1. A non-transitory computer storage medium comprising computer-useable instructions that, when used by one or more computing devices, cause the one or more computing devices to perform operations comprising: identifying a letterform of a character image, the letterform being selected from a set of letterforms defined by a model; determining locations of keypoints on the character image based on a set of model keypoints defined on the letterform in the model; determining a detail shape at each of one or more areas of the character image corresponding with one or more detail areas defined for the letterform in the model; and generating a visual descriptor for the character image that contains information regarding the letterform, the locations of the keypoints, and the one or more detail shapes.