cv::face::FaceRecognizer Class Reference

Abstract base class for all face recognition models. More...

#include <opencv2/face.hpp>

Inheritance diagram for cv::face::FaceRecognizer:

Public Member Functions
virtual bool	empty () const CV_OVERRIDE=0
virtual String	getLabelInfo (int label) const
	Gets string information by label.
virtual std::vector< int >	getLabelsByString (const String &str) const
	Gets vector of labels by string.
virtual double	getThreshold () const =0
	threshold parameter accessor - required for default BestMinDist collector
int	predict (InputArray src) const
void	predict (InputArray src, int &label, double &confidence) const
	Predicts a label and associated confidence (e.g. distance) for a given input image.
virtual void	predict (InputArray src, Ptr< PredictCollector > collector) const =0
	if implemented - send all result of prediction to collector that can be used for somehow custom result handling
virtual void	read (const FileNode &fn) CV_OVERRIDE=0
virtual void	read (const String &filename)
	Loads a FaceRecognizer and its model state.
virtual void	setLabelInfo (int label, const String &strInfo)
	Sets string info for the specified model's label.
virtual void	setThreshold (double val)=0
	Sets threshold of model.
virtual void	train (InputArrayOfArrays src, InputArray labels)=0
	Trains a FaceRecognizer with given data and associated labels.
virtual void	update (InputArrayOfArrays src, InputArray labels)
	Updates a FaceRecognizer with given data and associated labels.
virtual void	write (const String &filename) const
	Saves a FaceRecognizer and its model state.
virtual void	write (FileStorage &fs) const CV_OVERRIDE=0
Public Member Functions inherited from cv::Algorithm
	Algorithm ()
virtual	~Algorithm ()
virtual void	clear ()
	Clears the algorithm state.
virtual bool	empty () const
	Returns true if the Algorithm is empty (e.g. in the very beginning or after unsuccessful read.
virtual String	getDefaultName () const
virtual void	read (const FileNode &fn)
	Reads algorithm parameters from a file storage.
virtual void	save (const String &filename) const
void	write (const Ptr< FileStorage > &fs, const String &name=String()) const
virtual void	write (FileStorage &fs) const
	Stores algorithm parameters in a file storage.
void	write (FileStorage &fs, const String &name) const

Protected Attributes
std::map< int, String >	_labelsInfo

Additional Inherited Members
Static Public Member Functions inherited from cv::Algorithm
template<typename _Tp >
static Ptr< _Tp >	load (const String &filename, const String &objname=String())
	Loads algorithm from the file.
template<typename _Tp >
static Ptr< _Tp >	loadFromString (const String &strModel, const String &objname=String())
	Loads algorithm from a String.
template<typename _Tp >
static Ptr< _Tp >	read (const FileNode &fn)
	Reads algorithm from the file node.
Protected Member Functions inherited from cv::Algorithm
void	writeFormat (FileStorage &fs) const

Detailed Description

Abstract base class for all face recognition models.

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

Description

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

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 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 cv::cvSetCaptureProperty, ocvcvGetCaptureProperty, VideoCapture::set and VideoCapture::get. Algorithm provides similar method where instead of integer id's you specify the parameter names as text Strings. See Algorithm::set and 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 FaceRecognizer supports the:

• Training of a FaceRecognizer with 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 Mat.
• Loading/Saving the model state from/to a given XML or YAML.
• Setting/Getting labels info, that is stored as a string. String labels info is useful for keeping names of the recognized people.

Note

When using the FaceRecognizer interface in combination with Python, please stick to Python 2. Some underlying scripts like create_csv will not work in other versions, like Python 3. 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, whether a face belongs to the training dataset or if it is unknown. You might wonder, why there's no public API in 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 FaceRecognizer algorithm. The appropriate place to set the thresholds is in the constructor of the specific FaceRecognizer and since every FaceRecognizer is a 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:

// 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 appropriate parameters:
Ptr<FaceRecognizer> model = EigenFaceRecognizer::create(num_components, threshold);

Sometimes it's impossible to train the model, just to experiment with threshold values. Thanks to 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:

// 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:

//
Mat img = imread("person1/3.jpg", IMREAD_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 FaceRecognizer is a Algorithm, you can use Algorithm::name to get the name of a FaceRecognizer:

// Create a FaceRecognizer:
Ptr<FaceRecognizer> model = EigenFaceRecognizer::create();
// And here's how to get its name:
String name = model->name();

Member Function Documentation

empty()

virtual bool cv::face::FaceRecognizer::empty ( ) const

pure virtual

This is an overloaded member function, provided for convenience. It differs from the above function only in what argument(s) it accepts.

Reimplemented from cv::Algorithm.

Implemented in cv::face::BasicFaceRecognizer.

getLabelInfo()

virtual String cv::face::FaceRecognizer::getLabelInfo ( int  label ) const

virtual

Gets string information by label.

If an unknown label id is provided or there is no label information associated with the specified label id the method returns an empty string.

getLabelsByString()

virtual std::vector< int > cv::face::FaceRecognizer::getLabelsByString ( const String &  str ) const

virtual

Gets vector of labels by string.

The function searches for the labels containing the specified sub-string in the associated string info.

getThreshold()

virtual double cv::face::FaceRecognizer::getThreshold ( ) const

pure virtual

threshold parameter accessor - required for default BestMinDist collector

Implemented in cv::face::BasicFaceRecognizer, and cv::face::LBPHFaceRecognizer.

predict() [1/3]

int cv::face::FaceRecognizer::predict

(

InputArray

src

)

const

This is an overloaded member function, provided for convenience. It differs from the above function only in what argument(s) it accepts.

predict() [2/3]

void cv::face::FaceRecognizer::predict	(	InputArray	src,
		int &	label,
		double &	confidence
	)		const

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

Parameters

src	Sample image to get a prediction from.
label	The predicted label for the given image.
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:

using namespace cv;
// Do your initialization here (create the cv::FaceRecognizer model) ...
// ...
// Read in a sample image:
Mat img = imread("person1/3.jpg", IMREAD_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):

using namespace cv;
// Do your initialization here (create the cv::FaceRecognizer model) ...
// ...
Mat img = imread("person1/3.jpg", IMREAD_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);

predict() [3/3]

virtual void cv::face::FaceRecognizer::predict ( InputArray  src, Ptr< PredictCollector >  collector  ) const

pure virtual

• if implemented - send all result of prediction to collector that can be used for somehow custom result handling

Parameters

src	Sample image to get a prediction from.
collector	User-defined collector object that accepts all results

To implement this method u just have to do same internal cycle as in predict(InputArray src, CV_OUT int &label, CV_OUT double &confidence) but not try to get "best@ result, just resend it to caller side with given collector

read() [1/2]

virtual void cv::face::FaceRecognizer::read ( const FileNode &  fn )

pure virtual

This is an overloaded member function, provided for convenience. It differs from the above function only in what argument(s) it accepts.

Reimplemented from cv::Algorithm.

Implemented in cv::face::BasicFaceRecognizer, and cv::face::BasicFaceRecognizer.

read() [2/2]

virtual void cv::face::FaceRecognizer::read ( const String &  filename )

virtual

Loads a FaceRecognizer and its model state.

Loads a persisted model and state from a given XML or YAML file . Every 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.

Reimplemented in cv::face::BasicFaceRecognizer.

setLabelInfo()

virtual void cv::face::FaceRecognizer::setLabelInfo ( int  label, const String &  strInfo  )

virtual

Sets string info for the specified model's label.

The string info is replaced by the provided value if it was set before for the specified label.

setThreshold()

virtual void cv::face::FaceRecognizer::setThreshold ( double  val )

pure virtual

Sets threshold of model.

Implemented in cv::face::BasicFaceRecognizer, and cv::face::LBPHFaceRecognizer.

train()

virtual void cv::face::FaceRecognizer::train ( InputArrayOfArrays  src, InputArray  labels  )

pure virtual

Trains a FaceRecognizer with given data and associated labels.

Parameters

src	The training images, that means the faces you want to learn. The data has to be given as a vector<Mat>.
labels	The labels corresponding to the images have to be given either as a vector<int> or a Mat of type CV_32SC1.

The following source code snippet shows you how to learn a Fisherfaces model on a given set of images. The images are read with 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 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 FaceRecognizer you don't have to pay any attention to the order of the labels, just make sure same persons have the same label:

// holds images and labels
vector<Mat> images;
vector<int> labels;
// using Mat of type CV_32SC1
// Mat labels(number_of_samples, 1, CV_32SC1);
// images for first person
images.push_back(imread("person0/0.jpg", IMREAD_GRAYSCALE)); labels.push_back(0);
images.push_back(imread("person0/1.jpg", IMREAD_GRAYSCALE)); labels.push_back(0);
images.push_back(imread("person0/2.jpg", IMREAD_GRAYSCALE)); labels.push_back(0);
// images for second person
images.push_back(imread("person1/0.jpg", IMREAD_GRAYSCALE)); labels.push_back(1);
images.push_back(imread("person1/1.jpg", IMREAD_GRAYSCALE)); labels.push_back(1);
images.push_back(imread("person1/2.jpg", IMREAD_GRAYSCALE)); labels.push_back(1);

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

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

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

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

update()

virtual void cv::face::FaceRecognizer::update ( InputArrayOfArrays  src, InputArray  labels  )

virtual

Updates a FaceRecognizer with given data and associated labels.

Parameters

src	The training images, that means the faces you want to learn. The data has to be given as a vector<Mat>.
labels	The labels corresponding to the images have to be given either as a vector<int> or a Mat of type CV_32SC1.

This method updates a (probably trained) FaceRecognizer, but only if the algorithm supports it. The Local Binary Patterns Histograms (LBPH) recognizer (see createLBPHFaceRecognizer) can be updated. For the Eigenfaces and Fisherfaces method, this is algorithmically not possible and you have to re-estimate the model with 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.

// 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 =  LBPHFaceRecognizer::create();
// 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 EigenFaceRecognizer::create), which doesn't support updating, will throw an error similar to:

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'

Note

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

write() [1/2]

virtual void cv::face::FaceRecognizer::write ( const String &  filename ) const

virtual

Saves a FaceRecognizer and its model state.

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

Parameters

filename

The filename to store this FaceRecognizer to (either XML/YAML).

Every 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.

Reimplemented in cv::face::BasicFaceRecognizer.

write() [2/2]

virtual void cv::face::FaceRecognizer::write ( FileStorage &  fs ) const

pure virtual

This is an overloaded member function, provided for convenience. It differs from the above function only in what argument(s) it accepts. Saves this model to a given FileStorage.

Parameters

fs	The FileStorage to store this FaceRecognizer to.

Reimplemented from cv::Algorithm.

Implemented in cv::face::BasicFaceRecognizer, and cv::face::BasicFaceRecognizer.

Member Data Documentation

_labelsInfo

std::map<int, String> cv::face::FaceRecognizer::_labelsInfo

protected

---

The documentation for this class was generated from the following file:

• opencv2/face.hpp