CIFilter输出图像随机显示先前的输出图像

Question

I've found a very weird behaviour for the CIFilter with the CIGaussianBlur filter.

I am performing this method multiple times in fast succession for different images. SOMETIMES, the "last processed image" will be returned instead of the one I send in. For example, if I have the images:

A , B and C .

If I perform the blurring in fast succession, SOMETIMES I get a result like:

Blurred A , Blurred A , Blurred C

+(UIImage *)applyBlurToImageAtPath:(NSURL *)imageUrlPath
{
    if (imageUrlPath == nil)
        return nil;

    //Tried to create new contexts each loop, and also tried to use a singleton context
    //    if(CIImageContextSingleton == nil)
    //    {
    //        CIImageContextSingleton = [CIContext contextWithOptions:nil];
    //    }
    CIContext *context = [CIContext contextWithOptions:nil];//[Domain sharedInstance].CIImageContextSingleton;

    CIFilter *gaussianBlurFilter = [CIFilter filterWithName:@"CIGaussianBlur"];
    [gaussianBlurFilter setDefaults];
    CIImage *inputImage = [CIImage imageWithContentsOfURL:imageUrlPath];
    [gaussianBlurFilter setValue:inputImage forKey:kCIInputImageKey];
    [gaussianBlurFilter setValue:@(1) forKey:kCIInputRadiusKey];

    //Tried both these methods for getting the output image
    CIImage *outputImage = [gaussianBlurFilter valueForKey:kCIOutputImageKey];
//    CIImage *outputImage = [gaussianBlurFilter outputImage];

    //If I'm doing this, the problem never occurs, so the problem is isolated to the gaussianBlurFilter:
    //outputImage = inputImage;

    CGImageRef cgimg     = [context createCGImage:outputImage fromRect:[inputImage extent]];
    UIImage *resultImage = [UIImage imageWithCGImage:cgimg];

    //Tried both with and without releasing the cgimg
    CGImageRelease(cgimg);

    return resultImage;
}

I've tried both in a loop, and by running the method when making a gesture or such and the same problem appears. (The image at the imageUrlPath is correct.) Also, see comments in the code for stuff I've tried.

Am I missing something? Is there some internal cache for the CIFilter? The method is always running on the main thread.

Answer 1

Based on the code given, and on the assumption that this method is always called on the main thread, you should be ok, but I do see some things that are ill-advised in the code:

• Do not re-create your CIContext every time the method is called. I would suggest structuring a different way, not as a Singleton. Keep your CIContext around and re-use the same context when performing a lot of rendering.
• If your CIFilter does not change, it is not necessary to re-create it every time either. If you are calling the method on the same thread, you can simply set the inputImage key on the filter. You will need to get a new outputImage from the filter whenever the input image is changed.

My guess is the problem is likely surrounding the Core Image Context rendering to the same underlying graphics environment(probably GPU rendering), but since you are constantly recreating a CIContext, perhaps there is something wonky going on.

Just a guess really, since I don't have code handy to test out myself. If you have a test project that demonstrates the problem, it would be easier to debug. Also- I'm still skeptical of the threading. The fact that it works without applying the blur does not necessarily prove that it is the blur causing the issue-- randomness is more likely to involve threading problems in my experience.