1. A method performed by data processing apparatus, the method comprising: identifying a set of images for a root node of a decision tree, visual characteristics of each image being represented by image feature values for the image, the set of images including one or more positive images that have been deemed to include a particular object and one or more negative images that have been deemed to not include the particular object; identifying, for each of a plurality of locations in one or more positive images from the set of images, image filters, the image filter for each location representing visual features of the location in positive images; determining, for each of two or more image locations, a positive location feature score and a negative location feature score, the positive location feature score being determined based on a measure of similarity between the image filter and the positive image feature values for each of two or more different positive images, the negative location feature score being determined based on a measure of similarity between the image filter and the negative image feature values for each of two or more different negative images; identifying a first distinctive location from the two or more image locations, the first distinctive location being identified based on a difference between the positive location feature score at the first distinctive location and the negative location feature score at the first distinctive location meeting a difference threshold; and selecting a first set of distinguishing feature values for identifying the particular object based on image feature values for the first distinctive location.