Marking engine and method to optimize tone levels in a digital output system

A method for optimizing tone level gradations in an image output device capable of receiving a number of input levels and outputting a halftone cell comprising a plurality of pixels arranged to define a number of output levels, said method comprising:

• creating a tone reproduction curve defining a relationship between the number of output levels and a level of darkness, said creating including:
• producing a calibration sample of individually measurable calibration elements representative of a plurality of the output levels, the calibration elements each defining a level of darkness;
• empirically measuring the level of darkness of each of the calibration elements;
• and,
• plotting the measured level of darkness relative to the output levels;
• correlating each one of the number of input levels to a corresponding one of the number of output levels based on a desired percentage change in the level of darkness, said correlating including:
• assigning each input level to one of the measured levels of darkness;
• determining a percentage change in the level of darkness between successive input levels;
• where the determined percentage change is substantially less than the desired percentage change, removing the input level assignment of one of the successive input levels;
• and,
• where the determined percentage change is substantially greater than the desired percentage change, repeating the producing, measuring, plotting, assigning and determining steps between the successive input levels until the determined percentage change is substantially equal to the desired percentage change between the successive input levels;
• and,
• storing the correlation of input levels to output levels.

An image output apparatus for receiving one of a predefined set of sequential input levels, and for outputting, based on the input level received, one of a set of predefined halftone cell configurations of subpixels, where each one of the set of input levels produces an appreciable change in darkness in the output halftone cell, the apparatus comprising:

• a look up table preprogrammed such that for each pair of consecutive input levels, a first of the pair corresponds to a first number of subpixels, and a second of the pair corresponds to a second number of subpixels, wherein for at least two pairs a different between the first and second numbers of subpixels is not the same;
• and
• a halftone pattern generator for arranging the number of subpixels into a pattern of neighboring subpixels, said pattern starting at a substantially central location in the halftone cell and extending in multiple directions to a periphery of the halftone cell;
• wherein the appreciable change in darkness is substantially uniform between consecutive input levels.

A method of generating a halftone output comprising:

• receiving an input intensity value for an input pixel;
• providing a matrix of subpixels corresponding to the input pixel, each subpixel having a separate selectable output value;
• dividing the matrix into a plurality of distinct subsets each including a number of subpixels, said plurality of distinct subsets including at least two subsets having numbers of subpixels different from one another;
• and,
• selecting the subpixel output values such that in each subset all the subpixels therein have the same output value, and the number of subsets having like subpixel output values corresponds to the input intensity value of the input pixel, said selecting being carried out such that a boundary of neighboring subsets having like subpixel output values fluctuates in location between a center region of the matrix and a periphery of the matrix in accordance with the input intensity value.