1. A method of moving a point with respect to a guide surface in a computer graphics system, the method comprising: projecting a point expressed in a first coordinate frame into a parametric space defined by parametric directions on a guide surface and a normal direction of the guide surface, wherein the guide surface is in an unposed configuration; determining coordinates of the projected point in the parametric space of the guide surface in the unposed configuration; performing an operation to change the coordinates of the projected point within the parametric space to define a modified point; using a processor, evaluating the guide surface at the modified point in the parametric space to determine a corresponding position in the first coordinate frame; projecting a second point expressed in the first coordinate frame into the parametric space of the guide surface; determining second coordinates of the second projected point in the parametric space of the guide surface; performing the operation to change the second coordinates of the second projected point within the parametric space to define a second modified point; and using the processor, evaluating the guide surface at the second modified point in the parametric space to determine a second corresponding position in the first coordinate frame.