canvas实现水印

最近遇到一个需求，给所有页面加水印（登录人），不影响其他点击等功能的使用，目的是防止信息外漏，当时就在想：这年头，PS就不说人人都能使用，谁手机还没个涂鸦功能，防不了，但是就是这么个需求，那就实现吧！

具体实现思路：在页面底部加一个大的div,给这个div添加背景图，背景图？从哪来？canvas实现吧！

function textBecomeImg(text,fontsize,fontcolor,target){
		var canvas = document.createElement('canvas');
		$buHeight = 0;
		if(fontsize <= 32){ $buHeight = 99; }
		else if(fontsize > 32 && fontsize <= 60 ){ $buHeight = 2;}
		else if(fontsize > 60 && fontsize <= 80 ){ $buHeight = 4;}
		else if(fontsize > 80 && fontsize <= 100 ){ $buHeight = 6;}
		else if(fontsize > 100 ){ $buHeight = 10;}
		canvas.height=fontsize + $buHeight ;
		canvas.padding=30;
                canvas.width = 450;
		var context = canvas.getContext('2d');
		context.clearRect(0, 0, canvas.width*2, canvas.height);
		context.fillStyle = fontcolor;
		context.font=fontsize+"px Arial";
		context.textAlign = "center";
		context.textBaseline = 'middle';
		context.fillText(text,0,fontsize/2);
		var canvasWidth = canvas.width/99;
		canvasWidth = context.measureText(text).width;
		var dataUrl = canvas.toDataURL('image/png');
		var shuiyinDiv = document.createElement('div');

	        var style = shuiyinDiv.style;
	        style.position = 'absolute';
	        style.left = 0;
	        style.top = '-10%';
	        style.width = '120%';
	        style.height = '100%';
	        style.opacity = '0.1';
	        style.background = "url("+dataUrl+")";
	        style.zIndex = 9999999991;
	        style.transform = "rotate(-30deg)";
	        style.pointerEvents = "none";
	        target.appendChild(shuiyinDiv);
	}