A process for specifying mean-preserving multi-level halftone matrices with varying amounts of modulation

Apparatus and a method for generating a multi-level halftone image (104) from a digitally sampled continuous tone (contone) image (101). The apparatus includes a control circuit (140), a preference matrix (150) having as its matrix elements addresses of a plurality of look-up tables (160) and a plurality of look-up tables (160) in the form of a look-up table stack (155). The control circuit instructs the preference matrix to select a specific look-up table from the look-up table stack in a pre-defined manner. The selected table is used to convert an intensity value (102) into each multi-level pixel value (106) in the halftone image. To accomplish the conversion, each look-up table contains a quantized one-dimensional transfer function (165') having as an input the magnitude of the intensity value. The transfer functions are automatically generated such that the sum of the derivatives of the transfer functions equals the number of pixel values in a halftone cell. The magnitude of each intensity value is mapped into an output level by the quantized transfer function as one of a plurality of available levels. The resulting output level is assigned a pixel location in a halftone cell (103). The look-up table selection is repeated for each element of the preference matrix such that the magnitude of each different intensity value in the contone image is mapped into a corresponding halftone cell to generate a complete multi-level halftone image.