A precision Z-interpolator for use in an interactive graphics system multiplies the line slope by a value f.sub.S in computing the first Z ordinate value. This eliminates the offset error in Z inherent in previously known interpolators. For extremely large slopes, the slope is not used directly in the computations. Instead the value used is (Z.sub.E -Z.sub.S)/(1+f.sub.E /f.sub.S) (see FIG. 4).