Classifying CNNs as just a form of preprocessing is not a legitimate view IMO. The breakthrough with CNNs is that they provide stronger gradient signals because

Classifying CNNs as just a form of preprocessing is not a legitimate view IMO. The breakthrough with CNNs is that they provide stronger gradient signals because of weight sharing. Essentially, CNNs made the first deep neural networks possible, they were trainable in areas where plain fully connected layers were not.