System and apparatus for tonal reproduction adjustment in an error diffusion process

A digital copier, comprising:

• scanning means for scanning a document and generating input grey image values corresponding to a plurality of pixels;
• modifying means for adding an error value to an input grey image value corresponding to a first pixel to produce a modified input grey image value;
• comparing means for comparing the modified input grey image value with a predetermined threshold value and for outputting a rendering value based on the comparison;
• error means, operatively connected to said comparing means for generating an error equal to the modified input grey value when the modified input grey value is less than the predetermined threshold value and for generating an error equal to the modified input grey value minus an effective spot area value when the modified input grey value is equal to or greater than the predetermined threshold value;
• diffusing means for fractionalizing the error into fractional error values and diffusing the fractional error values to pixels adjacent the first pixel;
• buffer means, operatively connected to said diffusing means and said modifying means, for accumulating the diffused fractional error values for individual pixels and generating the error value therefrom;
• and
• print means, operatively connected to said comparing means, for reproducing an image on a recording medium based on the output from said comparing means.

A method of processing an input grey image value corresponding to a pixel, comprising the steps of:

• (a) adding an error value to the input grey image value to produce a modified input grey image value;
• (b) comparing the modified input grey image value with a predetermined threshold value and outputting a rendering value based on the comparison;
• (c) generating an error equal to the modified input grey value when the modified input grey value is less than the predetermined threshold value;
• (d) generating an error equal to the modified input grey value minus an effective spot area value when the modified input grey value is equal to or greater than the predetermined threshold value;
• (e) fractionalizing the error into fractional error values;
• (f) diffusing the fractional error values to pixels adjacent the pixel;
• and
• (g) accumulating the diffused fractional error values for individual pixels and generating the error value therefrom.

A method of generating an effective spot area value to be used in an error diffusion process, comprising the steps of:

• (a) scanning a set of grey test patches corresponding to a set of known grey input values and generating image data therefrom;
• (b) printing a set of grey patches based on the generated image data;
• (c) measuring a reflectance of each grey test patch to generate a plurality of output grey values, one output grey value for each test patch and corresponding known grey input value;
• (d) generating a tonal reproduction curve from the set of known grey input values and corresponding output grey values;
• and
• (e) calculating a plurality of effective spot area values, each effective spot area value being a result of dividing one output grey value by the corresponding known grey input value.

A system for processing an input grey image value corresponding to a pixel, comprising:

• modifying means for adding an error value to the input grey image value to produce a modified input grey image value;
• comparing means for comparing the modified input grey image value with a predetermined threshold value and for outputting a rendering value based on the comparison;
• effective spot area value means for outputting one of a plurality of effective spot area values according to the input grey image value, the effective spot area values being created from a process of scanning a set of grey test patches corresponding to a set of known grey input values and generating image data therefrom, printing a set of grey patches based on the generated image data, measuring a reflectance of each grey test patch to generate a plurality of output grey values, one output grey value for each test patch and corresponding known grey input value, generating a tonal reproduction curve from the set of known grey input values and corresponding output grey values, calculating a plurality of effective spot area values, each effective spot area value being a result of dividing one output grey value by the corresponding known grey input value, and storing the plurality of effective spot area values as a function of the output grey value;
• error means, operatively connected to said comparing means and said effective spot area value means, for generating an error equal to the modified input grey value when the modified input grey value is less than the predetermined threshold value and for generating an error equal to the modified input grey value minus the effective spot area value when the modified input grey value is equal to or greater than the predetermined threshold value;
• diffusing means for fractionalizing the error into fractional error values and diffusing the fractional error values to pixels adjacent the pixel;
• and
• buffer means, operatively connected to said diffusing means and said modifying means, for accumulating the diffused fractional error values for individual pixels and generating the error value therefrom.