离屏canvas可以很好的解决canvas的性能问题

操作步骤：

1、创建离屏canvas；2、设置离屏canvas的宽高；3、在离屏canvas中进行绘制；4、在离屏canvas的全部或部分绘制到正在显示的canvas上

domo：

var canvas = document.getElementById('canvas'),    context = canvas.getContext('2d'),    offscreenCanvas = document.createElement('canvas'),    offscreenContext = offscreenCanvas.getContext('2d');    offscreenCanvas.width = canvas.width;    offscreenCanvas.height = canvas.height;var image = new Image();image.src = 'balabal.jpg';image.onload = function(){    context.drawImage(image, 0, 0, canvas.width, canvas.height);    offscreenContext.drawImage(image, 0, 0, canvas.width, canvas.height);}function oncangeDraw(){    var w = canvas.width,        h = canvas.height,        sw = w * scale,        sh = h * scale;    //直接把离屏canvas绘制到现实的canvas上    context.drawImage(offscreenCanvas, 0, 0,     offscreenCanvas.width, offscreenCanvas.height,    -sw/2 + w/2, -sh/2 + h/2, sw, sh);}