1. A drawing processing apparatus comprising: a texture memory part which stores data on a texture; a texture processing part which interpolates the data on the texture stored in the texture memory part by using a linear interpolator based on a specified value of a parametric coordinate defined for each of units of drawing processing, thereby calculating interpolated data on the texture corresponding to the specified value of the parametric coordinate; and a drawing processing part which processes the units of drawing processing by using the interpolated data on the texture, and wherein: the units of drawing processing are segments or component units of a parametric curve represented by a one-dimensional parametric coordinate, or patches or component units of a parametric surface represented by two-dimensional parametric coordinates; the texture memory part stores a group of representative values of an internal division factor at two points in a domain of a parameter as the data on the texture, the internal division factor depending on the parametric coordinate(s) defined for each of the segments or patches; the texture processing part interpolates the group of representative values of the internal division factor by using a linear interpolator with values of the parametric coordinate(s) successively specified by the drawing processing part as to a segment or patch to be processed as interpolation factors, thereby calculating interpolated values of the internal division factor corresponding to the specified values of the parametric coordinate(s); and the drawing processing part performs an internal division operation on a plurality of control points defining shape of the segment or patch to be processed at ratios based on the interpolated values of the internal division factor, thereby calculating a vertex coordinate or vertex coordinates on the parametric curve or the parametric surface corresponding to the specified values of the parametric coordinate(s) in the segment or patch to be processed, wherein: if the parametric curve or the parametric surface 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 or adjoining patches, 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 or the patch 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.