Describes a portion of a surface (a patch) limited by two values of the u parameter in the u parametric direction, and two values of the v parameter in the v parametric direction. The domain of the trimmed surface must be within the domain of the surface being trimmed. The trimmed surface is defined by: - the basis surface, and - the values (umin, umax) and (vmin, vmax) which limit it in the u and v parametric directions. The trimmed surface is built from a copy of the basis surface. Therefore, when the basis surface is modified the trimmed surface is not changed. Consequently, the trimmed surface does not necessarily have the same orientation as the basis surface. Modifies this patch by changing the trim values applied to the original surface