.. py:currentmodule:: bob.ip.facedetect

.. testsetup:: *

   from __future__ import print_function
   import bob.io.base
   import bob.io.base.test_utils
   import bob.io.image
   import bob.ip.facedetect
   import math

   face_image = bob.io.base.load(bob.io.base.test_utils.datafile('testimage.jpg', 'bob.ip.facedetect'))

=====================================
 Face Detection using Python and Bob
=====================================

As in most modern face detectors, we also apply a cascaded classifier for detecting faces.
In this package, we provide a pre-trained classifier for upright frontal faces, but the cascade can be re-trained using own data.


Face Detection
--------------

The most simple face detection task is to detect a single face in an image.
This task can be achieved using a single command:

.. doctest::

   >>> face_image = bob.io.base.load('testimage.jpg') # doctest: +SKIP
   >>> bounding_box, quality = bob.ip.facedetect.detect_single_face(face_image)
   >>> print (quality, bounding_box.topleft, bounding_box.size)
   39.209601948 (110, 82) (224, 187)

.. plot:: plot/detect_single_face.py
   :include-source: False

As you can see, the bounding box is **not** square as for other face detectors, but has an aspect ratio of :math:`5:6`.
Also, for each detection we provide a ``quality`` value, which specifies, how good the detection is, see :ref:`several` on how to use this ``quality`` value to differentiate between faces and non-faces.

The function :py:func:`detect_single_face` has several optional parameters with proper default values.
The first optional parameter specifies the :py:class:`bob.ip.facedetect.Cascade`, which contains the classifier cascade.
We will see later, how this cascade can be re-trained.
The second parameter is the sampler, which is explained in more detail in the following section :ref:`sampling`.

The ``minimum_overlap`` parameter defines the minimum overlap that patches of multiple detections of the same face might have.
If set to ``1`` (or ``None``), only the bounding box of the best detection is returned, while smaller values will compute the average over more detection, which usually makes the detection more stable.
The related ``relative_prediction_threshold`` parameter defines, which of the bounding boxes to account during averaging, see :py:func:`average_detections`.

.. _sampling:

Sampling
========

The :py:class:`Sampler` defines how the image is scanned.
The ``scale_factor`` (a value between 0.5 and 1) defines, in which scale granularity the image is scanned.
For higher scale factors like the default :math:`2^{-1/16}` many scales are tested and the detection time is increased.
For lower scale factors like :math:`2^{-1/4}`, fewer scales are tested, which might reduce the stability of the detection.

The ``distance`` parameter defines the distance in pixel units between two tested bounding boxes.
A lower distance improves stability, but needs more time.
Anyways, distances higher than 4 pixels are not recommended.

The ``lowest_scale`` parameter defines the size of the smallest bounding box, relative to the size of the image.
For example, for a given image of resolution :math:`640\times480` and a ``lowest_scale = 0.125`` (the default), the smallest detected face would be 60 (i.e. 480*0.125) pixels high.
Theoretically, this parameter might be set to ``None``, for which **all** possible scales are extracted, but this is not recommended.

Finally, the sampler has a given ``patch_size``, which is tightly connected to the cascade and should not be changed.

The :py:class:`Sampler` can return an `iterator` of bounding boxes that will be tested:

.. doctest::

   >>> sampler = bob.ip.facedetect.Sampler(scale_factor=math.pow(2., -1./4.), distance=2, lowest_scale = 0.125)
   >>> patches = list(sampler.sample(face_image))
   >>> print (face_image.shape)
   (3, 531, 354)
   >>> print (patches[0].topleft, patches[0].size)
   (0, 0) (357, 298)
   >>> print (patches[-1].topleft, patches[-1].size)
   (463, 300) (63, 53)
   >>> print (len(patches))
   14493

.. _several:

Detecting Several Faces
=======================

As you can see, there are a lot a lot of patches in different locations and scales that might contain faces.
In fact, when given an image with several faces, you might want to get the bounding boxes for all faces at once.
The classifiers in the cascade do not only provide a decision if a given patch contains a face, but it also returns a `quality` value.
For the pre-trained cascade, this quality value lies approximately between -100 and +100.
Higher values indicate that there is a face, while patches with smaller values usually contain background.

