1. A computer implemented method for learning part-based object models from a set of digital representations of images, the method comprising: receiving a set of digital representations of images, each image having at least one object; for each image: extracting one or more shape features and appearance features from the image; generating one or more shape models of the object based on the shape features, a shape model corresponding to a part of the object; computing an appearance model for each shape model of the object based on the appearance features; selecting one or more shape models and appearance models as reference shape models and appearance models of the object; and storing the reference shape models and appearance models of the images.