Facsimile and static presentation processing – Facsimile – Specific signal processing circuitry
Reexamination Certificate
1996-05-30
2001-06-19
Grant, II, Jerome (Department: 2624)
Facsimile and static presentation processing
Facsimile
Specific signal processing circuitry
C358S463000, C382S298000, C382S299000
Reexamination Certificate
active
06249357
ABSTRACT:
FIELD OF THE PRESENT INVENTION
The present invention is directed to an error diffusion process which allows for adjustment of the tonal reproduction curve (TRC) in a printer or printing system. More specifically, the present invention is directed to an error diffusion process for adjusting the tonal reproduction curve so as to compensate for spot overlap produced by a printing device.
BACKGROUND OF THE PRESENT INVENTION
A well known method of rendering grey images on a binary output device is error diffusion. Error diffusion is most commonly used in displaying continuous tone images on a bi-level display. However, error diffusion has been also utilized in digital copiers and binary printing devices to render grey and continuous tone images.
FIG. 31
illustrates a conventional error diffusion technique. In Step S
1
of this process, the video signal for pixel X is modified to include the accumulated error diffused to this pixel from previous threshold processes. The modified video signal value X is compared at Step S
2
with the value 128, assuming a video range between 0 and 255. If Step S
2
determines that the modified video signal value X is greater than or equal to 128, the process proceeds to Step S
4
wherein a value is output to indicate the turning ON of pixel X. The process then proceeds to calculate the error associated with the threshold process at Step S
6
wherein this error, Y, is calculate as being X−255.
On the other hand, if Step S
2
determines that the modified video signal value X is less than 128, a signal is output at Step S
3
indicating that the pixel X is to be turned OFF. The process then proceeds to Step S
5
wherein the error, Y, is calculated as being equal to the value X.
The error calculated in either Steps S
5
or S
6
is multiplied by weighting coefficients and distributed to downstream pixels in Step S
7
. Thus, the error from the threshold process is diffused to adjacent pixels. The coefficients conventionally used to diffuse the error to adjacent downstream pixels are illustrated in FIG.
32
.
In
FIG. 32
, X represents the current pixel being thresholded. The weighted errors from this threshold process are diffused to adjacent downstream pixels according to preselected coefficients. For example, the weighting coefficient for the next pixel in the same scanline conventionally is {fraction (7/16)}, whereas the coefficient for the pixel that is one over in the fastscan direction and one down in the slowscan direction from the currently processed pixel is ⅙.
In describing the error diffusion process, it is assumed that the video value is in a range between 0 and 255. However, any chosen range for the video signal can be utilized. As described above, in conventional error diffusion methods, the binarization of the pixel or the reduction of its grey level is determined by comparing a modified input with a threshold. The modified input video signal is the input video signal, V, plus an accumulated error term, e
i
, determined from the processing of previous pixels.
One problem with utilizing error diffusion in a printing environment is that the tonal reproduction curve (TRC) tends to be nonlinear. This nonlinear characteristic of the TRC presents various problems in attempting to render a grey image or continuous tone image on a binary printing device.
For example, if a grey wedge is to be reproduced on a digital copier utilizing a conventional error diffusion process, wherein the grey wedge image is printed at 300 spots per inch, the reproduced grey wedge tends to show a rapid increase in density when compared to the original grey wedge. Moreover, if a digital copier utilizing a standard conventional error diffusion method scans in a continuous tone image and reproduces a continuous tone image at 300 spots per inch, the reproduced continuous tone image tends to be too dark in comparison with the original continuous tone image. Thus, utilizing conventional error diffusion to render grey or continuous tone images on a binary device cannot render a reproduced copy having high image quality.
To address this problem with the standard error diffusion process, it has been proposed to process the image data through a compensating grey level confirmation before printing the image data with error diffusion. An example of such a proposal is disclosed in U.S. Pat No. 5,087,981. The entire contents of U.S. Pat. No. 5,087,981 are hereby incorporated by reference.
U.S. Pat. No. 5,087,981 discloses the utilization of a compensation grey level transformation wherein the area coverage of a new print spot minus that of the area coverage that corresponds to the overlap of the previously printed spots is calculated. This net spot coverage is then normalized to the pixel spacing and is square to determine the effective area A. It is noted that in this transformation, A is greater than 1. Once the effective area of the new print spot is calculated, the effective area is utilized in computing the total error for diffusing to downstream pixels. More specifically, the total error in printing a pixel with a grey level G is calculated as G−A.
If a grey wedge is scanned in by a digital copier and the grey edge image is reproduced at 300 spots per inch utilizing the compensating error diffusion process of U.S. Pat. No. 5,087,981, the reproduced grey wedge demonstrates a gradual increase in density when compared to the original grey wedge. However, the utilization of this compensating error diffusion process appears to produce clumps and streaks of dark pixels in the midtone to shadow areas of the reproduced grey edge. Therefore, it is desirable to provide a compensated error diffusion method wherein the density of a reproduced grey wedge gradually increases without the artifacts of clumps and streaks of dark pixels in the midtone to shadow areas.
SUMMARY OF THE INVENTION
One aspect of the present invention is a method of reducing a number of levels in a multi-level grey scale pixel value representing a pixel and diffusing an error generated from reducing the number of levels. The method receives the multi-level grey scale pixel value of a first resolution; generates a screened multi-level grey scale pixel value; reduces the number of levels in the screened multi-level grey scale pixel value; generates an error value as a result of the reduction process; modifies the generated error value based on an effective area value dependent on the multi-level grey scale pixel value; and diffuses the error value to multi-level grey scale pixel values of adjacent pixels.
A second aspect of the present invention is a system for reducing a number of levels in a multi-level grey scale pixel value representing a pixel and diffusing an error generated from reducing the number of levels. The system includes input means for receiving the multi-level grey scale pixel value, the multi-level grey scale pixel value having a first resolution; screening means for generating a screened multi-level grey scale pixel value; high addressability means for converting the screened multi-level grey scale pixel value to a second resolution, the second resolution being higher than the first resolution; reduction means for reducing the number of levels in the screened multi-level grey scale pixel value; spot area means for generating an effective spot area value based on the received multi-level grey scale pixel value; error means for generating a plurality of possible error values; means for multiplying each possible error value by the effective spot area value to produce a plurality of possible modified error values; selecting means for selecting an error value from the plurality of possible modified error values as a result of the reduction by said reduction means; and
error diffusing means for diffusing the selected modified error value to multi-level grey scale pixel values of adjacent pixels.
A third aspect of the present invention is a method of generating an error value. The method performs the steps of generating a screened multi-level grey scale pixel value representing a pixel havin
Metcalfe David J.
Shiau Jeng-Nan
Grant II Jerome
Xerox Corporation
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