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310 lines
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ReStructuredText
310 lines
16 KiB
ReStructuredText
FaceRecognizer
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==============
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.. highlight:: cpp
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FaceRecognizer
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--------------
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.. ocv:class:: FaceRecognizer
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All face recognition models in OpenCV are derived from the abstract base class :ocv:class:`FaceRecognizer`, which provides
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a unified access to all face recongition algorithms in OpenCV. ::
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class FaceRecognizer : public Algorithm
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{
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public:
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//! virtual destructor
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virtual ~FaceRecognizer() {}
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// Trains a FaceRecognizer.
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virtual void train(InputArray src, InputArray labels) = 0;
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// Gets a prediction from a FaceRecognizer.
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virtual int predict(InputArray src) const = 0;
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// Predicts the label and confidence for a given sample.
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virtual void predict(InputArray src, int &label, double &confidence) const = 0;
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// Serializes this object to a given filename.
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virtual void save(const string& filename) const;
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// Deserializes this object from a given filename.
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virtual void load(const string& filename);
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// Serializes this object to a given cv::FileStorage.
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virtual void save(FileStorage& fs) const = 0;
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// Deserializes this object from a given cv::FileStorage.
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virtual void load(const FileStorage& fs) = 0;
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};
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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.
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:ocv:class:`Algorithm` provides the following features for all derived classes:
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* 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.
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* 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.
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* 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.
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Moreover every :ocv:class:`FaceRecognizer` supports the:
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* **Training** of a :ocv:class:`FaceRecognizer` with :ocv:func:`FaceRecognizer::train` on a given set of images (your face database!).
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* **Prediction** of a given sample image, that means a face. The image is given as a :ocv:class:`Mat`.
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* **Loading/Saving** the model state from/to a given XML or YAML.
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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!
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Here is an example of setting a threshold for the Eigenfaces method, when creating the model:
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.. code-block:: cpp
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// Let's say we want to keep 10 Eigenfaces and have a threshold value of 10.0
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int num_components = 10;
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double threshold = 10.0;
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// Then if you want to have a cv::FaceRecognizer with a confidence threshold,
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// create the concrete implementation with the appropiate parameters:
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Ptr<FaceRecognizer> model = createEigenFaceRecognizer(num_components, threshold);
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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:
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.. code-block:: cpp
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// The following line reads the threshold from the Eigenfaces model:
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double current_threshold = model->getDouble("threshold");
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// And this line sets the threshold to 0.0:
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model->set("threshold", 0.0);
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If you've set the threshold to ``0.0`` as we did above, then:
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.. code-block:: cpp
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//
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Mat img = imread("person1/3.jpg", CV_LOAD_IMAGE_GRAYSCALE);
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// Get a prediction from the model. Note: We've set a threshold of 0.0 above,
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// since the distance is almost always larger than 0.0, you'll get -1 as
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// label, which indicates, this face is unknown
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int predicted_label = model->predict(img);
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// ...
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is going to yield ``-1`` as predicted label, which states this face is unknown.
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FaceRecognizer::train
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---------------------
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Trains a FaceRecognizer with given data and associated labels.
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.. ocv:function:: void FaceRecognizer::train(InputArray src, InputArray labels)
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:param src: The training images, that means the faces you want to learn. The data has to be given as a ``vector<Mat>``.
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:param labels: The labels corresponding to the images have to be given either as a ``vector<int>`` or a
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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:
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.. code-block:: cpp
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// holds images and labels
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vector<Mat> images;
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vector<int> labels;
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// images for first person
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images.push_back(imread("person0/0.jpg", CV_LOAD_IMAGE_GRAYSCALE)); labels.push_back(0);
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images.push_back(imread("person0/1.jpg", CV_LOAD_IMAGE_GRAYSCALE)); labels.push_back(0);
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images.push_back(imread("person0/2.jpg", CV_LOAD_IMAGE_GRAYSCALE)); labels.push_back(0);
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// images for second person
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images.push_back(imread("person1/0.jpg", CV_LOAD_IMAGE_GRAYSCALE)); labels.push_back(1);
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images.push_back(imread("person1/1.jpg", CV_LOAD_IMAGE_GRAYSCALE)); labels.push_back(1);
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images.push_back(imread("person1/2.jpg", CV_LOAD_IMAGE_GRAYSCALE)); labels.push_back(1);
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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:
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.. code-block:: cpp
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// Create a new Fisherfaces model and retain all available Fisherfaces,
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// this is the most common usage of this specific FaceRecognizer:
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//
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Ptr<FaceRecognizer> model = createFisherFaceRecognizer();
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And finally train it on the given dataset (the face images and labels):
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.. code-block:: cpp
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// This is the common interface to train all of the available cv::FaceRecognizer
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// implementations:
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//
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model->train(images, labels);
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FaceRecognizer::predict
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-----------------------
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.. ocv:function:: int FaceRecognizer::predict(InputArray src) const
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Predicts a label for a given input image.
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:param src: Sample image to get a prediction from.
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.. ocv:function:: void predict(InputArray src, int &label, double &confidence) const
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Predicts a label and associated confidence (e.g. distance) for a given input image.
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:param src: Sample image to get a prediction from.
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:param label: The predicted label for the given image.
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:param confidence: Associated confidence (e.g. distance) for the predicted label.
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The suffix ``const`` means that prediction does not affect the internal model
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state, so the method can be safely called from within different threads.
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The following example shows how to get a prediction from a trained model:
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.. code-block:: cpp
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using namespace cv;
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// Do your initialization here (create the cv::FaceRecognizer model) ...
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// ...
