Setting alpha transparency for an image

["Germán Diago" <germandiago gmail com>]

I want to set the alpha transparency of an image with this code, but
it does nothing.
Could anyone tell me the correct way to do it?


bool on_expose_event(GdkEventExpose* ev)
	{
		img_->add_alpha(false, 0, 0, 0);
		img_->composite(img_, 0, 0, img_->get_width(), img_->get_height()
							, 0, 0, 1, 1, Gdk::INTERP_BILINEAR, 10);
		img_->render_to_drawable(get_window(), get_style()->get_black_gc(),
		0, 0, 100, 80, img_->get_width(), img_->get_height(), // draw the
whole image (from 0,0 to the full width,height) at 100,80 in the
window
		Gdk::RGB_DITHER_NONE, 0, 0);
		return true;
	}

 img_ is a Glib::RefPtr<Gdk::Pixbuf>.

Thanks in advance.