CIDetector trackingID never present

问题

I'm working on some face detection code on OSX Mavericks and I'm trying to take advantage of the newish (as of 10.8) face tracking across multiple stills functionality that CIDetector offers.

I have basic face detection working fine, like so:

- (void)captureOutput:(AVCaptureOutput *)captureOutput
didOutputSampleBuffer:(CMSampleBufferRef)sampleBuffer
       fromConnection:(AVCaptureConnection *)connection {

    CVImageBufferRef imageBuffer = CMSampleBufferGetImageBuffer(sampleBuffer);
    CIImage *image =  [CIImage imageWithCVImageBuffer:imageBuffer];
    CIDetector *faceDetector = [CIDetector detectorOfType:CIDetectorTypeFace
                                                  context:nil
                                                  options:@{ CIDetectorAccuracy : CIDetectorAccuracyHigh,
                                                         CIDetectorTracking : @YES
                                                        }];
    NSArray *features = [faceDetector featuresInImage:image];
    for ( CIFaceFeature *feature in features ) {
        if (feature.hasTrackingID) {
            NSLog(@"tracking id: %@", @(feature.trackingID));
        }
    }
}

The features list does get populated correctly but that trackingID never seems to be present. Has anyone gotten this working on Mavericks? It fails the same way on Mountain Lion.

I have seen a similar question here (CIFaceFeature trackingID is always coming same for multiple faces) but I didn't learn anything new there.

For what it's worth it does seem to function correctly on iOS.

回答1:

I looked at this code again and the answer turned out to be pretty obvious: I was constantly re-initializing the CIDetector, which was bad for performance and also had the consequence of resetting its internal tracking data each frame. So the first time a face was detected was always the first time a face was detected for that particular CIDetector instance.

Also, CIDetector warns about this in the docs:

"This class can maintain many state variables that can impact performance. So for best performance, reuse CIDetector instances instead of creating new ones.", from https://developer.apple.com/library/mac/documentation/CoreImage/Reference/CIDetector_Ref/Reference/Reference.html.

来源：https://stackoverflow.com/questions/19983741/cidetector-trackingid-never-present