Re: Re: Re: Re: Re: Getting a Cairo context

[Emmanuele Bassi <ebassi gmail com>]

Hi;

On 26 March 2015 at 19:54, Lucas Levrel <llevrel yahoo fr> wrote:
> Le 26 mars 2015, Sergei Naumov a écrit :
>
> > Hmm. This is something I did not know I could do. But in this case I would
> > have to redraw everything in the widget's drawing area which is my axes,
> > tickmarks, labeles, etc... All right, I will try it this way because it is
> > much simpler.
>
>
> What I do in a similar situation is keeping a kind of carbon copy of the
> bare drawing:
>
> - get two pixmaps from the drawing area :
>   bare_pixmap=gdk_pixmap_new(d_area->window,width,height,-1);
>   full_pixmap=gdk_pixmap_new(d_area->window,width,height,-1);
>
> - draw your "background" on bare_pixmap
>
> - when you have to redraw, just copy bare_pixmap to full_pixmap, add stuff
>   to full_pixmap, then display it:
>
>   gdk_draw_drawable(full_pixmap,copy_gc,bare_pixmap,0,0,0,0,-1,-1);
>   (add stuff to full_pixmap)
>   gdk_draw_drawable(d_area->window, copy_gc, full_pixmap, x, y, x, y, width,
> height);
>
> Where copy_gc has been defined this way:
>   GdkGC *copy_gc=d_area->style->fg_gc[GTK_WIDGET_STATE (d_area)];

This is really GTK+ 2.x only, and even then it's pretty much X11-only.

Pixmaps are really Cairo surfaces, these days, and GCs are Cairo
contexts. The widget state is part of the GtkStyleContext, and the
gtk_render_* API substitute the background rendering. Even for GTK+
2.x API you should not be using gdk_draw_* and GdkGC API at all,
unless your code is supposed to work with GTK+ 2.6.

Again: I cannot recommend porting away from that API (even in GTK 2.x
code) strongly enough.

Ciao,
 Emmanuele.

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