A computer system obtains a digital 3-dimensional model of a body surface, such as a human face. Portions of the surface correspond to reference points in the model. The system obtains image data of a personal care device in use as well as image data of the surface on which the model is based. The system calculates a relative location of the device relative to the reference points based on the image data of the surface. The system determines a true position of the device with respect to the surface based on the image data of the device in use, known physical dimensions of the device, and its relative location, even when the device is partially occluded from view. The system may, based on the true position of the device, cause device to perform actions such as application of a cosmetic or administration of a skin therapy.