MegaPixImage插件代码（new MegaPixImage）

/**
            * Mega pixel image rendering library for iOS6 Safari
            *
            * Fixes iOS6 Safari's image file rendering issue for large size image (over mega-pixel),
            * which causes unexpected subsampling when drawing it in canvas.
            * By using this library, you can safely render the image with proper stretching.
            *
            * Copyright (c) 2012 Shinichi Tomita <shinichi.tomita@gmail.com>
            * Released under the MIT license
            */
            (function () {

            /**
            * Detect subsampling in loaded image.
            * In iOS, larger images than 2M pixels may be subsampled in rendering.
            */
            function detectSubsampling(img) {
                var iw = img.naturalWidth, ih = img.naturalHeight;
                if (iw * ih > 1024 * 1024) { // subsampling may happen over megapixel image
                    var canvas = document.createElement('canvas');
                    canvas.width = canvas.height = 1;
                    var ctx = canvas.getContext('2d');
                    ctx.drawImage(img, -iw + 1, 0);
                    // subsampled image becomes half smaller in rendering size.
                    // check alpha channel value to confirm image is covering edge pixel or not.
                    // if alpha value is 0 image is not covering, hence subsampled.
                    return ctx.getImageData(0, 0, 1, 1).data[3] === 0;
                } else {
                    return false;
                }
            }

            /**
            * Detecting vertical squash in loaded image.
            * Fixes a bug which squash image vertically while drawing into canvas for some images.
            */
            function detectVerticalSquash(img, iw, ih) {
                var canvas = document.createElement('canvas');
                canvas.width = 1;
                canvas.height = ih;
                var ctx = canvas.getContext('2d');
                ctx.drawImage(img, 0, 0);
                var data = ctx.getImageData(0, 0, 1, ih).data;
                // search image edge pixel position in case it is squashed vertically.
                var sy = 0;
                var ey = ih;
                var py = ih;
                while (py > sy) {
                    var alpha = data[(py - 1) * 4 + 3];
                    if (alpha === 0) {
                        ey = py;
                    } else {
                        sy = py;
                    }
                    py = (ey + sy) >> 1;
                }
                var ratio = (py / ih);
                return (ratio === 0) ? 1 : ratio;
            }

            /**
            * Rendering image element (with resizing) and get its data URL
            */
            function renderImageToDataURL(img, options, doSquash) {
                var canvas = document.createElement('canvas');
                renderImageToCanvas(img, canvas, options, doSquash);
                return canvas.toDataURL("image/jpeg", options.quality || 0.8);
            }

            /**
            * Rendering image element (with resizing) into the canvas element
            */
            function renderImageToCanvas(img, canvas, options, doSquash) {
                var iw = img.naturalWidth, ih = img.naturalHeight;
                var width = options.width, height = options.height;
                var ctx = canvas.getContext('2d');
                ctx.save();
                transformCoordinate(canvas, ctx, width, height, options.orientation);
                var subsampled = detectSubsampling(img);
                if (subsampled) {
                    iw /= 2;
                    ih /= 2;
                }
                var d = 1024; // size of tiling canvas
                var tmpCanvas = document.createElement('canvas');
                tmpCanvas.width = tmpCanvas.height = d;
                var tmpCtx = tmpCanvas.getContext('2d');
                var vertSquashRatio = doSquash ? detectVerticalSquash(img, iw, ih) : 1;
                var dw = Math.ceil(d * width / iw);
                var dh = Math.ceil(d * height / ih / vertSquashRatio);
                var sy = 0;
                var dy = 0;
                while (sy < ih) {
                    var sx = 0;
                    var dx = 0;
                    while (sx < iw) {
                        tmpCtx.clearRect(0, 0, d, d);
                        tmpCtx.drawImage(img, -sx, -sy);
                        ctx.drawImage(tmpCanvas, 0, 0, d, d, dx, dy, dw, dh);
                        sx += d;
                        dx += dw;
                    }
                    sy += d;
                    dy += dh;
                }
                ctx.restore();
                tmpCanvas = tmpCtx = null;
            }

