data_generator.py

from keras.preprocessing.image import ImageDataGenerator

class FlippedImageDataGenerator(ImageDataGenerator):
    flip_indices = [
        (0, 2), (1, 3),
        (4, 8), (5, 9), (6, 10), (7, 11),
        (12, 16), (13, 17), (14, 18), (15, 19),
        (22, 24), (23, 25),
        ]

    def next(self):
        X_batch, y_batch = super(FlippedImageDataGenerator, self).next()
        batch_size = X_batch.shape[0]
        indices = np.random.choice(batch_size, batch_size/2, replace=False)
        X_batch[indices] = X_batch[indices, :, :, ::-1]

        if y_batch is not None:
            # x座標をフリップ
            y_batch[indices, ::2] = y_batch[indices, ::2] * -1

            # left_eye_center_x -> right_eye_center_x のようにフリップ
            for a, b in self.flip_indices:
                y_batch[indices, a], y_batch[indices, b] = (
                    y_batch[indices, b], y_batch[indices, a]
                )

        return X_batch, y_batch