ImageProcessingService.swift

func getFaceRects(in image: UIImage, normalizedTo rect: CGRect) -> [CGRect] {
    guard var ciImage = CIImage(image: image) else {
        print("Couldn't create CIImage.")
        return []
    }

    // correct the orientation becuase `CIImage.init` can lose it
    ciImage = ciImage.oriented(.init(image.imageOrientation))

    let scaleFactorX = rect.width / ciImage.extent.width
    let scaleFactorY = rect.height / ciImage.extent.height

    var normalizedRects: [CGRect] = []

    // request the results from our detector
    for faceFeature in ciDetector.features(in: ciImage) {
        // every `CIFeature` has a `bounds` property, which is relative to the image dimensions.
        // keep in mind that we work here in CoreImage's coordinate system, meaning that the origin
        // is bottom left, instead of UIKit's top left.
        let faceRect = faceFeature.bounds

        // normalize the face feature bounds to the given rect
        let normalizedRect = CGRect(x: faceRect.minX * scaleFactorX,
                                    y: rect.height - (faceRect.maxY * scaleFactorY),
                                    width: faceRect.width * scaleFactorX,
                                    height: faceRect.height * scaleFactorY)
        normalizedRects.append(normalizedRect)
    }

    return normalizedRects
}