图片缩略图

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;

}