1. A drawing processing apparatus for making an approximation to an object to be drawn with a parametric curve represented by coordinate points associated with a single parameter, the apparatus comprising: a factor data memory part which stores a group of representative values of an internal division factor at two points in a domain of the parameter in a one-dimensional parametric coordinate system defined for each of segments, or component units, of the parametric curve, the internal division factor being given by a linear function of the parameter; an interpolation processing part which interpolates the group of representative values of the internal division factor stored in the factor data memory part by using a linear interpolator 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 a subdivision processing part which performs 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 non-uniform 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 data memory part stores groups of representative values of internal division factors at two points in 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, the interpolation processing part interpolates the group of representative values 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.