1. One or more computer storage media having stored thereon multiple instructions that, when executed by one or more processors, cause the one or more processors to: identify a plane in a 3D coordinate system in which a 3D model is to be generated based on one or more 2D images; identify a direction of extrusion for the plane; receive a user identification of a region of interest on a 2D image; and automatically identify a location in the 3D model of the region of interest 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.