1. A method for tracking the location of a moveable three-dimensional convex shape with a video camera, comprising the steps of: step a: defining the location of an X-spot on said convex shape, said location being selected from the group consisting of a point on said convex shape closest to the video camera, and a point on said convex shape closest to a fixed point in space, said location being moveable on said convex shape as said convex shape changes location and orientation in space; step b: storing a digitized video image of said convex shape in a vicinity of said X-spot, the size of said vicinity being defined by the surface area of said convex shape with a substantially constant spherical curvature, said digitized video image having a luminance pattern referred to as a stored X-luminance pattern, said stored X-luminance pattern being stored as a stored matrix; step c: defining the location of a reference point selected from the group consisting of the centre of said stored X-luminance pattern, and a location within said stored X-luminance pattern; step d: registering a plurality of digitized video images, each one of said plurality of video images containing a video image of said convex shape which is referred to as a registered X-luminance pattern, said registered X-luminance pattern being of the same size as said stored X-luminance pattern, each one of said plurality of video images being registered as a registered matrix; step e: comparing said stored matrix and said registered matrix in each one of said plurality of video images on a pixel-by-pixel basis to determine the two-dimensional location of said reference point in each one of said plurality of video images with pixel accuracy; and step g: producing the two-dimensional location of said reference point as information useable by a computer in each one of said plurality of video images; said method being substantially invariant to rotation of said convex shape.