            /**
            * Transform canvas coordination according to specified frame size and orientation
            * Orientation value is from EXIF tag
            */
            function transformCoordinate(canvas, ctx, width, height, orientation) {
                switch (orientation) {
                    case 5:
                    case 6:
                    case 7:
                    case 8:
                        canvas.width = height;
                        canvas.height = width;
                        break;
                    default:
                        canvas.width = width;
                        canvas.height = height;
                }
                switch (orientation) {
                    case 2:
                        // horizontal flip
                        ctx.translate(width, 0);
                        ctx.scale(-1, 1);
                        break;
                    case 3:
                        // 180 rotate left
                        ctx.translate(width, height);
                        ctx.rotate(Math.PI);
                        break;
                    case 4:
                        // vertical flip
                        ctx.translate(0, height);
                        ctx.scale(1, -1);
                        break;
                    case 5:
                        // vertical flip + 90 rotate right
                        ctx.rotate(0.5 * Math.PI);
                        ctx.scale(1, -1);
                        break;
                    case 6:
                        // 90 rotate right
                        ctx.rotate(0.5 * Math.PI);
                        ctx.translate(0, -height);
                        break;
                    case 7:
                        // horizontal flip + 90 rotate right
                        ctx.rotate(0.5 * Math.PI);
                        ctx.translate(width, -height);
                        ctx.scale(-1, 1);
                        break;
                    case 8:
                        // 90 rotate left
                        ctx.rotate(-0.5 * Math.PI);
                        ctx.translate(-width, 0);
                        break;
                    default:
                        break;
                }
            }

            /**
            * MegaPixImage class
            */
            function MegaPixImage(srcImage) {
                if (window.Blob && srcImage instanceof Blob) {
                    var img = new Image();
                    var URL = window.URL && window.URL.createObjectURL ? window.URL :
                        window.webkitURL && window.webkitURL.createObjectURL ? window.webkitURL :
                        null;
                    if (!URL) { throw Error("No createObjectURL function found to create blob url"); }
                    img.src = URL.createObjectURL(srcImage);
                    this.blob = srcImage;
                    srcImage = img;
                }
                if (!srcImage.naturalWidth && !srcImage.naturalHeight) {
                    var _this = this;
                    srcImage.onload = function () {
                        var listeners = _this.imageLoadListeners;
                        if (listeners) {
                            _this.imageLoadListeners = null;
                            for (var i = 0, len = listeners.length; i < len; i++) {
                                listeners[i]();
                            }
                        }
                    };
                    this.imageLoadListeners = [];
                }
                this.srcImage = srcImage;
            }

            /**
            * Rendering megapix image into specified target element
            */
            MegaPixImage.prototype.render = function (target, options) {
                if (this.imageLoadListeners) {
                    var _this = this;
                    this.imageLoadListeners.push(function () { _this.render(target, options) });
                    return;
                }
                options = options || {};
                var imgWidth = this.srcImage.naturalWidth, imgHeight = this.srcImage.naturalHeight,
                width = options.width, height = options.height,
                maxWidth = options.maxWidth, maxHeight = options.maxHeight,
                doSquash = !this.blob || this.blob.type === 'image/jpeg';
                if (width && !height) {
                    height = (imgHeight * width / imgWidth) << 0;
                } else if (height && !width) {
                    width = (imgWidth * height / imgHeight) << 0;
                } else {
                    width = imgWidth;
                    height = imgHeight;
                }
                if (maxWidth && width > maxWidth) {
                    width = maxWidth;
                    height = (imgHeight * width / imgWidth) << 0;
                }
                if (maxHeight && height > maxHeight) {
                    height = maxHeight;
                    width = (imgWidth * height / imgHeight) << 0;
                }
                var opt = { width: width, height: height };
                for (var k in options) opt[k] = options[k];

                var tagName = target.tagName.toLowerCase();
                if (tagName === 'img') {
                    target.src = renderImageToDataURL(this.srcImage, opt, doSquash);
                } else if (tagName === 'canvas') {
                    renderImageToCanvas(this.srcImage, target, opt, doSquash);
                }
                if (typeof this.onrender === 'function') {
                    this.onrender(target);
                }
            };

            /**
            * Export class to global
            */
            if (typeof define === 'function' && define.amd) {
                define([], function () { return MegaPixImage; }); // for AMD loader
            } else {
                this.MegaPixImage = MegaPixImage;
            }

            })();

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