蘋果自帶的Core Image可以進行人臉識別，識別效率也很高，比使用OpenCV效果要好，而且Core Image不僅可以人臉識別，還能增加渲染效果，非常好用，但是相關資料比較少，這個項目是按照WWDC2012的一個Keynote視頻做的。

講一下大概流程，
首先換取到本地的圖片：

UIImage* image = [UIImage imageNamed:@"face"];

然后轉換成CIImage

CIImage* ciimage = [CIImage imageWithCGImage:image.CGImage];

下面創建CIDetector類實例來識別：

    CIImage* ciimage = [CIImage imageWithCGImage:image.CGImage];
    NSDictionary* opts = [NSDictionary dictionaryWithObject:
                          CIDetectorAccuracyHigh forKey:CIDetectorAccuracy];
    CIDetector* detector = [CIDetector detectorOfType:CIDetectorTypeFace
                                              context:nil options:opts];

對detector調用方法可以獲取到識別到的結果數組（可以識別多張臉）：

NSArray* features = [detector featuresInImage:ciimage];

下面使用for循環，分別對結果畫出臉的位置和眼睛嘴的位置

    for (CIFaceFeature *faceFeature in features){
        
        CGFloat faceWidth = testImage.bounds.size.width/4;
        
        //標出臉的位置
        UIView* faceView = [[UIView alloc] initWithFrame:[self verticalFlipFromRect:faceFeature.bounds inSize:image.size toSize:testImage.bounds.size]];
        faceView.layer.borderWidth = 1;
        faceView.layer.borderColor = [[UIColor redColor] CGColor];
        [testImage addSubview:faceView];
        // 標出左眼
        if(faceFeature.hasLeftEyePosition) {
            UIView* leftEyeView = [[UIView alloc] initWithFrame:
                                   CGRectMake(0,0, faceWidth*0.3, faceWidth*0.3)];
            [leftEyeView setBackgroundColor:[[UIColor blueColor] colorWithAlphaComponent:0.3]];
            [leftEyeView setCenter:[self verticalFlipFromPoint:faceFeature.leftEyePosition inSize:image.size toSize:testImage.bounds.size]];
            leftEyeView.layer.cornerRadius = faceWidth*0.15;
            [testImage addSubview:leftEyeView];
        }
        
        // 標出右眼
        if(faceFeature.hasRightEyePosition) {
            UIView* rightEyeView = [[UIView alloc] initWithFrame:
                               CGRectMake(0,0, faceWidth*0.3, faceWidth*0.3)];
            [rightEyeView setBackgroundColor:[[UIColor blueColor] colorWithAlphaComponent:0.3]];
            [rightEyeView setCenter:[self verticalFlipFromPoint:faceFeature.rightEyePosition inSize:image.size toSize:testImage.bounds.size]];
            rightEyeView.layer.cornerRadius = faceWidth*0.15;
            [testImage  addSubview:rightEyeView];
        }
        // 標出嘴
        if(faceFeature.hasMouthPosition) {
            UIView* mouth = [[UIView alloc] initWithFrame:
                             CGRectMake(faceFeature.mouthPosition.x-faceWidth*0.2,
                                        faceFeature.mouthPosition.y-faceWidth*0.2, faceWidth*0.4, faceWidth*0.4)];
            [mouth setBackgroundColor:[[UIColor greenColor] colorWithAlphaComponent:0.3]];
            [mouth setCenter:[self verticalFlipFromPoint:faceFeature.mouthPosition inSize:image.size toSize:testImage.bounds.size]];
            mouth.layer.cornerRadius = faceWidth*0.2;
            [testImage addSubview:mouth];
        }
    }

這里需要注意，識別結果的位置的坐標系，和蘋果普通UI的坐標系不同，這個識別結果的坐標系，是從左下角開始的，所以需要對結果位置需要變換（垂直翻轉）

-(CGRect)verticalFlipFromRect:(CGRect)originalRect inSize:(CGSize)originalSize toSize:(CGSize)finalSize{
    CGRect finalRect = originalRect;
    finalRect.origin.y = originalSize.height - finalRect.origin.y - finalRect.size.height;
    CGFloat hRate = finalSize.width / originalSize.width;
    CGFloat vRate = finalSize.height / originalSize.height;
    finalRect.origin.x *= hRate;
    finalRect.origin.y *= vRate;
    finalRect.size.width *= hRate;
    finalRect.size.height *= vRate;
    return finalRect;
    
}

- (CGPoint)verticalFlipFromPoint:(CGPoint)originalPoint inSize:(CGSize)originalSize toSize:(CGSize)finalSize{
    CGPoint finalPoint = originalPoint;
    finalPoint.y = originalSize.height - finalPoint.y;
    CGFloat hRate = finalSize.width / originalSize.width;
    CGFloat vRate = finalSize.height / originalSize.height;
    finalPoint.x *= hRate;
    finalPoint.y *= vRate;
    return finalPoint;
    
}

具體細節可以去GitHub看，里面還有實時視頻畫面的人臉識別