1. A computing device comprising: one or more input interfaces for receiving a sketch; one or more processors; and a computer-readable storage media storing instructions, which when executed by the one or more processors, cause the one or more processors to generate a model for identifying a subject of the sketch: performing an image-based search of a collection of images to identify a set of images with shape features similar to those of the sketch; filtering the set of images into a set of filtered images by removing images that have more than one subject, each image of the set of filtered images associated with at least one text-based label; deriving a shape feature for each image of the set of filtered images; generating a plurality of shape topics based on the shape features of the set of filtered images; generating a plurality of object topics based on the shape topics and associating each of the object topics with at least one of the plurality of shape topics; deriving a word distribution for each of the object topics from the text-based labels; identifying a word descriptive of the subject of the sketch from at least one of the word distributions; and applying the word as a tag to the sketch.