FaceRecognizer
==============

.. highlight:: cpp

FaceRecognizer
--------------

.. ocv:class:: FaceRecognizer : public Algorithm

All face recognition models in OpenCV are derived from the abstract base class :ocv:class:`FaceRecognizer`, which provides
a unified access to all face recongition algorithms in OpenCV. ::

  class FaceRecognizer : public Algorithm
  {
  public:
      //! virtual destructor
      virtual ~FaceRecognizer() {}

      // Trains a FaceRecognizer.
      virtual void train(InputArray src, InputArray labels) = 0;

      // Updates a FaceRecognizer.
      virtual void update(InputArrayOfArrays src, InputArray labels);

      // Gets a prediction from a FaceRecognizer.
      virtual int predict(InputArray src) const = 0;

      // Predicts the label and confidence for a given sample.
      virtual void predict(InputArray src, int &label, double &confidence) const = 0;

      // Serializes this object to a given filename.
      virtual void save(const string& filename) const;

      // Deserializes this object from a given filename.
      virtual void load(const string& filename);

      // Serializes this object to a given cv::FileStorage.
      virtual void save(FileStorage& fs) const = 0;

      // Deserializes this object from a given cv::FileStorage.
      virtual void load(const FileStorage& fs) = 0;
  };


Description
+++++++++++

I'll go a bit more into detail explaining :ocv:class:`FaceRecognizer`, because it doesn't look like a powerful interface at first sight. But: Every :ocv:class:`FaceRecognizer` is an :ocv:class:`Algorithm`, so you can easily get/set all model internals (if allowed by the implementation). :ocv:class:`Algorithm` is a relatively new OpenCV concept, which is available since the 2.4 release. I suggest you take a look at its description.

:ocv:class:`Algorithm` provides the following features for all derived classes:

* So called “virtual constructor”. That is, each Algorithm derivative is registered at program start and you can get the list of registered algorithms and create instance of a particular algorithm by its name (see :ocv:func:`Algorithm::create`). If you plan to add your own algorithms, it is good practice to add a unique prefix to your algorithms to distinguish them from other algorithms.

* Setting/Retrieving algorithm parameters by name. If you used video capturing functionality from OpenCV highgui module, you are probably familar with :ocv:cfunc:`cvSetCaptureProperty`, :ocv:cfunc:`cvGetCaptureProperty`, :ocv:func:`VideoCapture::set` and :ocv:func:`VideoCapture::get`. :ocv:class:`Algorithm` provides similar method where instead of integer id's you specify the parameter names as text strings. See :ocv:func:`Algorithm::set` and :ocv:func:`Algorithm::get` for details.

* Reading and writing parameters from/to XML or YAML files. Every Algorithm derivative can store all its parameters and then read them back. There is no need to re-implement it each time.

Moreover every :ocv:class:`FaceRecognizer` supports the:

* **Training** of a :ocv:class:`FaceRecognizer` with :ocv:func:`FaceRecognizer::train` on a given set of images (your face database!).

* **Prediction** of a given sample image, that means a face. The image is given as a :ocv:class:`Mat`.

* **Loading/Saving** the model state from/to a given XML or YAML.

Setting the Thresholds
+++++++++++++++++++++++

Sometimes you run into the situation, when you want to apply a threshold on the prediction. A common scenario in face recognition is to tell, wether a face belongs to the training dataset or if it is unknown. You might wonder, why there's no public API in :ocv:class:`FaceRecognizer` to set the threshold for the prediction, but rest assured: It's supported. It just means there's no generic way in an abstract class to provide an interface for setting/getting the thresholds of *every possible* :ocv:class:`FaceRecognizer` algorithm. The appropriate place to set the thresholds is in the constructor of the specific :ocv:class:`FaceRecognizer` and since every :ocv:class:`FaceRecognizer` is a :ocv:class:`Algorithm` (see above), you can get/set the thresholds at runtime!

Here is an example of setting a threshold for the Eigenfaces method, when creating the model:

.. code-block:: cpp

    // Let's say we want to keep 10 Eigenfaces and have a threshold value of 10.0
    int num_components = 10;
    double threshold = 10.0;
    // Then if you want to have a cv::FaceRecognizer with a confidence threshold,
    // create the concrete implementation with the appropiate parameters:
    Ptr<FaceRecognizer> model = createEigenFaceRecognizer(num_components, threshold);

Sometimes it's impossible to train the model, just to experiment with threshold values. Thanks to :ocv:class:`Algorithm` it's possible to set internal model thresholds during runtime. Let's see how we would set/get the prediction for the Eigenface model, we've created above:

.. code-block:: cpp

    // The following line reads the threshold from the Eigenfaces model:
    double current_threshold = model->getDouble("threshold");
    // And this line sets the threshold to 0.0:
    model->set("threshold", 0.0);

If you've set the threshold to ``0.0`` as we did above, then:

.. code-block:: cpp

    //
    Mat img = imread("person1/3.jpg", CV_LOAD_IMAGE_GRAYSCALE);
    // Get a prediction from the model. Note: We've set a threshold of 0.0 above,
    // since the distance is almost always larger than 0.0, you'll get -1 as
    // label, which indicates, this face is unknown
    int predicted_label = model->predict(img);
    // ...

is going to yield ``-1`` as predicted label, which states this face is unknown.

