  Understanding objects in terms of their individual parts is important,
because it enables a precise understanding of the objects' geometrical
structure, and enhances object recognition when the object is seen in a novel
pose or under partial occlusion. However, the manual annotation of parts in
large scale datasets is time consuming and expensive. In this paper, we aim at
discovering object parts in an unsupervised manner, i.e., without ground-truth
part or keypoint annotations. Our approach builds on the intuition that objects
of the same class in a similar pose should have their parts aligned at similar
spatial locations. We exploit the property that neural network features are
largely invariant to nuisance variables and the main remaining source of
variations between images of the same object category is the object pose.
Specifically, given a training image, we find a set of similar images that show
instances of the same object category in the same pose, through an affine
alignment of their corresponding feature maps. The average of the aligned
feature maps serves as pseudo ground-truth annotation for a supervised training
of the deep network backbone. During inference, part detection is simple and
fast, without any extra modules or overheads other than a feed-forward neural
network. Our experiments on several datasets from different domains verify the
effectiveness of the proposed method. For example, we achieve 37.8 mAP on
VehiclePart, which is at least 4.2 better than previous methods.
