This invention is a computer implemented method of approximating a gray scale tone with a more limited range image producer. One of a plurality of tone curves is associated with each pixel of a screening matrix. The plural tone curves are approximated by a polynomial and the polynominal coefficients are determined. The polynomial coefficients are stored in a look-up table. Each pixel of an image is mapped to a corresponding pixel of the screening matrix. For each pixel the corresponding polynomial coefficients approximating the tone curve are recalled and used to compute a pixel output value from a pixel input value. The polynomial is preferrably of the third degree polynomial and in a form easily computed using a digital signal processor with a hardware multiplier and arithmetic logic unit. Screening in this manner requires less memory storing the screening data than the prior art pure look-up table screening.