To extract all faces in a given image, the function :py:func:`detect_all_faces` requires that this threshold is given as well:

.. doctest::

   >>> bounding_boxes, qualities = bob.ip.facedetect.detect_all_faces(face_image, threshold=20, overlaps=1)
   >>> for i in range(len(bounding_boxes)):
   ...   print ("%3.4f"%qualities[i], bounding_boxes[i].topleft, bounding_boxes[i].size)
   39.9663 (110, 82) (224, 187)
   24.7024 (264, 192) (72, 60)
   22.6990 (379, 128) (117, 97)

The returned list of detected bounding boxes are sorted according to the quality values.
The detections are grouped using the :py:func:`group_detections`.
All groups that have less entries as the given number of ``overlaps`` are discarded, where the default value ``1`` will not discard any group.
Finally, each group is averaged by :py:func:`average_detections`.
Again, ``cascade``, ``sampler`` and ``minimum_overlap`` can be specified to the function.

.. note::
   The strategy for merging overlapping detections differ between the two detection functions.
   While :py:func:`detect_single_face` uses :py:func:`best_detection` to merge detections, :py:func:`overlapping_detections` simply uses :py:func:`group_detections` to keep only the detection with the highest quality in the overlapping area.
   The difference between :py:func:`overlapping_detections` and :py:func:`group_detections` is that the former uses only the bounding boxes that overlap with **the best detection**, while the latter first groups the detections, so that the **best group average** can be computed.


Iterating over the Sampler
==========================

In case you want to implement your own strategy of merging overlapping bounding boxes, you can simply get the detection qualities for all sampled patches.

.. note::
   For the low level functions, only gray-scale images are supported.

.. doctest::

   >>> cascade = bob.ip.facedetect.default_cascade()
   >>> gray_image = bob.ip.color.rgb_to_gray(face_image)
   >>> for quality, patch in sampler.iterate_cascade(cascade, gray_image):
   ...   if quality > 40:
   ...     print ("%3.4f"%quality, patch.topleft, patch.size)
   48.9983 (84, 84) (253, 210)
   51.7809 (105, 63) (253, 210)
   56.5325 (105, 84) (253, 210)
   47.9453 (106, 88) (212, 177)
   40.3316 (124, 71) (212, 177)
   43.7717 (134, 104) (179, 149)

As you can see, most of the patches with high quality values overlap.


Using the Command line
======================

Finally, we have developed a script, namely ``./bin/detect_faces.py``, which integrates most of the above functionality.
Given an image, the script will detect one or more faces in it, and display the bounding boxes around them.
When the script is run using default parameters, it will detect just the face in the image that comes with the highest confidence, as the result of :py:func:`detect_single_face` would do.

.. note::
   We are using `matplotlib.pyplot.imshow <http://matplotlib.org/api/pyplot_api.html#matplotlib.pyplot.imshow` to display the resulting image.
   We are aware that in some cases, no display shows up.
   In these cases, please try to change the display setup of matplotlib (which isn't easy, I have to admit), or use the ``--write-detection`` parameter to write the result to an image file, and inspect the image with your preferred application.

.. note::
   Each line of the bounding box is displayed as a single row.
   When your image resolution is too high, you might not be able to see the lines.
   Please zoom into the image to increase the visibility of the lines.

However, most of the parameters of the :py:class:`Sampler` that were discussed above, can be specified on command line such as:

* ``--distance`` : The distance between two offsets. Lower values will increase detection probability, but slow down detection speed.
* ``--scale-factor`` : The (logarithmic) distance between two tested scales. Must be in range ``]0, 1[``. Higher values (closer to 1) will increase detection probability, but slow down detection speed.
* ``--lowest-scale`` : The lowest image scale (relative to the image resolution), in which faces are detected. Must be in range ``[0,1]``. Lower values will slow down detection speed.
* ``--best-detection-overlap`` : If given, the bounding box is merged using several overlapping detections, where the given value specifies the minimum Jaccard :py:meth:`BoundingBox.similarity` value (which must be in range ``]0,1[``) between the bounding boxes that take part in the merging process. A good value for this parameter is ``0.2``.

Also, parameters to change the nature of the displayed results can be changed.
When the ``--prediction-threshold`` parameter is present, many bounding boxes will be displayed, where the color ranges from black (the lowest) to red (the highest prediction value):

* ``--prediction-threshold`` : Displays all detected bounding boxes that have a prediction value greater than the specified value. The lower the value, the more bounding boxes will be displayed. Good values (for the default cascade) might range in ``[20, 50]``.
* ``--prune-detections`` : Prunes the detected bounding boxes by eliminating all overlapping bounding boxes and keeping only the non-overlapping ones with the highest prediction values. The given parameter, again, specifies the amount of Jaccard :py:meth:`BoundingBox.similarity` for which two bounding boxes are considered to overlap. Anything in range ``[0,1]`` will work.

