def extract(self, sampler, feature_extractor, number_of_examples_per_scale = (100, 100), similarity_thresholds = (0.5, 0.8), parallel = None, mirror = False, use_every_nth_negative_scale = 1):
    """Extracts features from **all** images in **all** scales and writes them to file.

    This function iterates over all images that are present in the internally stored list, and extracts features using the given ``feature_extractor`` for every image patch that the given ``sampler`` returns.
    The final features will be stored in the ``feature_directory`` that is set in the constructor.

    For each image, the ``sampler`` samples patch locations, which cover the whole image in different scales.
    For each patch locations is tested, how similar they are to the face bounding boxes that belong to that image, using the Jaccard :py:meth:`BoundingBox.similarity`.
    The similarity is compared to the ``similarity_thresholds``.
    If it is smaller than the first threshold, the patch is considered as background, when it is greater the the second threshold, it is considered as a face, otherwise it is rejected.
    Depending on the image resolution and the number of bounding boxes, this will usually result in some positive and thousands of negative patches per image.
    To limit the total amount of training data, for all scales, only up to a given number of positive and negative patches are kept.
    Also, to further limit the number of negative samples, only every ``use_every_nth_negative_scale`` scale is considered (for the positives, always all scales are processed).

    To increase the number (especially of positive) examples, features can also be extracted for horizontally mirrored images.
    Simply set the ``mirror`` parameter to ``True``.
    Furthermore, this function is designed to be run using several parallel processes, e.g., using the `GridTK <https://pypi.python.org/pypi/gridtk>`_.
    Each of the processes will run on a particular subset of the images, which is defined by the ``SGE_TASK_ID`` environment variable.
    The ``parallel`` parameter defines the total number of parallel processes that are used.

    **Parameters:**

    ``sampler`` : :py:class:`Sampler`
      The sampler to use to sample patches of the images. Please assure that the sampler is set up such that it samples patch locations which can overlap with the face locations.

    ``feature_extractor`` : :py:class:`FeatureExtractor`
      The feature extractor to be used to extract features from image patches

    ``number_of_examples_per_scale`` : (int, int)
      The maximum number of positive and negative examples to extract for each scale of the image

    ``similarity_thresholds`` : (float, float)
      The Jaccard similarity threshold, below which patch locations are considered to be negative, and above which patch locations are considered to be positive examples.

    ``parallel`` : int or ``None``
      If given, the total number of parallel processes, which are used to extract features (the current process index is read from the ``SGE_TASK_ID`` environment variable)

    ``mirror`` : bool
      Extract positive and negative samples also from horizontally mirrored images?

    ``use_every_nth_negative_scale`` : int
      Skip some negative scales to decrease the number of negative examples, i.e., only extract and store negative features, when ``scale_counter % use_every_nth_negative_scale == 0``

      .. note::
         The ``scale_counter`` is not reset between images, so that we might get features from different scales in subsequent images.
    """

    feature_file = self._feature_file(parallel)
    bob.io.base.create_directories_safe(self.feature_directory)

    if parallel is None or "SGE_TASK_ID" not in os.environ or os.environ["SGE_TASK_ID"] == '1':
      extractor_file = os.path.join(self.feature_directory, "Extractor.hdf5")
      hdf5 = bob.io.base.HDF5File(extractor_file, "w")
      feature_extractor.save(hdf5)
      del hdf5

    total_positives, total_negatives = 0, 0

    indices = parallel_part(range(len(self)), parallel)
    if not indices:
      logger.warning("The index range for the current parallel thread is empty.")
    else:
      logger.info("Extracting features for images in range %d - %d of %d", indices[0], indices[-1], len(self))

    hdf5 = bob.io.base.HDF5File(feature_file, "w")
    for index in indices:
      hdf5.create_group("Image-%d" % index)
      hdf5.cd("Image-%d" % index)

      logger.debug("Processing file %d of %d: %s", index+1, indices[-1]+1, self.image_paths[index])

      # load image
      image = bob.io.base.load(self.image_paths[index])
      if image.ndim == 3:
        image = bob.ip.color.rgb_to_gray(image)
      # get ground_truth bounding boxes
      ground_truth = self.bounding_boxes[index]

      # collect image and GT for originally and mirrored image
      images = [image] if not mirror else [image, bob.ip.base.flop(image)]
      ground_truths = [ground_truth] if not mirror else [ground_truth, [gt.mirror_x(image.shape[1]) for gt in ground_truth]]
      parts = "om"

      # now, sample
      scale_counter = -1
      for image, ground_truth, part in zip(images, ground_truths, parts):
        for scale, scaled_image_shape in sampler.scales(image):
          scale_counter += 1
          scaled_gt = [gt.scale(scale) for gt in ground_truth]
          positives = []
          negatives = []
          # iterate over all possible positions in the image
          for bb in sampler.sample_scaled(scaled_image_shape):
            # check if the patch is a positive example
            positive = False
            negative = True
            for gt in scaled_gt:
              similarity = bb.similarity(gt)
              if similarity > similarity_thresholds[1]:
                positive = True
                break
              if similarity > similarity_thresholds[0]:
                negative = False
                break

            if positive:
              positives.append(bb)
            elif negative and scale_counter % use_every_nth_negative_scale == 0:
              negatives.append(bb)

          # per scale, limit the number of positive and negative samples
          positives = [positives[i] for i in quasi_random_indices(len(positives), number_of_examples_per_scale[0])]
          negatives = [negatives[i] for i in quasi_random_indices(len(negatives), number_of_examples_per_scale[1])]

          # extract features
          feature_extractor.prepare(image, scale)
          # .. negative features
          if negatives:
            negative_features = numpy.zeros((len(negatives), feature_extractor.number_of_features), numpy.uint16)
            for i, bb in enumerate(negatives):
              feature_extractor.extract_all(bb, negative_features, i)
            hdf5.set("Negatives-%s-%.5f" % (part,scale), negative_features)
            total_negatives += len(negatives)

          # positive features
          if positives:
            positive_features = numpy.zeros((len(positives), feature_extractor.number_of_features), numpy.uint16)
            for i, bb in enumerate(positives):
              feature_extractor.extract_all(bb, positive_features, i)
            hdf5.set("Positives-%s-%.5f" % (part,scale), positive_features)
            total_positives += len(positives)
      # cd backwards after each image
      hdf5.cd("..")

    hdf5.set("TotalPositives", total_positives)
    hdf5.set("TotalNegatives", total_negatives)