var type = 'png'; var canvas = document.createElement('canvas'); var imgData = canvas.toDataURL(type);var _fixType = function(type) {       type = type.toLowerCase().replace(/jpg/i, 'jpeg');       var r = type.match(/png|jpeg|bmp|gif/)[0];       return 'image/' + r;};imgData = imgData.replace(_fixType(type), 'image/octet-stream'); var saveFile = function(data, filename) {    var save_link = document.createElementNS('http://www.w3.org/1999/xhtml', 'a');        save_link.href = data;        save_link.download = filename;        var event = document.createEvent('MouseEvents');        event.initMouseEvent('click', true, false, window, 0, 0, 0, 0, 0, false, false, false, false, 0, null);         save_link.dispatchEvent(event);};var filename = 'weiyin_' + (new Date()).getTime() + '.' + type;saveFile(imgData, filename);為什么我最后下載出來的png是空白的呢