OpenCV 3.0 Can't load neural network

I need to use a neural network in my OpenCV (version 3.0) project. I've created and trained neural network and it works, but if I want to load neural network from YML file, it doesn't predict.

This is a code where I creat, train and save my neural network:

FileStorage fs("nn.yml", FileStorage::WRITE);

int input_neurons = 7;
int hidden_neurons = 100;
int output_neurons = 5;

Ptr<TrainData> train_data = TrainData::loadFromCSV("data.csv", 10, 7, 12);

Ptr<ANN_MLP> neural_network = ANN_MLP::create();
neural_network->setTrainMethod(ANN_MLP::BACKPROP);
neural_network->setBackpropMomentumScale(0.1);
neural_network->setBackpropWeightScale(0.05);
neural_network->setTermCriteria(TermCriteria(TermCriteria::MAX_ITER, (int)10000, 1e-6));

Mat layers = Mat(3, 1, CV_32SC1);
layers.row(0) = Scalar(input_neurons);
layers.row(1) = Scalar(hidden_neurons);
layers.row(2) = Scalar(output_neurons);

neural_network->setLayerSizes(layers);
neural_network->setActivationFunction(ANN_MLP::SIGMOID_SYM, 1, 1); 

neural_network->train(train_data);

if (neural_network->isTrained()) { 
    neural_network->write(fs);
    cout << "It's OK!" << endl; 
}

But next time, if I want to load it from YML file:

Ptr<ANN_MLP> neural_network = Algorithm::load<ANN_MLP>("nn.yml", "neural_network");

I get the output:

[-1.#IND, -1.#IND, -1.#IND, -1.#IND, -1.#IND]

[-1.#IND, 1.0263158, 1.0263158, 1.0263158, 1.0263158]

[1.0263158, 1.0263158, 1.0263158, 1.0263158, 1.0263158]

[-1.#IND, -1.#IND, -1.#IND, -1.#IND, -1.#IND]

Ptr<ANN_MLP> neural_network = Algorithm::load<ANN_MLP>("nn.yml");

This line cause that I get an error:

OpenCV Error: Unspecified error (The node is neither a map nor an empty collecti on) in cvGetFileNodeByName, file C:\builds\master_PackSlave-win64-vc12-shared\op encv\modules\core\src\persistence.cpp, line 739

What am I doing wrong? Where is the problem?

Solution

You can use save and load, or write and read, but you shouldn't mix them.

So you either need to do:

// Save
neural_network->save("nn.yml");

// Load
Ptr<ANN_MLP> nn = Algorithm::load<ANN_MLP>("nn.yml");

or:

// Write
neural_network->write(fs);

// Read
FileStorage ffs("nn.yml", FileStorage::READ);
Ptr<ANN_MLP> nn = Algorithm::read<ANN_MLP>(ffs.root());