Writing a new Algorithm

Although Essentia comes pre-loaded with a considerable list of algorithms, we imagine that you will want to write your own algorithms at some point. By following the next steps, you should be able to roll up your own algorithm in very little time.

As an example, you should have a look at the implementation of the Centroid algorithm, which you can find in the src/algorithms/stats/centroid.h and the src/algorithms/stats/centroid.cpp files.

Detailed steps for the creation of a new Algorithm

Declaring basic information

To write a new Algorithm, you will have to inherit from the essentia::Algorithm class. The first thing that you will want to do is to give your algorithm a name and a description. There are two fields specifically dedicated to this, which you need to fill in, otherwise it will be impossible for you to register your algorithm in the factory (it will give you a compilation error if you try):

// in your header
class MyAlgo : public Algorithm {
  static const char* name;
  static const char* description;
};

// in your source file
const char* MyAlgo::name = "MyAlgo";
const char* MyAlgo::description = DOC("This is my new algorithm that does lots of stuff");

Note that the description should be surrounded by the DOC() directive.

Declaring/registering your inputs and outputs

To be able to pass data to/from your algorithm, you will then need to declare what goes in and what goes out. These will take the form of any number of Inputs and Outputs, which can be of any type you want. You will also have to assign names to each one of your input and output. To do this, you will have to declare in your derived class member variables that are wrapped by the Input<> and Output<> wrappers, and register them in the constructor.

For instance, if you have an input that is an audio frame (represented using a std::vector<Real>) and your output is the high-frequency coefficient (HFC) (represented using a Real), you would:

  1. declare the following as member variables of your class:

    Input<std::vector<Real> > _audioFrame;
    Output<Real> _hfc;
    
  2. register them in the constructor (otherwise they won’t be visible), with their (compulsory) description:

    MyAlgorithm::MyAlgorithm() {
      declareInput(_audioFrame, "audio", "the input audio frame");
      declareOutput(_hfc, "hfc", "the resulting high-frequency coefficient");
    }
    

    The names that you use when registering the inputs/outputs are the names that will later be used to access them.

Declaring needed parameters

There is a function that you will always have to fill-in, which is the void declareParameters() method. This is used to tell what are the parameters that are used by this algorithm, so that it is able to make sure you will get all the parameters you need. For this, you will need to declare the names and description of the parameters you are expecting, as well as an optional default value. There are 2 methods you can use in order to do this:

void declareParameter(const std::string& name, const std::string& description);
void declareParameter(const std::string& name, const std::string& description, const Parameter& default_value);

The first version is to declare a single Parameter, the second one does the same but also specifies a default value. An example of the declareParameters() method follows:

void declareParameters() const {
  declareParameter("sampleRate", "the sampling rate of the analyzed track");
  declareParameter("nbCoeffs", "the number of coefficients to be output", 12);
  declareParameter("floatParam", "a random floating point parameter", 23.8);
  declareParameter("windowType", "the type of window used before doing the FFT", "Hann");
}

Note: the Parameter type is sort-of dynamic, so when specifying a default value, you can do it using its native type (i.e., int, float, string, …) as the conversion to the Parameter type is done automatically.

The configure() method

This method (with no parameters) will be called each time the object is configured. This is intended if you have some setup actions to do before starting to process that you only want to be done once (e.g., setting planes in an FFT, preparing cos&sin tables for MFCC computation, etc…). As a rule of thumb, you can (and should) initialize everything you can in the constructor (i.e., when not knowing any parameters) and initializes the rest of it (that is dependent on parameters) in the configure method.

You will be given as input a ParameterMap containing all of the parameters that you declared using the declareParameters statements.

Checking if the object is configured

For user convenience and more consistency, the configure() method will be called with default values upon creation, so that the object is always configured. If you declared some parameters in your declareParameters() method which do not have default values, when creating the object there will be some parameters missing upon entering the configure() method. You should make sure in that case that configure() still returns correctly and that the object is not in an invalid state.

The reset method

When doing batch computation (i.e., multiples files/sounds in a row), it might be usefull (or necessary) to reinitialize your algorithm between different files if it keeps a state of itself. This is the purpose of the reset() method.

Note: most of the people won’t need to use this, as the descriptor calculation won’t have any state.

The compute method

This is the main entry point for your Algorithm. It is the generic function that is used to tell an Algorithm to compute the things it is supposed to. This method will be called once the inputs and outputs are set. Basically the first thing you will want to do is get the inputs and outputs into local variables and then do your processing. This is done through the get() method that is defined for both the Input<> and Output<> classes, and it returns a reference to the type they are wrapping. Inputs are const references, Outputs are non-const references, so you can write to them.

Example:

Input<vector<Real> > _audio;
const vector<Real>& audioVector = _audio.get();

Output<string> _label;
string& genreLabel = _label.get();

Notice that genreLabel is not const, so that you can write to it, ie:

genreLabel = "Electro";

Another way to write your compute() method (and if the parameterless way of calling it disturbs you), is to write your function in the ‘classic’ way, passing the inputs as arguments to the function call, and then wrapping this call with the parameterless compute() method.

Example:

void compute() {
  // inputs and parameters
  const vector<Real>& array = _array.get()
  Real frequencyRange = parameter("frequencyRange").asReal();

  // output
  Real& centroid = _centroid.get();

  // do the actual work
  centroid = centroid_function(array, frequencyRange);
}

Real centroid_function(const vector<Real>& array, Real frequencyRange) {
  // your implementation here
}

Note: make sure that when using get, you always use references (&), and not a copy, otherwise

  1. your outputs won’t be stored
  2. you’ll be making unnecessary copies of your inputs, which can considerably slow down the execution time.

Here are some examples that you can have a look at to get you started: