MLKit Firebase android - How to convert FirebaseVisionFace to Image Object (like Bitmap)?

You created the FirebaseVisionImage from a bitmap. After detection returns, each FirebaseVisionFace describes a bounding box as a Rect that you can use to extract the detected face from the original bitmap, e.g. using Bitmap.createBitmap().

Since the accepted answer was not specific enough I will try to explain what I did.

1.- Create an ImageView on LivePreviewActivity like this:

private ImageView imageViewTest;

2.- Create it on the Activity xml and link it to java file. I placed it right before the the sample code had, so it can be visible on top of the camera feed.

3.-When they create a FaceDetectionProcessor pass an instance of the imageView to be able to set the source image inside the object.

FaceDetectionProcessor processor = new FaceDetectionProcessor(imageViewTest);

4.-Change the constructor of FaceDetectionProcessor to be able to receive ImageView as a parameter and create a global variable that saves that instance.

public FaceDetectionProcessor(ImageView imageView) {
    FirebaseVisionFaceDetectorOptions options =
            new FirebaseVisionFaceDetectorOptions.Builder()
                    .setClassificationType(FirebaseVisionFaceDetectorOptions.ALL_CLASSIFICATIONS)
                    .setTrackingEnabled(true)
                    .build();

    detector = FirebaseVision.getInstance().getVisionFaceDetector(options);
    this.imageView  = imageView;
}

5.- I created a crop method that takes a bitmap and a Rect to focus only on the face. So go ahead and do the same.

    public static Bitmap cropBitmap(Bitmap bitmap, Rect rect) {
    int w = rect.right - rect.left;
    int h = rect.bottom - rect.top;
    Bitmap ret = Bitmap.createBitmap(w, h, bitmap.getConfig());
    Canvas canvas = new Canvas(ret);
    canvas.drawBitmap(bitmap, -rect.left, -rect.top, null);
    return ret;
}

6.- Modify detectInImage method to save an instance of the bitmap being detected and save it in a global variable.

    @Override
protected Task<List<FirebaseVisionFace>> detectInImage(FirebaseVisionImage image) {
    imageBitmap = image.getBitmapForDebugging();
    return detector.detectInImage(image);
}

7.- Finally, modify OnSuccess method by calling the cropping method and assign result to the imageView.

    @Override
protected void onSuccess(
        @NonNull List<FirebaseVisionFace> faces,
        @NonNull FrameMetadata frameMetadata,
        @NonNull GraphicOverlay graphicOverlay) {
    graphicOverlay.clear();
    for (int i = 0; i < faces.size(); ++i) {
        FirebaseVisionFace face = faces.get(i);
        FaceGraphic faceGraphic = new FaceGraphic(graphicOverlay);
        graphicOverlay.add(faceGraphic);
        faceGraphic.updateFace(face, frameMetadata.getCameraFacing());
        croppedImage = cropBitmap(imageBitmap, face.getBoundingBox());
    }
    imageView.setImageBitmap(croppedImage);
}