上传图片，JS中等比压缩返回BASE64格式String

微信公众号里，需要上传照片，由于手机直接拍照上传，照片比较大，直接转BASE64上传会很慢

所以最好在JS中压缩后，再上传

网上找了个方法，照搬试了一下，可以用，但是，图片会被裁剪掉一大块

于是改了改，现在会根据图片是纵向还是横向等比例的进行压缩

哦！对了，这样转出来的BASE64字符串会有23位的文件头

如果后台用JDK的BASE64Decoder再转成图片的话，需要先把文件头去掉，不然无法转换的

 

HTML部分

<input type="file"  name="businessLicence" id="businessLicence" onchange="start_upload(this);"/>                                                    
<input type="hidden"  name="businessLicenceBase64" id="businessLicenceBase64"/>

 

Js部分

　　　　function start_upload(obj){ 
             if(!isCanvasSupported){  
                console.log("画布不存在，压缩失败");  
             }else{  
                compress(event, function(base64Img){
                    document.getElementById(obj.name+"Base64").value=base64Img; 
                });  
             }  
        }
        
        //判断是否存在画布  
        function isCanvasSupported() {  
            var elem = document.createElement('canvas');  
            return !!(elem.getContext && elem.getContext('2d'));  
        }  
          
        //压缩方法  
        function compress(event, callback) {  
            if ( typeof (FileReader) === 'undefined') {  
                console.log("当前浏览器内核不支持base64图标压缩");  
            } else {  
                try {  
                    var file = event.currentTarget.files[0];  
                     if(!/image\/\w+/.test(file.type)){     
                            alert("请确保文件为图像类型");                            
                            return false;    
                     }   
                    var reader = new FileReader();  
                    reader.onload = function (e) {  
                    var image = $('<img/>');  
                    image.load(function () {  
                    console.log("开始压缩");
                    //压缩的像素大小，square越大，像素越高  
                    var square = 500;  
                    var canvas = document.createElement('canvas');
                    if (this.width > this.height) {                         
                      canvas.width = Math.round(square * this.width / this.height);  
                      canvas.height = square; 
                    } else {    
                      canvas.width = square;  
                      canvas.height = Math.round(square * this.height / this.width);                    
                    }    
                    var context = canvas.getContext('2d');  
                    context.clearRect(0, 0, canvas.width, canvas.height);  
                    var imageWidth;  
                    var imageHeight;  
                    var offsetX = 0;  
                    var offsetY = 0;  
                    console.log(this.width);
                    console.log(this.height);
                    if (this.width > this.height) {                         
                      imageWidth = Math.round(square * this.width / this.height);  
                      imageHeight = square; 
                    } else {    
                      imageWidth = square;
                      imageHeight = Math.round(square * this.height / this.width);  
                    } 
                    console.log(imageWidth);
                    console.log(imageHeight);
                    context.drawImage(this, 0, 0, imageWidth, imageHeight);  
                    var data = canvas.toDataURL('image/jpeg');  
                        //压缩完成执行回调  
                       callback(data);  
                    });  
                    image.attr('src', e.target.result);  
                    };  
                    reader.readAsDataURL(file);  
                } catch(e) {  
                    console.log("压缩失败!");  
                }  
            }  
        }