 It is also possible to start with an expression for formula_131 and derive an expression for formula_37 according to

In the ideal case, when the camera maps the 3D points according to a perfect pinhole camera and the resulting 2D points can be detected without any noise, the two expressions for formula_37 are equal.In practice, however, they are not and it may be advantageous to combine the two estimates of formula_37, for example, in terms of some sort of average.There are also other types of extensions of the above computations which are possible.