The Design of Facial Expression Detection System to Determine the Level of Customer Satisfaction using K-Nearest Neighbor MethodDiana Rahmawati, Koko Joni, Muhammad Iqbal Maulana, Rosida Vivin Nahari, Achmad Fiqhi Ibadillah, Heri Setiawan and Adi Kurniawan SaputroMATEC Web Conf., 372 (2022) 06002DOI: https://doi.org/10.1051/matecconf/202237206002