canvas 显示模糊问题

//=== canvas 模糊问题 start
     var ratio = getPixelRatio(ct);
     if(ratio == 1) { // PC端 PixelRatio
  ratio = 2;
     }
     cs.style.width = cs.width + 'px';
     cs.style.height = cs.height + 'px';
     cs.width = cs.width * ratio;
     cs.height = cs.height * ratio;
     ct.scale(ratio, ratio);
     //=== canvas 模糊问题 end

var getPixelRatio = function (context) {
     var backingStore = context.backingStorePixelRatio ||
  context.webkitBackingStorePixelRatio ||
  context.mozBackingStorePixelRatio ||
  context.msBackingStorePixelRatio ||
  context.oBackingStorePixelRatio ||
  context.backingStorePixelRatio || 1;
     return (window.devicePixelRatio || 1) / backingStore;
 };