// 用到的库
https://html2canvas.hertzen.com/

// div转图片
<script src="https://html2canvas.hertzen.com/dist/html2canvas.min.js"></script>
<script type="text/javascript">
    $(document).ready(function() {
        // canvas生成图片
        $(".poster-down-load").on("click", function() {
            var getPixelRatio = function(context) { // 获取设备的PixelRatio
                var backingStore = context.backingStorePixelRatio ||
                    context.webkitBackingStorePixelRatio ||
                    context.mozBackingStorePixelRatio ||
                    context.msBackingStorePixelRatio ||
                    context.oBackingStorePixelRatio ||
                    context.backingStorePixelRatio || 0.5;
                return (window.devicePixelRatio || 0.5) / backingStore;
            };
            //生成的图片名称
            var imgName = "cs.jpg";// 输出图片名称
            var shareContent = document.getElementById("poster-box-left"); // 需要转图片的div
            var width = shareContent.offsetWidth;
            var height = shareContent.offsetHeight;
            var canvas = document.createElement("canvas");
            var context = canvas.getContext('2d');
            var scale = getPixelRatio(context); //将canvas的容器扩大PixelRatio倍，再将画布缩放，将图像放大PixelRatio倍。
            canvas.width = width * scale;
            canvas.height = height * scale;
            canvas.style.width = width + 'px';
            canvas.style.height = height + 'px';
            context.scale(scale, scale);

            var opts = {
                scale: 1,
                canvas: canvas,
                width: width,
                height: height,
                dpi: window.devicePixelRatio
            };
            html2canvas(shareContent, opts).then(function(canvas) {
                context.imageSmoothingEnabled = false;
                context.webkitImageSmoothingEnabled = false;
                context.msImageSmoothingEnabled = false;
                context.imageSmoothingEnabled = false;
                var dataUrl = canvas.toDataURL('image/jpeg', 1.0);
                dataURIToBlob(imgName, dataUrl, callback);
            });
        });
    })
    var dataURIToBlob = function(imgName, dataURI, callback) {
        var binStr = atob(dataURI.split(',')[1]),
            len = binStr.length,
            arr = new Uint8Array(len);

        for (var i = 0; i < len; i++) {
            arr[i] = binStr.charCodeAt(i);
        }

        callback(imgName, new Blob([arr]));
    }

    var callback = function(imgName, blob) {
        var triggerDownload = $("<a>").attr("href", URL.createObjectURL(blob)).attr("download", imgName).appendTo(
            "body").on("click", function() {
            if (navigator.msSaveBlob) {
                return navigator.msSaveBlob(blob, imgName);
            }
        });
        triggerDownload[0].click();
        triggerDownload.remove();
    };
</script>