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，用它可以达到以上目的。

- (UIImage *)fixOrientation:(UIImage *)aImage {// No-op if the orientation is already correctif (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 contextCGImageRef cgimg = CGBitmapContextCreateImage(ctx);UIImage *img = [UIImage imageWithCGImage:cgimg];CGContextRelease(ctx);CGImageRelease(cgimg);return img;
}