1. A method for rigid registration of ear impression models, comprising: extracting a canal region from an undetailed ear impression model, the undetailed ear impression model representing an undetailed surface of an ear canal and outer ear geometry; extracting a canal region from a detailed ear impression model, the detailed ear impression model representing a detailed surface of the ear canal; generating an orientation histogram for the canal region of the undetailed ear impression model and an orientation histogram for the canal region of the detailed ear impression model; performing a rotational alignment between the orientation histograms; computing a translational shift between the extracted canal regions after performing the rotational alignment; and performing a registration between the undetailed and detailed ear impression models after computing the translational shift, wherein performing a registration between the undetailed and detailed ear impression models comprises: performing a ray cast surface distance measure to identify cloud points of the detailed ear impression model that are near a tip of the canal region of the detailed ear impression model and cloud points of the detailed ear impression model that do not closely correspond to cloud points of the undetailed ear impression model; representing the cloud points of the undetailed ear impression model in a k-dimensional (KD)-Tree data structure; and performing an iterative closest point (ICP) algorithm on the cloud points of the undetailed ear impression model in the KD-Tree data structure and on the cloud points of the detailed ear impression model except the cloud points of the detailed ear impression model identified by the ray cast surface distance measure.