Face Detection in iOS 8 using OpenCV gets slow after several seconds - opencv

Question

I am developing and iOS app that uses OpenCV to detect Faces on realtime from the camera.
On iOS 7 devices it works without any kind of problem. However on iOS 8 devices it starts detecting (or trying to detect) the faces and after 4-5 seconds the FPS decrements from 30FPS to 20FPS (in the case of iPhone 6, other devices change the FPS).
I am using:
OpenCV 2.4.9
CvVideoCamera to get every frame from the camera.
Cascade Classifier for face detection.
My VieController.mm has the following code to setup the camera and process the image for face detection.
- (void)viewDidLoad {
[super viewDidLoad];
//Init camera and configuration
self.videoCamera = [[CvVideoCamera alloc] initWithParentView:self.imageVideoView];
self.videoCamera.delegate = self;
self.videoCamera.defaultAVCaptureDevicePosition = AVCaptureDevicePositionFront;
self.videoCamera.defaultAVCaptureSessionPreset = AVCaptureSessionPresetMedium;
self.videoCamera.defaultAVCaptureVideoOrientation = AVCaptureVideoOrientationPortrait;
self.videoCamera.defaultFPS = 30;
self.videoCamera.grayscaleMode = NO;
//Load Cascade Model for Face Detection
NSString* cascadeModel = [[NSBundle mainBundle] pathForResource:#"haarcascade_frontalface_alt2" ofType:#"xml"];
const char* cascadeModel_path = [cascadeModel fileSystemRepresentation];
if(face_cascade.load(cascadeModel_path)){
cascadeLoad = true;
}else{
cascadeLoad = false;
}
//Start camera
[self.videoCamera start];
// Init frames to 0 and start timer to refresh the frames each second
totalFrames = 0;
[NSTimer scheduledTimerWithTimeInterval:1.0 target:self selector:#selector(computeFPS) userInfo:nil repeats:YES];
}
- (void)didReceiveMemoryWarning {
[super didReceiveMemoryWarning];
// Dispose of any resources that can be recreated.
}
- (void) processImage:(Mat&)image {
try{
if(cascadeLoad && !image.empty()){
// Opencv Face Detection
cv::Mat frameH;
pyrDown(image, frameH);
face_cascade.detectMultiScale(frameH, faces, 1.2, 2, 0, cv::Size(50, 50));
//Draw an ellipse on each detected Face
for( size_t i = 0; i < faces.size(); i++ )
{
cv::Point center( faces[i].x + faces[i].width*0.5, faces[i].y + faces[i].height*0.5 );
ellipse( image, center, cv::Size( faces[i].width*0.5, faces[i].height*0.5), 0, 0, 360, Scalar( 255, 0, 255 ), 4, 8, 0 );
}
}
} catch (cv::Exception e) {
NSLog(#"CV EXCEPTION!");
}
//Add one frame
totalFrames++;
}
//Function called every second to update the frames and update the FPS label
- (void) computeFPS {
[self.lblFPS setText:[NSString stringWithFormat:#"%d fps", totalFrames]];
totalFrames = 0;
}
That's all my code.
With the following test project the image does not look on Portrait (I have not figure out yet why). But the problem with decreasing the FPS is happening.
If you want to test it, the link to my complete test project is here.
Has anyone faced this problem? Any idea what might be happening or what I am doing wrong?
Thanks a lot!
PS: I have also read that there is other Face Detectors for iOS like the CIDetector, but I am using OpenCV as I want to do some more processing in OpenCV with the faces.