UIImage的imageOrientation属性

在做人脸识别时，发现相机拍摄的图片直接拿去识别，不成功；然而把图片保存到本地相册，再选取出来，便可成功识别，百思不得其解？既然相册图片可以识别，而拍照图片不可，二者之间必有差异，查看后发现属性 imageOrientation 的值不一样，选取相册中的图片 imageOrientation 为 UIImageOrientationUp，而拍摄出来的试了几次均为 UIImageOrientationRight，便想直接将拍照所得图片的 imageOrientation 改为 UIImageOrientationUp，奈何 imageOrientation 为只读属性。后网上搜寻得到下面方法，记录在此。

下面才实用：

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。

+ (UIImage *)fixOrientation:(UIImage *)image
{
    // No-op if the orientation is already correct
    if (image.imageOrientation == UIImageOrientationUp) return image;
    
    // 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 (image.imageOrientation) {
        case UIImageOrientationDown:
        case UIImageOrientationDownMirrored:
            transform = CGAffineTransformTranslate(transform, image.size.width, image.size.height);
            transform = CGAffineTransformRotate(transform, M_PI);
            break;
            
        case UIImageOrientationLeft:
        case UIImageOrientationLeftMirrored:
            transform = CGAffineTransformTranslate(transform, image.size.width, 0);
            transform = CGAffineTransformRotate(transform, M_PI_2);
            break;
            
        case UIImageOrientationRight:
        case UIImageOrientationRightMirrored:
            transform = CGAffineTransformTranslate(transform, 0, image.size.height);
            transform = CGAffineTransformRotate(transform, -M_PI_2);
            break;
            
        default:
            break;
    }
    
    switch (image.imageOrientation) {
        case UIImageOrientationUpMirrored:
        case UIImageOrientationDownMirrored:
            transform = CGAffineTransformTranslate(transform, image.size.width, 0);
            transform = CGAffineTransformScale(transform, -1, 1);
            break;
            
        case UIImageOrientationLeftMirrored:
        case UIImageOrientationRightMirrored:
            transform = CGAffineTransformTranslate(transform, image.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, image.size.width, image.size.height,
                                             CGImageGetBitsPerComponent(image.CGImage), 0,
                                             CGImageGetColorSpace(image.CGImage),
                                             CGImageGetBitmapInfo(image.CGImage));
    CGContextConcatCTM(ctx, transform);
    switch (image.imageOrientation) {
        case UIImageOrientationLeft:
        case UIImageOrientationLeftMirrored:
        case UIImageOrientationRight:
        case UIImageOrientationRightMirrored:
            // Grr...
            CGContextDrawImage(ctx, CGRectMake(0, 0, image.size.height, image.size.width), image.CGImage);
            break;
            
        default:
            CGContextDrawImage(ctx, CGRectMake(0, 0, image.size.width, image.size.height), image.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;
}

拍照图片识别成功！