Examples of intra-personal and extra-personal difference images. In the top row, two different images of the same person are subtracted, yielding the difference image at the far right that captures intra-personal variation. In the bottom row, two images of two different individuals are subtracted, yielding an extra-personal difference image. In the model, these two classes of difference image are used to create separate face-spaces so that new difference images can be analyzed probabilistically.