Correlated secondary TRC calibration method
View Patent ↗This invention is a method of producing a set of TRC's for a color printer's secondary halftone screens that is correlated with the printer's primary halftone screens. The method makes use of the printer/screen characteristic data that is normally gathered during screen calibration. However, instead of progressing from the data to a normal calibration for the secondary screens, the method goes backward through the data starting with the finished primary screen TRC's. The method insures that for every primary screen density, the closest possible secondary screen density is used when the same digital value is specified.
1. In a printing system, a method for minimizing artifacts produced at a border produced by printing with a primary halftone screen on one side of the border and printing with a secondary halftone screen at the other side of the border, comprising:
providing a document to be printed;
rendering the document into a full color bit raster;
converting the rendered document to the printing system's color space;
halftoning the converted document, wherein halftoning include calibration of primary and secondary halftone screens for each color separation, comprising:
generating a first tone reproduction curve (TRC) associated with the primary halftone screen using input values from a selected color separation and primary halftone screen output characteristics, wherein each primary halftone screen output characteristic is a function of measured luminance (delta-e from paper);
generating a second TRC associated with the secondary halftone screen using the first TRC, the primary halftone screen characteristics and secondary halftone screen output characteristics, wherein each secondary halftone screen output characteristic is a function of measured luminance (delta-e from paper), comprising:
for each input value in the selected color separation, determining a first TRC output value and an effective primary halftone screen output characteristic value associated with the first TRC output value; and
determining a secondary screen characteristic value having the same luminance at the effective primary halftone screen output characteristic value associated with the first TRC output value;
determining a second TRC output value based upon the secondary screen characteristic value having the same luminance as the effective primary halftone screen output characteristic value and the input value; and
printing the halftoned document.
2. The method of claim 1 , wherein the secondary screen frequency is n times the frequency of the primary screen frequency and wherein generating a second TRC associated with the secondary halftone screen comprises:
choosing a point P 2 of the primary TRC which corresponds to an input value P 1 ,
using point P 2 to determined a point P 3 corresponding to the primary screen characteristic output value,
using point P 3 to determine a point P 4 corresponding to the secondary screen characteristic output value having the same luminance,
determining a point P 5 on the second TRC having an output value that is 1/n 2 of the P 1 and the same output value of P 4 , and
rounding off the output value of point P 5 to the nearest output value integer.
3. The method of claim 1 , using a normalized Jones diagram, comprising:
choosing a point P 2 of the first TRC which has an input value P 1 ,
using point P 2 to determine a point P 3 on the primary screen characteristic,
using point P 3 to determine a point P 4 on the secondary screen characteristic that has the same luminance,
using point P 4 to determine a point P 5 on the second TRC having the same input value P 1 , and
rounding off the output value of point P 5 to the nearest output value integer.