function hidePresentation(){
	var p_overlay = document.getElementById('presentation_overlay');
	var p_div = document.getElementById('presentation_div');
	var p_frame = document.getElementById('presentation_frame');
	p_frame.src= '';
	
	p_div.style.display = 'none';
	p_overlay.style.display = 'none';
}

function showPresentation(width,height,src){
	var p_overlay = document.getElementById('presentation_overlay');
	var p_div = document.getElementById('presentation_div');
	var p_frame = document.getElementById('presentation_frame');
	
	scroll(0,0);
	
	p_div.style.display = '';
	p_overlay.style.display = '';
	
	p_div.style.width = width+'px';
	p_div.style.height = height+'px';
	p_div.style.marginLeft = (width / 2 * -1 ) + 'px';
	p_div.style.marginTop = (height / 2 * -1 ) + 'px';
	
	p_frame.src= src;
}