canvas解决画图模糊

/**
 * HiDPI Canvas Polyfill (1.0.9)
 *
 * Author: Jonathan D. Johnson (http://jondavidjohn.com)
 * Homepage: https://github.com/jondavidjohn/hidpi-canvas-polyfill
 * Issue Tracker: https://github.com/jondavidjohn/hidpi-canvas-polyfill/issues
 * License: Apache 2.0
 */ (function(prototype) {

    var pixelRatio = (function(context) {
        var backingStore = context.backingStorePixelRatio || context.webkitBackingStorePixelRatio || context.mozBackingStorePixelRatio || context.msBackingStorePixelRatio || context.oBackingStorePixelRatio || context.backingStorePixelRatio || 1;

        return (window.devicePixelRatio || 1) / backingStore;
    })(prototype),

        forEach = function(obj, func) {
            for (var p in obj) {
                if (obj.hasOwnProperty(p)) {
                    func(obj[p], p);
                }
            }
        },

        ratioArgs = {
            ‘fillRect‘: ‘all‘,
            ‘clearRect‘: ‘all‘,
            ‘strokeRect‘: ‘all‘,
            ‘moveTo‘: ‘all‘,
            ‘lineTo‘: ‘all‘,
            ‘arc‘: [0, 1, 2],
            ‘arcTo‘: ‘all‘,
            ‘bezierCurveTo‘: ‘all‘,
            ‘isPointinPath‘: ‘all‘,
            ‘isPointinStroke‘: ‘all‘,
            ‘quadraticCurveTo‘: ‘all‘,
            ‘rect‘: ‘all‘,
            ‘translate‘: ‘all‘,
            ‘createRadialGradient‘: ‘all‘,
            ‘createLinearGradient‘: ‘all‘
        };

    if (pixelRatio === 1) return;

    forEach(ratioArgs, function(value, key) {
        prototype[key] = (function(_super) {
            return function() {
                var i, len,
                args = Array.prototype.slice.call(arguments);

                if (value === ‘all‘) {
                    args = args.map(function(a) {
                        return a * pixelRatio;
                    });
                } else if (Array.isArray(value)) {
                    for (i = 0, len = value.length; i < len; i++) {
                        args[value[i]] *= pixelRatio;
                    }
                }

                return _super.apply(this, args);
            };
        })(prototype[key]);
    });

    // Stroke lineWidth adjustment
    prototype.stroke = (function(_super) {
        return function() {
            this.lineWidth *= pixelRatio;
            _super.apply(this, arguments);
            this.lineWidth /= pixelRatio;
        };
    })(prototype.stroke);

    // Text
    //
    prototype.fillText = (function(_super) {
        return function() {
            var args = Array.prototype.slice.call(arguments);

            args[1] *= pixelRatio; // x
            args[2] *= pixelRatio; // y

            this.font = this.font.replace(/(\d+)(px|em|rem|pt)/g,

            function(w, m, u) {
                return (m * pixelRatio) + u;
            });

            _super.apply(this, args);

            this.font = this.font.replace(/(\d+)(px|em|rem|pt)/g,

            function(w, m, u) {
                return (m / pixelRatio) + u;
            });
        };
    })(prototype.fillText);

    prototype.strokeText = (function(_super) {
        return function() {
            var args = Array.prototype.slice.call(arguments);

            args[1] *= pixelRatio; // x
            args[2] *= pixelRatio; // y

            this.font = this.font.replace(/(\d+)(px|em|rem|pt)/g,

            function(w, m, u) {
                return (m * pixelRatio) + u;
            });

            _super.apply(this, args);

            this.font = this.font.replace(/(\d+)(px|em|rem|pt)/g,

            function(w, m, u) {
                return (m / pixelRatio) + u;
            });
        };
    })(prototype.strokeText);
})(CanvasRenderingContext2D.prototype);;
(function(prototype) {
    prototype.getContext = (function(_super) {
        return function(type) {
            var backingStore, ratio,
            context = _super.call(this, type);

            if (type === ‘2d‘) {

                backingStore = context.backingStorePixelRatio || context.webkitBackingStorePixelRatio || context.mozBackingStorePixelRatio || context.msBackingStorePixelRatio || context.oBackingStorePixelRatio || context.backingStorePixelRatio || 1;

                ratio = (window.devicePixelRatio || 1) / backingStore;

                if (ratio > 1) {
                    this.style.height = this.height + ‘px‘;
                    this.style.width = this.width + ‘px‘;
                    this.width *= ratio;
                    this.height *= ratio;
                }
            }

            return context;
        };
    })(prototype.getContext);
})(HTMLCanvasElement.prototype);

      var canvas = document.querySelector(‘canvas‘);
      var ctx = canvas.getContext(‘2d‘);

      var getPixelRatio = function(context) {
        var backingStore = context.backingStorePixelRatio ||
          context.webkitBackingStorePixelRatio ||
          context.mozBackingStorePixelRatio ||
          context.msBackingStorePixelRatio ||
          context.oBackingStorePixelRatio ||
          context.backingStorePixelRatio || 1;

        return (window.devicePixelRatio || 1) / backingStore;
      };

      var ratio = getPixelRatio(ctx);  
　　　ctx.drawImage(document.querySelector(‘img‘), 0, 0, 300 * ratio, 90 * ratio);