1. A method of generating 3D images for computer simulations, in a computer-based simulation, comprising: displaying a 3D image having a subject and a user-selected object, movement of the subject in the 3D image being defined with respect to a first set of axes in 3 dimensional space, and movement of the user-selected object in the 3D image being defined with respect to a second set of axes in 3 dimensional space, the displayed 3D image showing an intersection of the subject and the user-selected object; and for images succeeding the displayed 3D image in the simulation, automatically using the first set of axes to define movement of the intersection including movement of the subject and movement of the user-selected object, in a manner omitting use of the second set of axes to define movement of the user-selected object, said automatic using the first set of axes resulting in further generated 3D images for the simulation.