1. A method, comprising: using a computer to implement: determining, from a single image obtained from a single real-world camera and as a result of manipulating one or more geometric primitives superimposed over the single image, one or more camera parameters of the single real-world camera from which the image was obtained such that a projection of a computer-generated three-dimensional model, which includes the one or more geometric primitives, through the determined camera parameters approximates the projection of a real-world object depicted in the image as seen through the real-world camera, wherein the one or more camera parameters include focal length, wherein the one or more camera parameters are refined based upon manipulating the one or more geometric primitives, wherein the one or more geometric primitives are three-dimensional objects, and wherein the single image is obtained from a scanner, or from memory.