function resizeImage(src,callback,w,h){

    

var canvas = document.createElement("canvas"),

        

ctx = canvas.getContext("2d"),

        

im = new Image();

        

w = w || 0,

        

h = h || 0;

    

im.onload = function(){

        

//为传入缩放尺寸用原尺寸

        

!w && (w = this.width);

        

!h && (h = this.height);

        

//以长宽最大值作为最终生成图片的依据

        

if(w !== this.width || h !== this.height){

            

var ratio;

            

if(w>h){

                

ratio = this.width / w;

                

h = this.height / ratio;

            

}else if(w===h){

                

if(this.width>this.height){

                    

ratio = this.width / w;

                    

h = this.height / ratio;

                

}else{

                    

ratio = this.height / h;

                    

w = this.width / ratio;

                

}

            

}else{

                

ratio = this.height / h;

                

w = this.width / ratio;

            

}

        

}

        

//以传入的长宽作为最终生成图片的尺寸

        

if(w>h){

            

var offset = (w - h) / 2;

            

canvas.width = canvas.height = w;

            

ctx.drawImage(im,0,offset,w,h);

        

}else if(w<h){

            

var offset = (h - w) / 2;

            

canvas.width = canvas.height = h;

            

ctx.drawImage(im,offset,0,w,h);

        

}else{

            

canvas.width = canvas.height = h;

            

ctx.drawImage(im,0,0,w,h);

        

}

        

callback(canvas.toDataURL("image/png"));

    

}

    

im.src = src;

}