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;
}