将CIFilter应用于图像会产生空的UIImage

Question

我正在尝试将CIFilter应用于我从UIImagePickerController收到的图像。这是我正在使用的代码：

- (void)imagePickerController:(UIImagePickerController *)picker didFinishPickingMediaWithInfo:(NSDictionary *)info {

    [picker dismissViewControllerAnimated:YES completion:nil];

    UIImage *userImage = info[UIImagePickerControllerOriginalImage];
    UIImage *croppedImage = [UIImage imageWithCGImage:CGImageCreateWithImageInRect(userImage.CGImage, CGRectMake(0, 320, 640, 640))];

    CIContext *context = [CIContext contextWithOptions:nil];
    CIImage *img = [CIImage imageWithCGImage:croppedImage.CGImage];
    CIFilter *adjustFilter = [CIFilter filterWithName:@"CILineOverlay"];

    [adjustFilter setDefaults];
    [adjustFilter setValue:img forKey:kCIInputImageKey];
    [adjustFilter setValue:[NSNumber numberWithFloat:1.0] forKey:@"inputNRSharpness"];
    CIImage *adjustedImage = [adjustFilter valueForKey:kCIOutputImageKey];
    UIImage *newImage = [UIImage imageWithCIImage:[context createCGImage:adjustedImage fromRect:adjustedImage.extent]];

    NSLog(@"%@", newImage);

    self.myImageView.image = newImage;

}

现在，当完成应用过滤器的过程（没有弹出错误）时，图像视图中没有可见的图像（尽管控制台会记录<UIImage: 0x8b91810>）。如果我跳过处理部分并将未经过滤的croppedImage分配给图像视图，则显示正常。

那我在这里做错了什么？我想用这个代码来处理截图，但是我用照片试了一下，得到了相同的（零）结果。