In many applications of digital signal processing – such as speech recognition and computer vision – the essential objective is to distinguish objects of interest, such as words or faces. This can be very challenging in real-world situations where objects of interest are distorted (e.g., a person is speaking with an accent, or a face is turned at an angle from the camera) or obscured (for example, a voice is competing with background noise, or a face is partially covered by a hand).