1. A method of generating a 3D model from an image using a computer system, said image being an image of physical objects, said method comprising: displaying said image; creating a primitive, said primitive representing at least a portion of a first physical object of said physical objects, said primitive having a set of parameters defining a size and location of said primitive in said 3D model, said set of parameters causing parts of said primitive to correspond to a first set of locations on said image, said first set of locations corresponding to a first approximate location of said first physical object in said image; modifying said set of parameters to cause said parts of said primitive to correspond to a second set of locations on said image; and, displaying a 2D representation of said primitive including displaying a difference between said first set of locations and said second set of locations.