1. A method comprising: obtaining two-dimensional image data of a surface of a subject using a two-dimensional imager; obtaining volumetric data representing both an internal and external structure of the subject using a volumetric scanner, wherein the volumetric scanner is separate from the two-dimensional imager, wherein the two-dimensional imager is placed at a user-adjustable physical position relative to the volumetric scanner; registering the coordinate of the volumetric data with the coordinate of the two-dimensional image data based on the user-adjustable physical position of the two-dimensional imager relative to the volumetric scanner; setting an opacity threshold of the volumetric data to render an external surface of the subject; extending an array ray from a view point of the two-dimensional image data until the array ray encounters the external surface of the subject that has an opacity level higher than the opacity threshold; mapping the two-dimensional image data to the external surface of the volumetric data based on the extended array ray; generating display image data comprising the two-dimensional image data and the volumetric data; and visualizing the external surface of the volumetric data of the subject combined with color texture of the two-dimensional image data without generating external surface mesh data.