今天要做一个读取摄像头图片功能，发现拍摄之后的图片会在拍摄水平的基础上自动发生90°旋转。通过下面的方法 确实修正了，未避免选照片也发生该案例，我觉得在选取图片的时候也要进行修正。

 //get original photo from iOS photos 
//如果该图片大于2M，会自动旋转90度；否则不旋转

网络上说：用相机拍摄出来的照片含有EXIF信息，UIImage的imageOrientation属性指的就是EXIF中的orientation信息。如果我们忽略orientation信息，而直接对照片进行像素处理或者drawInRect等操作，得到的结果是翻转或者旋转90之后的样子。这是因为我们执行像素处理或者drawInRect等操作之后，imageOrientaion信息被删除了，imageOrientaion被重设为0，造成照片内容和imageOrientaion不匹配。所以，在对照片进行处理之前，先将照片旋转到正确的方向，并且返回的imageOrientaion为0。

修正代码如下：

1 – (UIImage *)fixOrientation:(UIImage *)aImage {
2
3   // No-op if the orientation is already correct
4   if (aImage.imageOrientation == UIImageOrientationUp)
5     return aImage;
6
7   // We need to calculate the proper transformation to make the image upright.
8   // We do it in 2 steps: Rotate if Left/Right/Down, and then flip if Mirrored.
9   CGAffineTransform transform = CGAffineTransformIdentity;
10
11   switch (aImage.imageOrientation) {
12     case UIImageOrientationDown:
13     case UIImageOrientationDownMirrored:
14       transform = CGAffineTransformTranslate(transform, aImage.size.width, aImage.size.height);
15       transform = CGAffineTransformRotate(transform, M_PI);
16       break;
17
18     case UIImageOrientationLeft:
19     case UIImageOrientationLeftMirrored:
20       transform = CGAffineTransformTranslate(transform, aImage.size.width, 0);
21       transform = CGAffineTransformRotate(transform, M_PI_2);
22       break;
23
24     case UIImageOrientationRight:
25     case UIImageOrientationRightMirrored:
26       transform = CGAffineTransformTranslate(transform, 0, aImage.size.height);
27       transform = CGAffineTransformRotate(transform, -M_PI_2);
28       break;
29     default:
30       break;
31   }
32
33   switch (aImage.imageOrientation) {
34     case UIImageOrientationUpMirrored:
35     case UIImageOrientationDownMirrored:
36       transform = CGAffineTransformTranslate(transform, aImage.size.width, 0);
37       transform = CGAffineTransformScale(transform, -1, 1);
38       break;
39
40     case UIImageOrientationLeftMirrored:
41     case UIImageOrientationRightMirrored:
42       transform = CGAffineTransformTranslate(transform, aImage.size.height, 0);
43       transform = CGAffineTransformScale(transform, -1, 1);
44       break;
45     default:
46       break;
47   }
48
49   // Now we draw the underlying CGImage into a new context, applying the transform
50   // calculated above.
51   CGContextRef ctx = CGBitmapContextCreate(NULL, aImage.size.width, aImage.size.height,
52                        CGImageGetBitsPerComponent(aImage.CGImage), 0,
53                        CGImageGetColorSpace(aImage.CGImage),
54                        CGImageGetBitmapInfo(aImage.CGImage));
55   CGContextConcatCTM(ctx, transform);
56   switch (aImage.imageOrientation) {
57     case UIImageOrientationLeft:
58     case UIImageOrientationLeftMirrored:
59     case UIImageOrientationRight:
60     case UIImageOrientationRightMirrored:
61       // Grr…
62       CGContextDrawImage(ctx, CGRectMake(0,0,aImage.size.height,aImage.size.width), aImage.CGImage);
63       break;
64
65     default:
66       CGContextDrawImage(ctx, CGRectMake(0,0,aImage.size.width,aImage.size.height), aImage.CGImage);
67       break;
68   }
69
70   // And now we just create a new UIImage from the drawing context
71   CGImageRef cgimg = CGBitmapContextCreateImage(ctx);
72   UIImage *img = [UIImage imageWithCGImage:cgimg];
73   CGContextRelease(ctx);
74   CGImageRelease(cgimg);
75   return img;
76 }