本文轉載自這裡：http://blog.csdn.net/hitwhylz/article/details/39518463

今天寫demo的時候發現, 如果把通過相機獲取到的圖片，直接進行操作, 比如裁剪, 縮放, 則會把原圖片向又旋轉90度。
剛開始覺得莫名其妙, 不知所措。 後來百度了一下，找到了解決辦法。

查詢過程中, 碰到了一種說法:

//get original photo from iOS photos 
//如果該圖片大於2M，會自動旋轉90度；否則不旋轉
UIImage* originalImg=[dict objectForKey:UIImagePickerControllerOriginalImage
 
];

至於是否正確, 還沒確定。 先Mark。

下面的解決辦法親測可行。 原文:http://www.cnblogs.com/jiangyazhou/archive/2012/03/22/2412343.html
用相機拍攝出來的照片含有EXIF資訊，UIImage的imageOrientation屬性指的就是EXIF中的orientation資訊。
如果我們忽略orientation資訊，而直接對照片進行畫素處理或者drawInRect等操作，得到的結果是翻轉或者旋轉90之後的樣子。這是因為我們執行畫素處理或者drawInRect等操作之後，imageOrientaion資訊被刪除了，imageOrientaion被重設為0，造成照片內容和imageOrientaion不匹配。
所以，在對照片進行處理之前，先將照片旋轉到正確的方向，並且返回的imageOrientaion為0。
下面這個方法就是一個UIImage category中的方法，用它可以達到以上目的。

- (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;
}