In accordance with one or more aspects, a plane in a 3D coordinate system in which a 3D model is to be generated based on one or more 2D images is identified. A direction of extrusion for the plane is also identified. Additionally, a user identification of a region of interest on a 2D image is received and projected onto the plane. A location in the 3D model of the region of interest is then automatically identified by extruding the plane along the direction of extrusion until the region of interest in the plane matches a corresponding region of at least one of the one or more 2D images.