1. A method comprising: obtaining two-dimensional image data of a surface of the subject using a two-dimensional imager; obtaining volumetric data representing both an internal and external structure of the subject using a volumetric scanner; setting an opacity threshold of the volumetric data to render an external surface of the subject without generating external surface mesh data; extending an array ray from a view point of the two-dimensional image data to the external surface of the subject, wherein the external surface of the subject has an opacity level higher than the opacity threshold; mapping the two-dimensional image data to the external surface of the subject based on the extended array ray; and generating combined image data comprising the two-dimensional image data and the volumetric data.