Multi-class Focal Loss in Keras

focalloss.py

from keras import backend as K


def focal_loss(gamma=2):
    def loss(y_true, y_pred):
        epsilon = K.epsilon()
        y_pred = K.clip(y_pred, epsilon, 1 - epsilon)
        pt = K.max(y_true * y_pred, axis=-1)
        batch_loss = -K.pow((1 - pt), gamma) * K.log(pt)
        return K.mean(batch_loss)
    return loss