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 S1 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 S2 with the value 128, assuming a video range between 0 and 255. If Step S2 determines that the modified video signal value X is greater than or equal to 128, the process proceeds to Step S4 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 S6 wherein this error, Y, is calculate as being X-255.
On the other hand, if Step S2 determines that the modified video signal value X is less than 128, a signal is output at Step S3 indicating that the pixel X is to be turned OFF. The process then proceeds to Step S5 wherein the error, Y, is calculated as being equal to the value X.
The error calculated in either Steps S5 or S6 is multiplied by weighting coefficients and distributed to downstream pixels in Step S7. 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 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 1/6.
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.sub.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.