How to detect face in image using Face Detection api in iOS

by abdus on April 24, 2013

Apple introduced powerful Face detection api in iOS 5. It uses quartz core framework (more specifically CoreImage) to detect faces in given image. Face Detection api in iOS detect the face, wrap it in CIFaceFeature object and add it in an array. Array contains faces detected in that image. Each CIFaceFeature object contains detail of detected face. That includes

  • Face bounds (CGRect)
  • leftEyePosition (CGPoint)
  • rightEyePosition (CGPoint)
  • mouthPosition (CGPoint)

Face Detection

Face Detection api in iOS also take accuracy factor as parameter. You have to set this parameter against CIDetectorAccuracy key. There are two possible values you can give to this parameter i.e.

  • CIDetectorAccuracyLow
  • CIDetectorAccuracyHigh

For obvious reasons CIDetectorAccuracyHigh will take more time to fetch results from given image. Anyways you can decide the best fit according to your scenario. I would recommend to do the face detection process in background thread if you are going for CIDetectorAccuracyHigh.

One thing to be mentioned here, the coordinate system used by UIKit is different from Coordinate System used by Quartz Core. Quartz Core origin is located in lower left corner of screen. Its y-axis increases upwards and x-axis increases on right side. UIKit origin is located in upper left corner of screen. Its y-axis increases downwards and x-axis increases on right side.

Face Detection api in iOS uses quartz core framework hence the CGPoint calculated for (leftEye, rightEye, mouth) cannot be used

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as it is in UIImageView to spot face features. If you do so it will look inverted with the reference of image displayed in UIImageView. You need to perform appropriate calculations in order to synchronize Quartz Core and UIKit coordinates to get accurate points in UIImageView.

Following is the function that takes UIImage and spot position of left/right eye and mouth detected throughout the image.

-(void)detectForFacesInUIImage:(UIImage *)facePicture
    CIImage* image = [CIImage imageWithCGImage:facePicture.CGImage];

    CIDetector* detector = [CIDetector detectorOfType:CIDetectorTypeFace
                                              context:nil options:[NSDictionary dictionaryWithObject:CIDetectorAccuracyLow forKey:CIDetectorAccuracy]];

    NSArray* features = [detector featuresInImage:image];

    for(CIFaceFeature* faceObject in features)
        CGRect modifiedFaceBounds = faceObject.bounds;
        modifiedFaceBounds.origin.y = facePicture.size.height-faceObject.bounds.size.height-faceObject.bounds.origin.y;

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            CGRect leftEye = CGRectMake(faceObject.leftEyePosition.x,(facePicture.size.height-faceObject.leftEyePosition.y), 10, 10);
            [self addSubViewWithFrame:leftEye];

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            CGRect rightEye = CGRectMake(faceObject.rightEyePosition.x, (facePicture.size.height-faceObject.rightEyePosition.y), 10, 10);
            [self addSubViewWithFrame:rightEye];

           CGRect  mouth = CGRectMake(faceObject.mouthPosition.x,facePicture.size.height-faceObject.mouthPosition.y,10, 10);
            [self addSubViewWithFrame:mouth];
    UIView* highlitView = [[UIView alloc] initWithFrame:frame];
    highlitView.layer.borderWidth = 1;
    highlitView.layer.borderColor = [[UIColor whiteColor] CGColor];
    [imageView addSubview:highlitView];

You can download working example from my Git repository. Here is the link Face Detection api in iOS

{ 6 comments… read them below or add one }

Jyotsana April 1, 2014 at 11:59 am

I want help regarding face recognition in objective C


Дмитрий Лобанов January 15, 2014 at 1:36 pm

I like your blog, really interesting topics here. Thank you!

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Question about face detection:
you use as size.width and size.height constants like 10 ?
Why so? I expect that every face feature (eye, mouth) will have their bounds.


Mahendra yadav December 3, 2013 at 6:18 am
abdul rehman September 18, 2013 at 10:42 am

nice post bu ti want to detect simple object like cake bread focussed .. possible ?


Zeeshan Mirza April 27, 2013 at 4:06 am

nice post


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