~MyArea()
{
}
bool MyArea::on_expose_event(GdkEventExpose* event)
{
// This is where we draw on the window
    if(!surf_){
        Glib::RefPtr<Gdk::Window> window = get_window();
        Cairo::RefPtr<Cairo::Context> cr = window->create_cairo_context();
        surf_ = cr->get_target();
    }
    Cairo::RefPtr<Cairo::Context> cr = Cairo::Context::create(surf_);
    cr->set_line_width(10.0);
    cr->move_to(0, 0);
    cr->line_to(100, 100);
    cr->stroke();   
    return true;
}


--
Best Regards
Bicen.Zhu

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--
Best Regards
Bicen.Zhu

-- 
Please note that according to the German law on data retention,
information on every electronic information exchange with me is
retained for a period of six months.
[Bitte beachten Sie, dass dem Gesetz zur Vorratsdatenspeicherung zufolge
jeder elektronische Kontakt mit mir sechs Monate lang gespeichert wird.]