UIImageの回転

- (UIImage *) rotateImage:(UIImage *)img angle:(int)angle
{
    CGImageRef imgRef = [img CGImage];
    CGContextRef context;
 
    switch (angle) {
        case 90:
            UIGraphicsBeginImageContext(CGSizeMake(img.size.height, img.size.width));
            context = UIGraphicsGetCurrentContext();
            CGContextTranslateCTM(context, img.size.height, img.size.width);
            CGContextScaleCTM(context, 1.0, -1.0);
            CGContextRotateCTM(context, M_PI/2.0);
            break;
        case 180:
            UIGraphicsBeginImageContext(CGSizeMake(img.size.width, img.size.height));
            context = UIGraphicsGetCurrentContext();
            CGContextTranslateCTM(context, img.size.width, 0);
            CGContextScaleCTM(context, 1.0, -1.0);
            CGContextRotateCTM(context, -M_PI);
            break;
        case 270:
            UIGraphicsBeginImageContext(CGSizeMake(img.size.height, img.size.width));
            context = UIGraphicsGetCurrentContext();
            CGContextScaleCTM(context, 1.0, -1.0);
            CGContextRotateCTM(context, -M_PI/2.0);
            break;
        default:
            NSLog(@"you can select an angle of 90, 180, 270");
            return nil;
    }  
 
    CGContextDrawImage(context, CGRectMake(0, 0, img.size.width, img.size.height), imgRef);
    UIImage *ret = UIGraphicsGetImageFromCurrentImageContext();  
 
    UIGraphicsEndImageContext();
    return ret;
}