A fill surface is used with a passing point in the two examples above. At certain panels, the base triangle is not wide enough and the fill surface breaks, leaving a flat surface.
Below, the fill surface is cut by an offset plane, and the bulginess of the surface is coded to correspond with the count of the curve and surface. Colors are made less gradient-like by switching the "j" variable with "i", allowing the red value to step incrementally every so many counts.
Finally, a simple cylinder replaces the bulging fill surface to demonstrate the changing bulginess more clearly.