 Compute the 2dcurve. Try to solve the particular case if possible. Otherwise, an approximation is done. For approximation some parameters are used, including required tolerance of approximation. Tolerance is maximal possible value of 3d deviation of 3d projection of projected curve from "exact" 3d projection. Since algorithm searches 2d curve on surface, required 2d tolerance is computed from 3d tolerance with help of U,V resolutions of surface. 3d and 2d tolerances have sense only for curves on surface, it defines precision of projecting and approximation and have nothing to do with distance between the projected curve and the surface. More...
