The purpose of my code is just showing the error. I was wonering about the
reason why its failed. Is the error related to the expose event?
And Is there any ways to create a context that i can use it many times?But
currently When i want to draw something every time , i shoud create it
again.
2008/12/3 Milosz Derezynski <[EMAIL PROTECTED]>
> Hey Diego,
>
> The thing with this is that I don't think it makes any sense.
>
> Just keeping an old Cairo::Context arouind is not really useful, Context
> creation is very cheap
> and I think there might be implications regarding Widget double-buffering
> for keeping a Context around,
> (but someone else would have to make a definitive statement about that, I'm
> not 100% sure right now).
>
> As for keeping another surface around: to show something on the Widget's
> window
> you need to draw on that, and not on another surface. If you do in fact
> draw on another surface then, yes, you will
> have to copy that over the actual Widget's surface every time in expose,
> which can be indeed useful if your
> drawings are very complex and expensive to redraw, but is generally not
> recommended if you can instead keep a state
> in your app so that in expose, the drawing can be redrawn based on the
> state.
>
> The initial code sent here by BC, again, doesn't make really any sense at
> all (no offense). It just added an extra step
> of getting the surface associated with the Widget's window, and then
> getting a context for that surface again, which is
> equivalent to just getting the context with window->create_cairo_context();
>
>
> M.
>
>
> On Wed, Dec 3, 2008 at 12:49 PM, Diego A. Fons <[EMAIL PROTECTED]>wrote:
>
>> Milosz Derezynski wrote:
>>
>>> 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 <[EMAIL PROTECTED] <mailto:[EMAIL PROTECTED]>>
>>>
>>>
>>> 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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>>> gtkmm-list@gnome.org <mailto:gtkmm-list@gnome.org>
>>> http://mail.gnome.org/mailman/listinfo/gtkmm-list
>>>
>>>
>>>
>>>
>>> --
>>> 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.]
>>> ------------------------------------------------------------------------
>>>
>>> _______________________________________________
>>> gtkmm-list mailing list
>>> gtkmm-list@gnome.org
>>> http://mail.gnome.org/mailman/listinfo/gtkmm-list
>>>
>>>
>> Hi,
>>
>> I also have this problem, my reason to keep the context (or surface) is
>> that i want to draw and i don't want to loose the previous drawings i made
>> in the context.
>>
>> The last code works fine but when you call
>> Cairo::RefPtr<Cairo::Context> cr = window->create_cairo_context();
>> in the on_expose_event(), you are creating a new context, an empty
>> context.
>>
>> My solution was drawing in a context i create and then copy this context
>> over the one i get from the component, the problem is that this method is
>> _very_ inefficient.
>>
>> Regards,
>> Diego A. Fons.
>>
>
>
>
> --
> 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.]
>
> _______________________________________________
> gtkmm-list mailing list
> gtkmm-list@gnome.org
> http://mail.gnome.org/mailman/listinfo/gtkmm-list
>
>
--
Best Regards
Bicen.Zhu
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