1. A computer-readable medium carrying one or more sequences of instructions, wherein execution of the one or more sequences of instructions by one or more processors causes the one or more processors to form an object model based on a set of training images by performing the steps comprising: forming an interim object model based at least in part on the set of training images; for each object image in a set of images selected from the set of training images, generating an object model estimate by fitting the object image to the interim object model; and calculating a fitting error between the object image and the object model estimate; and responsive to identifying a subset of training images having associated fitting errors that are larger than an error threshold value, balancing the set of training images based on the associated fitting errors; and forming the object model based on the balanced set of training images.