Paint from a surface to other doesn't work?

Hi!
I'm absolutely getting crazy and I need someone that give me a clue on this:

In this section of code:

void studio::Widget_NavView::on_finish_render()

{

if(studio::App::navigator_uses_cairo)

{

if(!cairo_surface)

{

synfig::warning("dock_navigator: Bad cairo surface");

return;

}

 if(cairo_surface_status(cairo_surface))

return;

 Target_Cairo::gamma_filter(cairo_surface);


 int w = 0, h = 0;

int dw = cairo_image_surface_get_width(cairo_surface);

int dh = cairo_image_surface_get_height(cairo_surface);


 if(cairo_prev)

{

w = cairo_image_surface_get_width(cairo_prev);

h = cairo_image_surface_get_height(cairo_prev);

}


 if( w!=dw || h!=dh || !cairo_prev)

{

cairo_prev=cairo_image_surface_create(CAIRO_FORMAT_ARGB32, dw, dh);

}

synfig::info("cairo_surface is mapped %d",
cairo_surface_is_mapped(cairo_surface)); // is not mapped

static int pass2=0;

// Image written to file is correct

cairo_surface_write_to_png(cairo_surface, strprintf(
"dock_navigator_on_finish_render_cairo_surface-pass%d.png", pass2).c_str());


 cairo_t* cr=cairo_create(cairo_prev);

cairo_save(cr);

cairo_set_source_surface(cr, cairo_surface, 0, 0);

cairo_set_operator(cr, CAIRO_OPERATOR_SOURCE);

cairo_paint(cr);

cairo_restore(cr);

cairo_destroy(cr);


 cairo_surface_write_to_png(cairo_prev, strprintf(
"dock_navigator_on_finish_render_cairo_prev-pass%d.png", pass2).c_str());

pass2++;

}

}



the written to file images cairo_prev and cairo_surface doesn't coincide,
how's that possible???
In particular cairo_surface is correct (what I expect to be) and cairo_prev
is simply transparent.
If I can write a surface to png correctly, why I can't paint it to other
surface??
Thanks in advance!

-- 
Carlos
http://synfig.org

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