I am trying to make a 3d globe so that the user will be able to mark locations on it. However for each location i need to assign exact coordinates the mark is at. I want to go about it by storing the rotation of the globe as well as mouse coordinates and sphere position. However i run into problems when the user wants to zoom in, i.e. increase Z axis position. How am I able then to get the width of the sphere on the screen? I am sure there is some ration in which the sphere visually gets bigger when the z axis increases in relation to its radius but I am not aware of it. Could someone give me advice on it?