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function  compressImage(maxWidth,maxHeight,srcOriginalImage,callback){
     // 创建 canvas
 
     var  canvasId = "canvas_" + new  Date().getTime()+ "" +parseInt(Math.random());
     $( "<canvas></canvas>" ).hide().attr( "id" ,canvasId).appendTo( "body" );
 
     var  c=$( "#" +canvasId)[0];
     var  ctx=c.getContext( "2d" );
 
     // 释放canvas
     function  releaseCanvas(){
         $( "#" +canvasId).remove();
     }
 
 
     // 创建要绘制的Image对象
     var  img =  new  Image();
     img.src = srcOriginalImage;
     // 等待img加载完毕
     img.onload =  function (){
         // 与backgournd-size:contain效果相同
         if (img.width/img.height<maxWidth/maxHeight){
             c.height = img.height;
             if (img.height>maxHeight){
                 c.height = maxHeight;
             }
             c.width = img.width/img.height*c.height;
         } else {
             c.width = img.width;
             if (img.width>maxWidth){
                 c.width = maxWidth;
             }
             c.height = img.height/img.width*c.width;
         }
         ctx.drawImage(img,0,0,img.width,img.height,0,0,c.width,c.height);
         callback(c.toDataURL());
 
         releaseCanvas();
     };
 
 
 
}