How to convert a 2d point into a 3d point ?

To create 3d graphism i use a formula that convert a 3d point into a 2d point that i can draw on the screen.

Now how can i do the inverse ? : convert a 2d point into a 3d point.

Here is the formula :

d and r are focal parameters.

n and n1 are used to center the drawing on the form.

(I use : d=2000, r=2000, n=400, n1=400)

xe and ye are the coordinates and the result of the formula.

x,y and z are the 3d coordinates of the point.

d,r,n and n1 are known.

b=y-z+r

xe=(d*x)/b+n

ye=n1-(d*y)/b

Thanks.