做项目的时候发现, 如果把通过相机获取到的图片，直接进行操作, 比如裁剪, 缩放, 可能会把原图片向右旋转90度。
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。
下面这个方法就是一个UIImage category中的方法，用它可以达到获取正确照片方向的目的。
- (UIImage *)fixOrientation:(UIImage *)aImage {
// No-op if the orientation is already correct
if (aImage.imageOrientation == UIImageOrientationUp)
return aImage;
// We need to calculate the proper transformation to make the image upright.
// We do it in 2 steps: Rotate if Left/Right/Down, and then flip if Mirrored.
CGAffineTransform transform = CGAffineTransformIdentity;
switch (aImage.imageOrientation) {
case UIImageOrientationDown:
case UIImageOrientationDownMirrored:
transform = CGAffineTransformTranslate(transform, aImage.size.width, aImage.size.height);
transform = CGAffineTransformRotate(transform, M_PI);
break;
case UIImageOrientationLeft:
case UIImageOrientationLeftMirrored:
transform = CGAffineTransformTranslate(transform, aImage.size.width, 0);
transform = CGAffineTransformRotate(transform, M_PI_2);
break;
case UIImageOrientationRight:
case UIImageOrientationRightMirrored:
transform = CGAffineTransformTranslate(transform, 0, aImage.size.height);
transform = CGAffineTransformRotate(transform, -M_PI_2);
break;
default:
break;
}
switch (aImage.imageOrientation) {
case UIImageOrientationUpMirrored:
case UIImageOrientationDownMirrored:
transform = CGAffineTransformTranslate(transform, aImage.size.width, 0);
transform = CGAffineTransformScale(transform, -1, 1);
break;
case UIImageOrientationLeftMirrored:
case UIImageOrientationRightMirrored:
transform = CGAffineTransformTranslate(transform, aImage.size.height, 0);
transform = CGAffineTransformScale(transform, -1, 1);
break;
default:
break;
}
// Now we draw the underlying CGImage into a new context, applying the transform
// calculated above.
CGContextRef ctx = CGBitmapContextCreate(NULL, aImage.size.width, aImage.size.height,
CGImageGetBitsPerComponent(aImage.CGImage), 0,
CGImageGetColorSpace(aImage.CGImage),
CGImageGetBitmapInfo(aImage.CGImage));
CGContextConcatCTM(ctx, transform);
switch (aImage.imageOrientation) {
case UIImageOrientationLeft:
case UIImageOrientationLeftMirrored:
case UIImageOrientationRight:
case UIImageOrientationRightMirrored:
// Grr...
CGContextDrawImage(ctx, CGRectMake(0,0,aImage.size.height,aImage.size.width), aImage.CGImage);
break;
default:
CGContextDrawImage(ctx, CGRectMake(0,0,aImage.size.width,aImage.size.height), aImage.CGImage);
break;
}
// And now we just create a new UIImage from the drawing context
CGImageRef cgimg = CGBitmapContextCreateImage(ctx);
UIImage *img = [UIImage imageWithCGImage:cgimg];
CGContextRelease(ctx);
CGImageRelease(cgimg);
return img;
}
但有时候我们获取图片时用的是AGImagePickerController，在其从ALAsset转为UIImage的时候可能原图就发生了旋转，因此，这里也给出了一个处理办法：
for (ALAsset * asset in self.assetArray) {
CGImageRef ref = [[asset defaultRepresentation]fullResolutionImage];
UIImage *orgImage = [UIImage imageWithCGImage:[asset.defaultRepresentation fullScreenImage]
scale:[asset.defaultRepresentation scale] orientation:
(UIImageOrientation)[asset.defaultRepresentation orientation]];
// 这句解决了问题
orgImage = [UIImage imageWithCGImage:ref scale:1 orientation:orgImage.imageOrientation];
// XMLog(@"原始选完imageOrientation===%ld",orgImage.imageOrientation);
// 让图片的旋转方向向上（即正确的方向）
orgImage = [self fixOrientation: orgImage];
[self.previewImageArray addObject:orgImage];
}
[delegate imagePickerDidSelectImagearray:self.previewImageArray];
但如果一开始用以下这种写法从ALAsset转为UIImage，就不会发生旋转的问题：
ALAssetRepresentation *assetRep = [asset defaultRepresentation];
CGImageRef imgRef = [assetRep fullResolutionImage];
UIImage *orgImage = [UIImage imageWithCGImage:imgRef
scale:assetRep.scale
orientation:(UIImageOrientation)assetRep.orientation];
