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; // 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; }; 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. 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 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. FaceRecognizer::train --------------------- Trains a FaceRecognizer with given data and associated labels. .. ocv:function:: void FaceRecognizer::train(InputArray 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``. :param labels: The labels corresponding to the images have to be given either as a ``vector`` 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``. The labels of each image are stored within a ``std::vector`` (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 images; vector 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 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::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 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. 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 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. 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 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. 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.