canvas字体，图片模糊

我们在用canvas画图的时候，经常会发现canvas写的文字或线条会特别模糊，那么我们可以根据屏幕retina对canvas图片进行放大

var c=document.getElementById("myCanvas");
var ctx=c.getContext("2d");

var w = c.width
var h = c.height

var devicePixelRatio = window.devicePixelRatio || 1
var backingStoreRatio = ctx.webkitBackingStorePixelRatio || 1
var ratio = devicePixelRatio / backingStoreRatio
c.width = w * ratio;
c.height = h * ratio;
//按屏幕比率进行放大
ctx.scale(ratio,ratio);