1. A computer-implemented method, comprising: obtaining a surface patch that represents a surface as an N-sided patch including one or more internal constraint curves; receiving input selecting at least one of the internal constraint curves; generating an internal N-sided patch from the selected internal constraint curve, an external boundary of the internal N-sided patch defined by the selected internal constraint curve; displaying the surface as the surface patch that includes the internal N-sided patch, the surface patch and the internal N-sided patch each configured to be separately modified to affect the displayed surface; receiving a selection input selecting the internal N-sided patch; receiving a location input specifying a different location on the displayed surface; and generating a copy of the selected internal N-sided patch at the specified different location on the displayed surface.