Getting the name of a FaceRecognizer
+++++++++++++++++++++++++++++++++++++

Since every :ocv:class:`FaceRecognizer` is a :ocv:class:`Algorithm`, you can use :ocv:func:`Algorithm::name` to get the name of a :ocv:class:`FaceRecognizer`:

.. code-block:: cpp

    // Create a FaceRecognizer:
    Ptr<FaceRecognizer> model = createEigenFaceRecognizer();
    // And here's how to get its name:
    std::string name = model->name();


FaceRecognizer::train
---------------------

Trains a FaceRecognizer with given data and associated labels.

.. ocv:function:: void FaceRecognizer::train( InputArrayOfArrays src, InputArray labels ) = 0

    :param src: The training images, that means the faces you want to learn. The data has to be given as a ``vector<Mat>``.

    :param labels: The labels corresponding to the images have to be given either as a ``vector<int>`` or a

The following source code snippet shows you how to learn a Fisherfaces model on a given set of images. The images are read with :ocv:func:`imread` and pushed into a ``std::vector<Mat>``. The labels of each image are stored within a ``std::vector<int>`` (you could also use a :ocv:class:`Mat` of type `CV_32SC1`). Think of the label as the subject (the person) this image belongs to, so same subjects (persons) should have the same label. For the available :ocv:class:`FaceRecognizer` you don't have to pay any attention to the order of the labels, just make sure same persons have the same label:

.. code-block:: cpp

    // holds images and labels
    vector<Mat> images;
    vector<int> labels;
    // images for first person
    images.push_back(imread("person0/0.jpg", CV_LOAD_IMAGE_GRAYSCALE)); labels.push_back(0);
    images.push_back(imread("person0/1.jpg", CV_LOAD_IMAGE_GRAYSCALE)); labels.push_back(0);
    images.push_back(imread("person0/2.jpg", CV_LOAD_IMAGE_GRAYSCALE)); labels.push_back(0);
    // images for second person
    images.push_back(imread("person1/0.jpg", CV_LOAD_IMAGE_GRAYSCALE)); labels.push_back(1);
    images.push_back(imread("person1/1.jpg", CV_LOAD_IMAGE_GRAYSCALE)); labels.push_back(1);
    images.push_back(imread("person1/2.jpg", CV_LOAD_IMAGE_GRAYSCALE)); labels.push_back(1);

Now that you have read some images, we can create a new :ocv:class:`FaceRecognizer`. In this example I'll create a Fisherfaces model and decide to keep all of the possible Fisherfaces:

.. code-block:: cpp

    // Create a new Fisherfaces model and retain all available Fisherfaces,
    // this is the most common usage of this specific FaceRecognizer:
    //
    Ptr<FaceRecognizer> model =  createFisherFaceRecognizer();

And finally train it on the given dataset (the face images and labels):

.. code-block:: cpp

    // This is the common interface to train all of the available cv::FaceRecognizer
    // implementations:
    //
    model->train(images, labels);

FaceRecognizer::update
----------------------

Updates a FaceRecognizer with given data and associated labels.

.. ocv:function:: void FaceRecognizer::update( InputArrayOfArrays src, InputArray labels )

    :param src: The training images, that means the faces you want to learn. The data has to be given as a ``vector<Mat>``.

    :param labels: The labels corresponding to the images have to be given either as a ``vector<int>`` or a

This method updates a (probably trained) :ocv:class:`FaceRecognizer`, but only if the algorithm supports it. The Local Binary Patterns Histograms (LBPH) recognizer (see :ocv:func:`createLBPHFaceRecognizer`) can be updated. For the Eigenfaces and Fisherfaces method, this is algorithmically not possible and you have to re-estimate the model with :ocv:func:`FaceRecognizer::train`. In any case, a call to train empties the existing model and learns a new model, while update does not delete any model data.

