function createBlankPng(w, h) {
	var canvas = document.createElement('canvas');
	// http://code.google.com/p/explorercanvas/wiki/Instructions#Dynamically_created_elements
	if (!canvas.getContext) G_vmlCanvasManager.initElement(canvas);
	var ctx = canvas.getContext('2d');
	canvas.width = w;
	canvas.height = h;
	ctx.fillStyle = 'rgba(' + [0,0,0,0].join() + ')';
	ctx.fillRect(0,0,w,h);
	// 'data:image/png;base64,'.length => 22
	return canvas.toDataURL('image/png','');
}