Objective-C

Import the following to your ViewController

#import <CoreImage/CoreImage.h>
#import <CoreImage/CoreImage.h>
#import <QuartzCore/QuartzCore.h>

Call the function

[self faceDetector];

Function definition:

-(void)faceDetector
{
    // Load the picture for face detection
    UIImageView* image = [[UIImageView alloc] initWithImage:[UIImage imageNamed:@"download.jpeg"]];
    
    // Draw the face detection image
    [self.view addSubview:image];
    
    // Execute the method used to markFaces in background
    [self performSelectorInBackground:@selector(markFaces:) withObject:image];
    
    // flip image on y-axis to match coordinate system used by core image
    [image setTransform:CGAffineTransformMakeScale(1, -1)];
    
    // flip the entire window to make everything right side up
    [self.view setTransform:CGAffineTransformMakeScale(1, -1)];
    
    
}

Mark Face Function

//Adds face squares and color masks to eyes and mouth
-(void)markFaces:(UIImageView *)facePicture
{
    // draw a CI image with the previously loaded face detection picture
    CIImage* image = [CIImage imageWithCGImage:facePicture.image.CGImage];
    
    // create a face detector - since speed is not an issue we'll use a high accuracy
    // detector
    CIDetector* detector = [CIDetector detectorOfType:CIDetectorTypeFace
                                              context:nil options:[NSDictionary dictionaryWithObject:CIDetectorAccuracyHigh forKey:CIDetectorAccuracy]];
    
    // create an array containing all the detected faces from the detector
    NSArray* features = [detector featuresInImage:image];
    NSLog(@"Number of faces %d",[features count]);
    
    // we'll iterate through every detected face.  CIFaceFeature provides us
    // with the width for the entire face, and the coordinates of each eye
    // and the mouth if detected.  Also provided are BOOL's for the eye's and
    // mouth so we can check if they already exist.
    //    for (features in image)
    //    {
    for(CIFaceFeature* faceFeature in features)
    {
        // get the width of the face
        CGFloat faceWidth = faceFeature.bounds.size.width;
        
        // create a UIView using the bounds of the face
        UIView* faceView = [[UIView alloc] initWithFrame:faceFeature.bounds];
        
        // add a border around the newly created UIView
        faceView.layer.borderWidth = 1;
        faceView.layer.borderColor = [[UIColor redColor] CGColor];
        
        // add the new view to create a box around the face
        [self.view addSubview:faceView];
        
        if(faceFeature.hasLeftEyePosition)
        {
            // create a UIView with a size based on the width of the face
            UIView* leftEyeView = [[UIView alloc] initWithFrame:CGRectMake(faceFeature.leftEyePosition.x-faceWidth*0.15, faceFeature.leftEyePosition.y-faceWidth*0.15, faceWidth*0.3, faceWidth*0.3)];
            // change the background color of the eye view
            [leftEyeView setBackgroundColor:[[UIColor blueColor] colorWithAlphaComponent:0.3]];
            // set the position of the leftEyeView based on the face
            [leftEyeView setCenter:faceFeature.leftEyePosition];
            // round the corners
            leftEyeView.layer.cornerRadius = faceWidth*0.15;
            // add the view to the window
            [self.view addSubview:leftEyeView];
        }
        
        if(faceFeature.hasRightEyePosition)
        {
            // create a UIView with a size based on the width of the face
            UIView* leftEye = [[UIView alloc] initWithFrame:CGRectMake(faceFeature.rightEyePosition.x-faceWidth*0.15, faceFeature.rightEyePosition.y-faceWidth*0.15, faceWidth*0.3, faceWidth*0.3)];
            // change the background color of the eye view
            [leftEye setBackgroundColor:[[UIColor blueColor] colorWithAlphaComponent:0.3]];
            // set the position of the rightEyeView based on the face
            [leftEye setCenter:faceFeature.rightEyePosition];
            // round the corners
            leftEye.layer.cornerRadius = faceWidth*0.15;
            // add the new view to the window
            [self.view addSubview:leftEye];
        }
        
        if(faceFeature.hasMouthPosition)
        {
            // create a UIView with a size based on the width of the face
            UIView* mouth = [[UIView alloc] initWithFrame:CGRectMake(faceFeature.mouthPosition.x-faceWidth*0.2, faceFeature.mouthPosition.y-faceWidth*0.2, faceWidth*0.4, faceWidth*0.4)];
            // change the background color for the mouth to green
            [mouth setBackgroundColor:[[UIColor greenColor] colorWithAlphaComponent:0.3]];
            // set the position of the mouthView based on the face
            [mouth setCenter:faceFeature.mouthPosition];
            // round the corners
            mouth.layer.cornerRadius = faceWidth*0.2;
            // add the new view to the window
            [self.view addSubview:mouth];
        }
    }
    
    // }
}

The Simulator ScreenShot for the Function