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// Read in a sample image:
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Mat img = imread("person1/3.jpg", CV_LOAD_IMAGE_GRAYSCALE);
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// And get a prediction from the cv::FaceRecognizer:
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int predicted = model->predict(img);
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Or to get a prediction and the associated confidence (e.g. distance):
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.. code-block:: cpp
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using namespace cv;
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// Do your initialization here (create the cv::FaceRecognizer model) ...
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// ...
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Mat img = imread("person1/3.jpg", CV_LOAD_IMAGE_GRAYSCALE);
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// Some variables for the predicted label and associated confidence (e.g. distance):
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int predicted_label = -1;
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double predicted_confidence = 0.0;
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// Get the prediction and associated confidence from the model
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model->predict(img, predicted_label, predicted_confidence);
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FaceRecognizer::save
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--------------------
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Saves a :ocv:class:`FaceRecognizer` and its model state.
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.. ocv:function:: void FaceRecognizer::save(const string& filename) const
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Saves this model to a given filename, either as XML or YAML.
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:param filename: The filename to store this :ocv:class:`FaceRecognizer` to (either XML/YAML).
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.. ocv:function:: void FaceRecognizer::save(FileStorage& fs) const
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Saves this model to a given :ocv:class:`FileStorage`.
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:param fs: The :ocv:class:`FileStorage` to store this :ocv:class:`FaceRecognizer` to.
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Every :ocv:class:`FaceRecognizer` overwrites ``FaceRecognizer::save(FileStorage& fs)``
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to save the internal model state. ``FaceRecognizer::save(const string& filename)`` saves
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the state of a model to the given filename.
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The suffix ``const`` means that prediction does not affect the internal model
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state, so the method can be safely called from within different threads.
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FaceRecognizer::load
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--------------------
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Loads a :ocv:class:`FaceRecognizer` and its model state.
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.. ocv:function:: void FaceRecognizer::load(const string& filename)
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.. ocv:function:: void FaceRecognizer::load(FileStorage& fs)
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Loads a persisted model and state from a given XML or YAML file . Every
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:ocv:class:`FaceRecognizer` has to overwrite ``FaceRecognizer::load(FileStorage& fs)``
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to enable loading the model state. ``FaceRecognizer::load(FileStorage& fs)`` in
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turn gets called by ``FaceRecognizer::load(const string& filename)``, to ease
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saving a model.
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createEigenFaceRecognizer
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-------------------------
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.. ocv:function:: Ptr<FaceRecognizer> createEigenFaceRecognizer(int num_components = 0, double threshold = DBL_MAX)
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: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.
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:param threshold: The threshold applied in the prediciton.
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Notes:
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++++++
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* Training and prediction must be done on grayscale images, use :ocv:func:`cvtColor` to convert between the color spaces.
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* **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.
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Model internal data:
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++++++++++++++++++++
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* ``num_components`` see :ocv:func:`createEigenFaceRecognizer`.
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* ``threshold`` see :ocv:func:`createEigenFaceRecognizer`.
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* ``eigenvalues`` The eigenvalues for this Principal Component Analysis (ordered descending).
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* ``eigenvectors`` The eigenvectors for this Principal Component Analysis (ordered by their eigenvalue).
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* ``mean`` The sample mean calculated from the training data.
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* ``projections`` The projections of the training data.
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* ``labels`` The threshold applied in the prediction. If the distance to the nearest neighbor is larger than the threshold, this method returns -1.
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createFisherFaceRecognizer
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-------------------------
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.. ocv:function:: Ptr<FaceRecognizer> createFisherFaceRecognizer(int num_components = 0, double threshold = DBL_MAX)
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: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.
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:param threshold: The threshold applied in the prediction. If the distance to the nearest neighbor is larger than the threshold, this method returns -1.
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Notes:
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++++++
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* Training and prediction must be done on grayscale images, use :ocv:func:`cvtColor` to convert between the color spaces.
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* **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.
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Model internal data:
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++++++++++++++++++++
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* ``num_components`` see :ocv:func:`createFisherFaceRecognizer`.
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* ``threshold`` see :ocv:func:`createFisherFaceRecognizer`.
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* ``eigenvalues`` The eigenvalues for this Linear Discriminant Analysis (ordered descending).
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* ``eigenvectors`` The eigenvectors for this Linear Discriminant Analysis (ordered by their eigenvalue).
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* ``mean`` The sample mean calculated from the training data.
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* ``projections`` The projections of the training data.
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* ``labels`` The labels corresponding to the projections.
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createLBPHFaceRecognizer
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-------------------------
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.. ocv:function:: Ptr<FaceRecognizer> createLBPHFaceRecognizer(int radius=1, int neighbors=8, int grid_x=8, int grid_y=8, double threshold = DBL_MAX);
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:param radius: The radius used for building the Circular Local Binary Pattern. The greater the radius, the
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: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.
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: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.
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: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.
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:param threshold: The threshold applied in the prediction. If the distance to the nearest neighbor is larger than the threshold, this method returns -1.
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Notes:
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++++++
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* 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.
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Model internal data:
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++++++++++++++++++++
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* ``radius`` see :ocv:func:`createLBPHFaceRecognizer`.
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* ``neighbors`` see :ocv:func:`createLBPHFaceRecognizer`.
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* ``grid_x`` see :ocv:func:`createLBPHFaceRecognizer`.
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* ``grid_y`` see :ocv:func:`createLBPHFaceRecognizer`.
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* ``threshold see :ocv:func:`createLBPHFaceRecognizer`.``
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* ``histograms`` Local Binary Patterns Histograms calculated from the given training data (empty if none was given).
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* ``labels`` Labels corresponding to the calculated Local Binary Patterns Histograms.
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