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Marking engine and method to optimize tone levels in a digital output system
| Details |
Inventors: Crean, Peter A.; Maltz, Martin S.;
Assignee: Xerox Corporation (Stamford, CT)
Primary Examiner: Lee; Thomas D.
Assistant Examiner:
Attorney, Agent or Firm: Fay, Sharpe, Fagan, Minnich & McKee, LLP
A halftone cell is composed of a plurality of pixel groupings predefined to correspond to optimal tone level gradations. Selected output pixel groupings vary in number, and in shape of subpixels used from other pixel groupings, in order that successive input levels correspond to successive halftone cells defining substantially equal changes in a measured darkness value. Tone level gradations are optimized by purposeful correlation of input levels to output levels, especially where the number of output levels exceeds the number of input levels. During optimization, a calibration sample is produced by the image output device and calibration elements are empirically measured. Darkness change between successive levels is calculated and compared. Where the percentage of darkness change exceeds a desired amount, additional calibration elements are produced until the change calculated between successive levels is substantially the same. Successive input levels are associated with successive output levels such that a change between any adjacent input levels produces a change in darkness in the desired amount. The correlation is stored in a look up table for use during operation of the image output device. |
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DETAILED DESCRIPTION In accordance with the present invention, a halftone cell configuration of a plurality of output pixels has optimal tone level gradations.
The halftone cell comprises output pixel patterns corresponding to the tone level gradations where selected patterns vary in form and number of output pixels from other patterns.
In accordance with another aspect of the present invention, each of the output pixel patterns corresponds to a substantially equal change in a measured darkness value.
In accordance with another aspect of the present invention, the output pixel patterns comprise a predetermined number of subpixels comprising a fractional portion of each pixel.
In accordance with another embodiment of the present invention, a method for optimizing tone level gradations in an image output device is provided.
The method includes the steps of creating a tone reproduction curve defining a relationship between the number of output levels and a level of darkness.
The method also includes 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.
In accordance with another aspect of the present invention, the tone reproduction curve is created by producing a calibration sample of individually measurable calibration elements representative of a plurality of the output levels.
The level of darkness associated with the calibration elements is then empirically measured and plotted relative to the output levels.
In accordance with another aspect of the present invention, the correlation comprises determining a percentage change in the level of darkness between successive input levels.
Where the determined percentage is substantially less than the desired percentage change, one of the input levels is unassigned from one of the successive input levels.
When the determined change is substantially greater than the desired change, additional calibration elements are produced and measured until the determined change is substantially equal to the desired change
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