.. code-block:: cpp

    // Create a new LBPH model (it can be updated) and use the default parameters,
    // this is the most common usage of this specific FaceRecognizer:
    //
    Ptr<FaceRecognizer> model =  createLBPHFaceRecognizer();
    // This is the common interface to train all of the available cv::FaceRecognizer
    // implementations:
    //
    model->train(images, labels);
    // Some containers to hold new image:
    vector<Mat> newImages;
    vector<int> newLabels;
    // You should add some images to the containers:
    //
    // ...
    //
    // Now updating the model is as easy as calling:
    model->update(newImages,newLabels);
    // This will preserve the old model data and extend the existing model
    // with the new features extracted from newImages!

Calling update on an Eigenfaces model (see :ocv:func:`createEigenFaceRecognizer`), which doesn't support updating, will throw an error similar to:

.. code-block:: none

    OpenCV Error: The function/feature is not implemented (This FaceRecognizer (FaceRecognizer.Eigenfaces) does not support updating, you have to use FaceRecognizer::train to update it.) in update, file /home/philipp/git/opencv/modules/contrib/src/facerec.cpp, line 305
    terminate called after throwing an instance of 'cv::Exception'

Please note: The :ocv:class:`FaceRecognizer` does not store your training images, because this would be very memory intense and it's not the responsibility of te :ocv:class:`FaceRecognizer` to do so. The caller is responsible for maintaining the dataset, he want to work with.

FaceRecognizer::predict
-----------------------

.. ocv:function:: int FaceRecognizer::predict( InputArray src ) const = 0
.. ocv:function:: void FaceRecognizer::predict( InputArray src, int & label, double & confidence ) const = 0

    Predicts a label and associated confidence (e.g. distance) for a given input image.

    :param src: Sample image to get a prediction from.
    :param label: The predicted label for the given image.
    :param confidence: Associated confidence (e.g. distance) for the predicted label.

The suffix ``const`` means that prediction does not affect the internal model
state, so the method can be safely called from within different threads.

The following example shows how to get a prediction from a trained model:

.. code-block:: cpp

    using namespace cv;
    // Do your initialization here (create the cv::FaceRecognizer model) ...
    // ...
    // Read in a sample image:
    Mat img = imread("person1/3.jpg", CV_LOAD_IMAGE_GRAYSCALE);
    // And get a prediction from the cv::FaceRecognizer:
    int predicted = model->predict(img);

Or to get a prediction and the associated confidence (e.g. distance):

.. code-block:: cpp

    using namespace cv;
    // Do your initialization here (create the cv::FaceRecognizer model) ...
    // ...
    Mat img = imread("person1/3.jpg", CV_LOAD_IMAGE_GRAYSCALE);
    // Some variables for the predicted label and associated confidence (e.g. distance):
    int predicted_label = -1;
    double predicted_confidence = 0.0;
    // Get the prediction and associated confidence from the model
    model->predict(img, predicted_label, predicted_confidence);

FaceRecognizer::save
--------------------

Saves a :ocv:class:`FaceRecognizer` and its model state.

.. ocv:function:: void FaceRecognizer::save(const string& filename) const

    Saves this model to a given filename, either as XML or YAML.

    :param filename: The filename to store this :ocv:class:`FaceRecognizer` to (either XML/YAML).

.. ocv:function:: void FaceRecognizer::save(FileStorage& fs) const

    Saves this model to a given :ocv:class:`FileStorage`.

    :param fs: The :ocv:class:`FileStorage` to store this :ocv:class:`FaceRecognizer` to.


Every :ocv:class:`FaceRecognizer` overwrites ``FaceRecognizer::save(FileStorage& fs)``
to save the internal model state. ``FaceRecognizer::save(const string& filename)`` saves
the state of a model to the given filename.

The suffix ``const`` means that prediction does not affect the internal model
state, so the method can be safely called from within different threads.

FaceRecognizer::load
--------------------

Loads a :ocv:class:`FaceRecognizer` and its model state.

.. ocv:function:: void FaceRecognizer::load( const string& filename )
.. ocv:function:: void FaceRecognizer::load( const FileStorage& fs ) = 0

Loads a persisted model and state from a given XML or YAML file . Every
:ocv:class:`FaceRecognizer` has to overwrite ``FaceRecognizer::load(FileStorage& fs)``
to enable loading the model state. ``FaceRecognizer::load(FileStorage& fs)`` in
turn gets called by ``FaceRecognizer::load(const string& filename)``, to ease
saving a model.

createEigenFaceRecognizer
-------------------------

.. ocv:function:: Ptr<FaceRecognizer> createEigenFaceRecognizer(int num_components = 0, double threshold = DBL_MAX)

    :param num_components: The number of components (read: Eigenfaces) kept for this Prinicpal Component Analysis. As a hint: There's no rule how many components (read: Eigenfaces) should be kept for good reconstruction capabilities. It is based on your input data, so experiment with the number. Keeping 80 components should almost always be sufficient.

    :param threshold: The threshold applied in the prediciton.

