html2canvas.js网页截图功能

需求：将网页生成图片，用户自行长按图片进行保存图片，再分享朋友圈。其中，都可识别图中的二维码。（二维码过小会识别不出）

首先，先来科普一下微信网页识别二维码原理：截屏识别，当客户端发现用户在网页的img标签内进行长按操作时，会立刻截屏并且启动二维码识别算法。https://www.cnblogs.com/daipi...

发现官网中的html2canvas.js插件存在一些bug：

1.截图不全，不完整
    解决方案：
    //修改源码，添加自定设置高度宽度
    

var width = options.width != null ? options.width : node.ownerDocument.defaultView.innerWidth;

var height = options.height != null ? options.height : node.ownerDocument.defaultView.innerHeight;
return renderDocument(node.ownerDocument, options, width, height, index).then(function (canvas) {**

    if (typeof(options.onrendered) === "function") {
        log("options.onrendered is deprecated, html2canvas returns a Promise containing the canvas");
        options.onrendered(canvas);
    }
    return canvas;
});

 2.图片像素模糊：
 解决方案：添加dpi参数
 
 function CanvasRenderer(width, height) {
                Renderer.apply(this, arguments);
                this.canvas = this.options.canvas || this.document.createElement("canvas");
                if (!this.options.canvas) {
                    if (this.options.dpi) {
                        this.options.scale = this.options.dpi / 96;   // 1 CSS inch = 96px.
                    }
                    if (this.options.scale) {
                        this.canvas.style.width = width + 'px';
                        this.canvas.style.height = height + 'px';
                        this.canvas.width = Math.floor(width * this.options.scale);
                        this.canvas.height = Math.floor(height * this.options.scale);
                        this.ctx.scale(this.options.scale, this.options.scale);
                    } else {
                        this.canvas.width = width;
                        this.canvas.height = height;
                    }
                }
                this.ctx = this.canvas.getContext("2d");
                this.taintCtx = this.document.createElement("canvas").getContext("2d");
                this.ctx.textBaseline = "bottom";
                this.variables = {};
                log("Initialized CanvasRenderer with size", width, "x", height)
  }
  
  
   return parser.ready.then(function() {
        log("Finished rendering");
        var canvas;

        if (options.type === "view") {
            canvas = crop(renderer.canvas, {width: renderer.canvas.width, height: renderer.canvas.height, top: 0, left: 0, x: 0, y: 0});
        } else if (node === clonedWindow.document.body || node === clonedWindow.document.documentElement || options.canvas != null) {
            canvas = renderer.canvas;
        } else if (options.scale) {
            var origBounds = {width: options.width != null ? options.width : bounds.width, height: options.height != null ? options.height : bounds.height, top: bounds.top, left: bounds.left, x: 0, y: 0};
            var cropBounds = {};
            for (var key in origBounds) {
                if (origBounds.hasOwnProperty(key)) { cropBounds[key] = origBounds[key] * options.scale; }
            }
            canvas = crop(renderer.canvas, cropBounds);
            canvas.style.width = origBounds.width + 'px';
            canvas.style.height = origBounds.height + 'px';
        } else {
            canvas = crop(renderer.canvas, {width:  options.width != null ? options.width : bounds.width, height: options.height != null ? options.height : bounds.height, top: bounds.top, left: bounds.left, x: 0, y: 0});
        }

        cleanupContainer(container, options);
        return canvas;
    });

参考链接：https://www.jianshu.com/p/f40...
最后写写html2canvas的使用方法：

 var height = $('.teacher_page').scrollTop() + $('.teacher_page').outerHeight(true);
     html2canvas($(".teacher_page"),{ 
         height:height,
        // window.devicePixelRatio是设备像素比
         dpi: window.devicePixelRatio,
     }).then(function(canvas) {
         var imgUri = canvas.toDataURL("image/png").replace("image/png", "image/octet-stream"); 
         $('.popBody').html('<img alt="二维码" src="'+imgUri+'" id="canvas"/>');
    });

官网：http://html2canvas.hertzen.com/
下载地址：https://www.bootcdn.cn/html2c...