1. A tessellation method performed by a computer system for subdividing a parametric curve represented by coordinate points associated with a single parameter into lines, the method comprising: storing in advance an initial value and a final value of an internal division factor at both ends of a domain of a parameter in a one-dimensional parametric coordinate system defined for each of segments, or component units, of the parametric curve into a factor table, the internal division factor having a value depending on the parameter, and interpolating the initial value and the final value of the internal division factor stored in the factor table with a value of the parameter specified in a segment to be processed as an interpolation factor, thereby calculating an interpolated value of the internal division factor corresponding to the specified value of the parameter; and performing an internal division operation on a plurality of control points defining shape of the segment to be processed at a ratio based on the interpolated value of the internal division factor, thereby calculating a vertex coordinate on the parametric curve corresponding to the specified value of the parameter in the segment to be processed, wherein if the parametric curve has knots at nonuniform pitches, a plurality of types of knot patterns having the knots of different pitches are determined based on connection relationship with adjoining segments, and the factor table stores the initial value and the final value of internal division factors at both ends of the domain of the parameter, the internal division factors corresponding to the plurality of types of knot patterns, respectively; and when one of the knot patterns to be applied to the segment to be processed is specified, said interpolating interpolates the initial value and the final value of the internal division factors corresponding to the specified knot pattern at a ratio based on the specified value of the parameter, thereby calculating an interpolated value of the internal division factor corresponding to the specified value of the parameter.