Re: Using existing cairo context for GtkDrawingArea

[Ivan Baldo <ibaldo adinet com uy>]

El 09/12/06 08:18, asubedi escribió:
> Hi,
>
> Thanks for the reply.
>
> On 12/8/06, Ivan Baldo <ibaldo adinet com uy> wrote:
> >     Hello.
> >     You can't AFAIK.
> >     GTK is double buffered, for every expose event, it draws to a
> > different place. When you draw on the expose event, you are not drawing
> > into the visible area of the screen, when the expose event is finished,
> > GTK switches the drawing you just did offscreen to appear on the screen
> > instead of the old one.
> >     So you must create a new Cairo context for every expose event, but
> > don't worry, this is highly optimized and no time is wasted on the new
> > context for every expose event.
> >     If you are having performance problems, you may try to use a cache
> > for some parts of your drawing, you can use the
> > cairo_surface_create_similar function for that, you can create a Cairo
> > context for that surface and draw things there, then you can use that
> > surface on the Cairo context of the expose event. You can cache mostly
> > static things and the more dynamic ones you can draw in the expose 
> > event.
> >     HTH, goodbye.
>
> This seems reasonable, but my problem is different. I do not want the
> user of my widget to put his/her drawing commands in the expose_event
> handler.  The member functions of the widget will be things like
> draw_line(x1, y1, x2, y2), draw_point(x, y), etc., which will be
> converted to corresponding cairo calls. So I would like to create a
> cairo context, draw on it using the above functions. And in the expose
> event I would like to draw on the DrawingArea using the previously
> created cairo context--which seems impossible. But is there a way to
> copy a cairo context to the one created from DrawingArea?
   In the first expose event of your widget you can create a cairo 
surface with cairo_surface_create_similar() and then a cairo context for 
it, save pointers for both the surface and the cairo context. Then, when 
the user calls your functions you draw in that saved cairo context and 
generate a new expose event; in the expose event handler you just draw 
the saved surface and thats it.
   Thats one way to accomplish what you say, another way is to manually 
save all the operations that the user has requested for your widget in 
some sort of array of commands and parameters and in every expose event 
just draw that with the Cairo calls.
   Another way is to manually generate an svg file with the functions 
that the user calls for the widget and in the expose event render it 
with librsvg.
   HTH, goodbye.

P.s.: please, answer to the mailing list so other people can help too, 
maybe they have better ideas, etc. and also they can answer sooner 
sometimes.


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