[cairo] clear operator

[Andrea Canciani]

On Tue, May 31, 2011 at 2:20 PM, control H <control.h at gmail.com> wrote:
> Hi,
>
> I'm trying to use the CLEAR operator, but it doesn't give the expected
> results. I'm using gtk, the configure event I made looks like this:
>
> static cairo_surface_t *surface = NULL;
>
> static gboolean
> configure_event (GtkWidget         *widget,
>                  GdkEventConfigure *event,
>                  gpointer           data)
> {
>  GtkAllocation allocation;
>  cairo_t *cr,*cr1,*cr2;
>  cairo_surface_type_t st;
>  cairo_surface_t *surf1,*surf2;
>
>  int w,h,r;
>
>  if (surface)
>    cairo_surface_destroy (surface);
>
>  gtk_widget_get_allocation (widget, &allocation);
>  w=allocation.width;
>  h=allocation.height;
>  r=h/2;
>
>  surface = gdk_window_create_similar_surface (gtk_widget_get_window (widget),
>                                               CAIRO_CONTENT_COLOR,
>                                               allocation.width,
>                                               allocation.height);
>  /* Initialize the surface to white */
>  cr=cairo_create (surface);
>  cairo_set_source_rgb (cr, 1, 1, 1);
>  cairo_paint (cr);

surface is now completely white

>
>  surf1=cairo_surface_create_similar(cairo_get_target(cr),
>                                     CAIRO_CONTENT_COLOR_ALPHA,
>                                     w,h);
>  cr1=cairo_create(surf1);
>  cairo_set_source_rgba(cr1,0.,1.,0.,.8);
>  cairo_move_to(cr1,w/2,h/2);
>  cairo_arc(cr1,w/2,h/2,MIN(w,h)/3,0,2*G_PI);
>  cairo_fill(cr1);
>
>  surf2=cairo_surface_create_similar(cairo_get_target(cr),
>                                     CAIRO_CONTENT_COLOR_ALPHA,
>                                     w,h);
>  cr2=cairo_create(surf2);
>  cairo_set_source_rgba(cr2,1.,1.,0.,.7);
>  cairo_arc(cr2,w/2.5,h/2,MIN(w,h)/4,0,2*G_PI);
>  cairo_fill(cr2);
>
>  cairo_set_operator(cr1,CAIRO_OPERATOR_CLEAR);
>  //cairo_set_operator(cr1,CAIRO_OPERATOR_DEST_OUT);
>  cairo_set_source_surface(cr1,surf2,0,0);
>  cairo_paint(cr1);

surf1 is now completely transparent

>
>  cairo_set_source_surface(cr,surf1,0,0);
>  cairo_set_operator(cr,CAIRO_OPERATOR_OVER);
>  cairo_paint(cr);

This is actually a no-op.

>
>  cairo_surface_destroy(surf1);
>  cairo_surface_destroy(surf2);
>  cairo_destroy(cr1);
>  cairo_destroy(cr2);
>
>  cairo_destroy (cr);
>
>  return TRUE;
> }
>
> The result is just a white screen. However, if I change

This is expected. When you paint() cr1 with a CLEAR
operator, you make it completely transparent.
Painting surf1 OVER cr has thus no effect at all
and correctly leaves surface white.

Andrea

> cairo_set_operator(cr1,CAIRO_OPERATOR_CLEAR);
> into
> cairo_set_operator(cr1,CAIRO_OPERATOR_DEST_OUT);
> or any other operator, it gives the expected result. What am I missing here?
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