  .. note::
     For large images or very tight sampling, the pruning process might take a while, as the implementation currently in in :math:`O(N^2)` with :math:`N` being the number of bounding boxes.

Finally, when you have trained your own cascade, you can specify it using the ``--cascade-file`` parameter.
How to train your own face detection cascade is described in the next section.


.. _retrain_detector:

Retrain the Detector
--------------------

As previously mentioned, there is a pre-trained classifier cascade included into this package.
However, this classifier is trained only to detect frontal or close-to-frontal upright faces, but no rotated or profile faces -- or even other objects.
Nevertheless, it is possible to train a cascade for your detection task.

Training Data
=============

The first thing that the cascade training requires is training data -- the more the better.
To ease the collection of positive and negative training data, a script ``./bin/collect_training_data.py`` is provided.
This script has several options:

- ``--image-directory``: This directory is scanned for images with the given ``--image-extension``, and all found images are considered.
- ``--output-file``: The file which will contain the information at the end.

To train the detector, both positive and negative training data needs to be present.
Positive data is defined by annotations of the images, which can be translated into bounding boxes.
E.g., for frontal facial images, bounding boxes can be defined by the eye coordinates (see :py:func:`bounding_box_from_annotation`) or directly by specifying the top-left and bottom-right coordinate.
There are two different ways, how annotations can be read.
One way is to read annotations from annotation file using the :py:func:`read_annotation_file` function, which can read various types of annotations.
To use this function, simply specify the command line options for the ``./bin/collect_training_data.py`` script:

- ``--annotation-directory``: For each image in the ``--image-directory``, an annotation file with the given ``--annotation-extension`` needs to be available in this directory.
- ``--annotation-type``: The way how annotations are stored in the annotation files (see :py:func:`read_annotation_file`).

The second way is to use one of our database interfaces (see https://gitlab.idiap.ch/bob/bob/wikis/Packages), which have annotations stored internally:

- ``--database``: The name of the database, e.g. ``banca`` for the ``bob.db.banca`` interface.
- ``--protocols``: If specified, only the images from these database protocols are used.
- ``--groups``: Images from these groups are used; by default, only the ``world`` group is used for training, but also ``dev`` and ``eval`` might be included.

Usually, it is also useful to include databases which do not contain target images at all.
For these, obviously, no annotations are required/available.
Hence, for pure background image databases, use the option:

- ``--no-annotations``

For example, to collect training data from three different databases, you could call:

.. code-block:: sh

   $ ./bin/collect_training_data.py --image-directory <...>/Yale-B/data --image-extension .pgm --annotation-directory <...>/Yale-B/annotations --annotation-type named --output-file Yale-B.txt
   $ ./bin/collect_training_data.py --database xm2vts --image-directory <...>/xm2vtsdb/images --protocols lp1 lp2 darkened-lp1 darkened-lp2 --groups world dev eval --output-file XM2VTS.txt
   $ ./bin/collect_training_data.py --image-directory <...>/FDHD-background/data --image-extension .jpeg --no-annotations --output-file FDHD.txt

The first scans the ``Yale-B/data`` directory for ``.pgm`` images and the ``Yale-B/annotations`` directory for annotations of the ``named`` type, the second uses the ``bob.db.xm2vts`` interface to collect images, whereas the third collects only background ``.jpeg`` data from the ``FDHD-background/data`` directory.

Training Feature Extraction
===========================

Training the classifier is split into two steps.
First, the ``./bin/extract_training_features.py`` can be used to extracted training features from a list of database files as generated by the ``./bin/collect_training_data.py`` script.
Again, several options can be selected:

- ``--file-lists``: The file lists to process
- ``--feature-directory``: A directory, where extracted features will be stored; this directory should be able to store several 100 GB of data
- ``--patch-size``: The size of the patches that should be extracted from the images; the default ``(24,20)`` has shown to be large enough
- ``--no-mirror-samples``: Turn off the horizontally mirroring of the sample images, which is enabled by default


Since the detector will use the :py:class:`Sampler` to extract image patches, we follow a similar approach to generate training data.
A sampler is used to iterate over the training images and extract image patches.
Depending on the overlap of the image patches, they are considered as positive or negative samples, or they are ignored, i.e., when the overlap has a value between the:

- ``--similarity-thresholds``: The upper bound to accept patches as negative and the lower bound to accept patches as positive training samples
- ``--distance``: The distance to scan the image with, see `Sampling`_.
- ``--lowest-scale``: The lowest image scale to scan, see `Sampling`_
- ``--scale-base``: The scale factor between two scales to scan, see Sampling_

