ImageProcessingService.swift

func blurFaces(in image: UIImage) -> UIImage? {
    guard let ciImage = CIImage(image: image)?
        .oriented(.init(image.imageOrientation)) else {
        return nil
    }

    // this image will use as the "mask map"
    var maskCanvasImage = CIImage.empty()
        .cropped(to: ciImage.extent)

    // request the features from `CIDetector`
    for faceFeature in ciDetector.features(in: ciImage) {
        let faceRect = faceFeature.bounds

        // position the mask in the center of the face rect, relative to the image extent.
        let maskCenter = CIVector(x: faceRect.maxX - (faceRect.width/2),
                                  y: faceRect.maxY - (faceRect.height/2))

        let radialGradientFilter = CIFilter(name: "CIRadialGradient", parameters: [
            "inputCenter": maskCenter,
            "inputRadius0": faceRect.height/2,
            "inputRadius1": faceRect.height,
            "inputColor0": CIColor.white,
            "inputColor1": CIColor(red: 1.0, green: 1.0, blue: 1.0, alpha: 0.0)
        ])

        if let gradientOutput = radialGradientFilter?.outputImage?
            .cropped(to: faceRect.insetBy(dx: -faceRect.width/2, dy: -faceRect.height/2)) {

            // "append" the new mask to our mask map
            maskCanvasImage = gradientOutput.composited(over: maskCanvasImage)
        }
    }

    // create the blurred copy of the image
    let blurRadius: Double = 30.0
    let blurredImage = ciImage
        .clampedToExtent()
        .applyingGaussianBlur(sigma: blurRadius)
        .cropped(to: ciImage.extent)

    // this is where the magic happens -
    // mask the blurred image with our mask map, over the original image
    let resultImage = blurredImage.applyingFilter("CIBlendWithMask", parameters: [
        kCIInputBackgroundImageKey: ciImage,
        kCIInputMaskImageKey: maskCanvasImage
    ])

    // create the final image and return it.
    if let cgImage = ciContext.createCGImage(resultImage, from: resultImage.extent) {
        return UIImage(cgImage: cgImage)
    }

    return nil
}