Notes:
++++++

* Training and prediction must be done on grayscale images, use :ocv:func:`cvtColor` to convert between the color spaces.
* **THE EIGENFACES METHOD MAKES THE ASSUMPTION, THAT THE TRAINING AND TEST IMAGES ARE OF EQUAL SIZE.** (caps-lock, because I got so many mails asking for this). You have to make sure your input data has the correct shape, else a meaningful exception is thrown. Use :ocv:func:`resize` to resize the images.
* This model does not support updating.

Model internal data:
++++++++++++++++++++

* ``num_components`` see :ocv:func:`createEigenFaceRecognizer`.
* ``threshold`` see :ocv:func:`createEigenFaceRecognizer`.
* ``eigenvalues`` The eigenvalues for this Principal Component Analysis (ordered descending).
* ``eigenvectors`` The eigenvectors for this Principal Component Analysis (ordered by their eigenvalue).
* ``mean`` The sample mean calculated from the training data.
* ``projections`` The projections of the training data.
* ``labels`` The threshold applied in the prediction. If the distance to the nearest neighbor is larger than the threshold, this method returns -1.

createFisherFaceRecognizer
--------------------------

.. ocv:function:: Ptr<FaceRecognizer> createFisherFaceRecognizer(int num_components = 0, double threshold = DBL_MAX)

    :param num_components: The number of components (read: Fisherfaces) kept for this Linear Discriminant Analysis with the Fisherfaces criterion. It's useful to keep all components, that means the number of your classes ``c`` (read: subjects, persons you want to recognize). If you leave this at the default (``0``) or set it to a value  less-equal ``0`` or greater ``(c-1)``, it will be set to the correct number ``(c-1)`` automatically.

    :param threshold: The threshold applied in the prediction. If the distance to the nearest neighbor is larger than the threshold, this method returns -1.

Notes:
++++++

* Training and prediction must be done on grayscale images, use :ocv:func:`cvtColor` to convert between the color spaces.
* **THE FISHERFACES METHOD MAKES THE ASSUMPTION, THAT THE TRAINING AND TEST IMAGES ARE OF EQUAL SIZE.** (caps-lock, because I got so many mails asking for this). You have to make sure your input data has the correct shape, else a meaningful exception is thrown. Use :ocv:func:`resize` to resize the images.
* This model does not support updating.

Model internal data:
++++++++++++++++++++

* ``num_components`` see :ocv:func:`createFisherFaceRecognizer`.
* ``threshold`` see :ocv:func:`createFisherFaceRecognizer`.
* ``eigenvalues`` The eigenvalues for this Linear Discriminant Analysis (ordered descending).
* ``eigenvectors`` The eigenvectors for this Linear Discriminant Analysis (ordered by their eigenvalue).
* ``mean`` The sample mean calculated from the training data.
* ``projections`` The projections of the training data.
* ``labels`` The labels corresponding to the projections.


createLBPHFaceRecognizer
-------------------------

.. ocv:function:: Ptr<FaceRecognizer> createLBPHFaceRecognizer(int radius=1, int neighbors=8, int grid_x=8, int grid_y=8, double threshold = DBL_MAX)

    :param radius: The radius used for building the Circular Local Binary Pattern. The greater the radius, the
    :param neighbors: The number of sample points to build a Circular Local Binary Pattern from. An appropriate value is to use `` 8`` sample points. Keep in mind: the more sample points you include, the higher the computational cost.
    :param grid_x: The number of cells in the horizontal direction, ``8`` is a common value used in publications. The more cells, the finer the grid, the higher the dimensionality of the resulting feature vector.
    :param grid_y: The number of cells in the vertical direction, ``8`` is a common value used in publications. The more cells, the finer the grid, the higher the dimensionality of the resulting feature vector.
    :param threshold: The threshold applied in the prediction. If the distance to the nearest neighbor is larger than the threshold, this method returns -1.

Notes:
++++++

* The Circular Local Binary Patterns (used in training and prediction) expect the data given as grayscale images, use :ocv:func:`cvtColor` to convert between the color spaces.
* This model supports updating.

Model internal data:
++++++++++++++++++++

* ``radius`` see :ocv:func:`createLBPHFaceRecognizer`.
* ``neighbors`` see :ocv:func:`createLBPHFaceRecognizer`.
* ``grid_x`` see :ocv:func:`createLBPHFaceRecognizer`.
* ``grid_y`` see :ocv:func:`createLBPHFaceRecognizer`.
* ``threshold`` see :ocv:func:`createLBPHFaceRecognizer`.
* ``histograms`` Local Binary Patterns Histograms calculated from the given training data (empty if none was given).
* ``labels`` Labels corresponding to the calculated Local Binary Patterns Histograms.