Since this sampling strategy would end up with a **huge** amount of negative samples, there are two options to limit them:

- ``--negative-examples-every``: limits the number of scales, from which negative examples are extracted
- ``--examples-per-image-scale``: limits the number of positive and negative examples for each image scale

Now, the type of LBP features that are extracted have to be defined.
Usually, LBP features in all possible sizes and aspect ratios that fit into the given ``--patch-size`` are generated.
Several options can be used to select a conglomerate of different kinds of LBP feature extractors, for more information please refer to [Atanasoaei2012]_:

- ``--lbp-variant``: Specifies LBP variants; a combination of several variants is possible, the single variants are:

  * ``ell``: circular LBP
  * ``u2``: uniform LBP
  * ``ri``: rotation invariant LBP
  * ``mct``: MCT codes (compare to the average instead of to the central bit)
  * ``dir``: Direction coded LBP
  * ``tran``: Transitional LBP

- ``--lbp-multi-block``: Use multi-block LBP (averaging over several pixels) instead of simple LBP features
- ``--lbp-overlap``: Should multi-block LBP overlap or not

- ``--lbp-square``: Limit the LBP sizes to square sizes, no rectangular LBPs will be extracted.
- ``--lbp-scale``: Do not generate all possible LBP feature sizes, but only one in the given size.

Interestingly, already a quite limited number of different LBP feature extractors might be sufficient.
For example, the pre-trained cascade uses the following options:

.. code-block:: sh

   $ ./bin/extract_training_features.py --file-lists Yale-B.txt XM2VTS.txt FDHD.txt ... --lbp-scale 1 --lbp-variant mct

Finally, there ``--parallel`` option can be used to run the feature extraction in parallel.
Particularly, in combination with the `GridTK <https://pypi.python.org/pypi/gridtk>`_, processing can be speed up tremendously:

.. code-block:: sh

   $ ./bin/jman submit --parallel 64  -- ./bin/extract_training_features.py ... --parallel 64


Cascade Training
================

To finally train the face detector cascade, the ``./bin/train_detector.py`` script is provided.
This script reads the training features as extracted by the ``./bin/extract_training_features.py`` script and generates a regular boosted cascade of weak classifiers.
Again, the script has several options:

- ``--feature-directory``: Reads all features from the given directory.
- ``--trained-file``: The cascade that will be generated.

The training is done in several bootstrapping rounds.
In the first round, a strong classifier is generated from randomly selected 5000 positive and 5000 negative samples.
After 8 weak classifiers have been selected, **all** remaining samples are classified with the current boosted machine.
Those 5000 positive and 5000 negative samples that are misclassified most strongly are added to the training samples.
A new bootstrapping round starts, which now selects 8*2 = 16 weak classifiers, until the 7th round has selected 512 weak classifiers.

These numbers can be modified on command line with the command line options:

- ``--bootstrapping-rounds``: Select the number of rounds of bootstrapping.
- ``--features-in-first-round``: The number of weak classifiers selected in the first round; will be doubled in each successive round.
- ``--training-examples``: The number of training examples to add for each round.

Finally, a regular cascade is created, which will reject patches with a value below the threshold -5 after each 25 weak classifiers are evaluated.
These numbers can be changed using the options:

- ``--classifiers-per-round``: The number of classifiers for each cascade step.
- ``--cascade-threshold``: The threshold, below which patches should be rejected (the same threshold for each cascade step).

This package also provides a script ``./bin/validate_cascade.py`` to automatically adapt the steps and thresholds of the cascade based on a validation set.
However, but the use of this script is not encouraged since I couldn't yet come up if a proper default configuration.

The Shipped Cascade
===================

For completeness it is worth mentioning that the default pre-trained cascade was trained on the following databases:

- BANCA: sets french, spanish and english (for the latter, we used the world set only)
- MOBIO: the world set of the hand-labeled images
- XM2VTS: all images of all protocols
- CMU-PIE: all images of all protocols
- MIT-CMU: training partition only
- MASH: all images of all protocols
- CINEMA: all images of all protocols
- Yale-B: all images of all protocols
- FDHD-background: background images without faces
- CalTech-background: background images without faces

Feature extraction was performed using a single scale MCT, as:

.. code-block:: sh

   $ ./bin/extract_training_features.py -vv --lbp-scale 1 --lbp-variant mct --negative-examples-every 1 --filelists [ALL of ABOVE]

Finally, the cascade training used default parameters:

.. code-block:: sh

  $ ./bin/extract_training_features.py -vv
