function compress(img) { var initSize = img.src.length; var width = img.width; var height = img.height; //如果图片大于四百万像素，计算压缩比并将大小压至400万以下 var ratio; if ((ratio = width * height / 4000000)>1) { ratio = Math.sqrt(ratio); width /= ratio; height /= ratio; }else { ratio = 1; } canvas.width = width; canvas.height = height; // 铺底色 ctx.fillStyle = "#fff"; ctx.fillRect(0, 0, canvas.width, canvas.height); //如果图片像素大于100万则使用瓦片绘制 var count; if ((count = width * height / 1000000) > 1) { count = ~~(Math.sqrt(count)+1); //计算要分成多少块瓦片 // 计算每块瓦片的宽和高 var nw = ~~(width / count); var nh = ~~(height / count); tCanvas.width = nw; tCanvas.height = nh; for (var i = 0; i < count; i++) { for (var j = 0; j < count; j++) { tctx.drawImage(img, i * nw * ratio, j * nh * ratio, nw * ratio, nh * ratio, 0, 0, nw, nh); ctx.drawImage(tCanvas, i * nw, j * nh, nw, nh); } } } else { ctx.drawImage(img, 0, 0, width, height); } //进行最小压缩 var ndata = canvas.toDataURL("image/jpeg", 0.1); console.log("压缩前：" + initSize); console.log("压缩后：" + ndata.length); console.log("压缩率：" + ~~(100 * (initSize - ndata.length) / initSize) + "%"); tCanvas.width = tCanvas.height = canvas.width = canvas.height = 0; return ndata; }
【三】图片上传
完成图片压缩后，就可以塞进formdata里进行上传了，先将base64数据转成字符串，再实例化一个ArrayBuffer，然后将字符 串以8位整型的格式传入ArrayBuffer，再通过BlobBuilder或者Blob对象，将8位整型的ArrayBuffer转成二进制对象 blob，然后把blob对象append到formdata里，再通过ajax发送给后台即可。
XmlHttpRequest2中不仅可以发送大数据，还多出了比如获取发送进度的API，我代码里也进行了简单的实现。
// 图片上传，将base64的图片转成二进制对象，塞进formdata上传 function upload(basestr, type, $li) { var text = window.atob(basestr.split(",")[1]); var buffer = new ArrayBuffer(text.length); var ubuffer = new Uint8Array(buffer); var pecent = 0 , loop = null; for (var i = 0; i < text.length; i++) { ubuffer[i] = text.charCodeAt(i); } var Builder = window.WebKitBlobBuilder window.MozBlobBuilder; var blob; if (Builder) { var builder = new Builder(); builder.append(buffer); blob = builder.getBlob(type); } else { blob = new window.Blob([buffer], {type: type}); } var xhr = new XMLHttpRequest(); var formdata = new FormData(); formdata.append("imagefile", blob); xhr.open("post", "/cupload"); xhr.onreadystatechange = function () { if (xhr.readyState == 4 && xhr.status == 200) { console.log("上传成功：" + xhr.responseText); clearInterval(loop); //当收到该消息时上传完毕 $li.find(".progress span").animate({"width": "100%"}, pecent < 95 ? 200 : 0, function () { $(this).html("上传成功"); }); $(".pic-list").append("<a href="" + xhr.responseText + " ">" + xhr.responseText + "<img src="" + xhr.responseText + "" /></a>") } }; //数据发送进度，前50%展示该进度 xhr.upload.addEventListener("progress", function (e) { if (loop) return; pecent = ~~(100 * e.loaded / e.total) / 2; $li.find(".progress span").css("width", pecent + "%"); if (pecent == 50) { mockProgress(); } }, false); //数据后50%用模拟进度 function mockProgress() { if (loop) return; loop = setInterval(function () { pecent++; $li.find(".progress span").css("width", pecent + "%"); if (pecent == 99) { clearInterval(loop); } }, 100) } xhr.send(formdata); }
至此，整个上传的前端图片压缩就完成了，因为是用了formdata提交，所以后台接数据的时候就跟普通form表单提交数据一样处理即可。
如果对该demo有兴趣的可以看这个demo的github地址：
前端代码：https://github.com/whxaxes/node-test/blob/master/server/upload/index_2.html
顺便也贴出后台的实现（nodejs）：https://github.com/whxaxes/node-test/blob/master/server/upload/upload_2.js