用html5编写图片裁切上传，在iphone手机上可能会遇到图片方向错误问题，在此把解决方法和大家分享一下，
用到了html5的 FileReader和Canvas，如果还没有接触的同学，先了解一下其方法。
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//此方法为file input元素的change事件
function change(){
var file = this.files[0];
var orientation;
//EXIF js 可以读取图片的元信息 https://github.com/exif-js/exif-js
EXIF.getData(file,function(){
orientation=EXIF.getTag(this,'Orientation');
});
var reader = new FileReader();
reader.onload = function(e) {
getImgData(this.result,orientation,function(data){
//这里可以使用校正后的图片data了
});
}
reader.readAsDataURL(file);
}
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// @param {string} img 图片的base64
// @param {int} dir exif获取的方向信息
// @param {function} next 回调方法，返回校正方向后的base64
function getImgData(img,dir,next){
var image=new Image();
image.onload=function(){
var degree=0,drawWidth,drawHeight,width,height;
drawWidth=this.naturalWidth;
drawHeight=this.naturalHeight;
//以下改变一下图片大小
var maxSide = Math.max(drawWidth, drawHeight);
if (maxSide > 1024) {
var minSide = Math.min(drawWidth, drawHeight);
minSide = minSide / maxSide * 1024;
maxSide = 1024;
if (drawWidth > drawHeight) {
drawWidth = maxSide;
drawHeight = minSide;
} else {
drawWidth = minSide;
drawHeight = maxSide;
}
}
var canvas=document.createElement('canvas');
canvas.width=width=drawWidth;
canvas.height=height=drawHeight;
var context=canvas.getContext('2d');
//判断图片方向，重置canvas大小，确定旋转角度，iphone默认的是home键在右方的横屏拍摄方式
switch(dir){
//iphone横屏拍摄，此时home键在左侧
case 3:
degree=180;
drawWidth=-width;
drawHeight=-height;
break;
//iphone竖屏拍摄，此时home键在下方(正常拿手机的方向)
case 6:
canvas.width=height;
canvas.height=width;
degree=90;
drawWidth=width;
drawHeight=-height;
break;
//iphone竖屏拍摄，此时home键在上方
case 8:
canvas.width=height;
canvas.height=width;
degree=270;
drawWidth=-width;
drawHeight=height;
break;
}
//使用canvas旋转校正
context.rotate(degree*Math.PI/180);
context.drawImage(this,0,0,drawWidth,drawHeight);
//返回校正图片
next(canvas.toDataURL("image/jpeg",.8));
}
image.src=img;
}
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