experiment_focal_loss.py

# Source: https://github.com/mkocabas/focal-loss-keras
# Usage: model_prn.compile(optimizer=optimizer, loss=[focal_loss(alpha=.25, gamma=2)])

import tensorflow as tf
from keras import backend as K
'''
Compatible with tensorflow backend
'''
def focal_loss(gamma=2., alpha=.25):
    def focal_loss_fixed(y_true, y_pred):
        pt_1 = tf.where(tf.equal(y_true, 1), y_pred, tf.ones_like(y_pred))
        pt_0 = tf.where(tf.equal(y_true, 0), y_pred, tf.zeros_like(y_pred))
        return -K.sum(alpha * K.pow(1. - pt_1, gamma) * K.log(pt_1))-K.sum((1-alpha) * K.pow( pt_0, gamma) * K.log(1. - pt_0))
    return focal_loss_fixed