Facsimile and static presentation processing – Static presentation processing – Attribute control
Reexamination Certificate
1999-08-16
2003-07-15
Grant, II, Jerome (Department: 2624)
Facsimile and static presentation processing
Static presentation processing
Attribute control
C358S003050, C382S251000
Reexamination Certificate
active
06594035
ABSTRACT:
TECHNICAL FIELD
The invention relates generally to halftoning techniques and more particularly to an error diffusion halftoning technique.
BACKGROUND ART
Digital images provide a convenient format for transmission, modification, and/or reproduction of images. When an image is captured digitally, grayscale information for each pixel of an image is extracted. Typically, 256 grayscale levels are utilized to extract the grayscale information. However, the majority of printers that are currently in use are binary with respect to their printing methods. That is, the printers operate to reproduce the original image either by depositing or by refraining to deposit a small amount of ink or toner for each pixel of the captured image. The binary nature of these printers allows only two levels of grayscale to be printed for each pixel. Thus, digitally captured images having more than two levels of grayscale cannot be reproduced by the binary printers, unless a special printing technique, such as halftoning, is utilized.
A halftoning technique is a process for printing different shades of grayscale by varying the density of “dark” pixels that have been deposited with ink or toner. A lower density of dark pixels equates to a lighter shade of grayscale, while a higher density of dark pixels equates to a darker shade of grayscale. Since the density of dark pixels can vary in numerous degrees, the number of grayscales that can be produced using the halftoning technique is far greater than two levels. As long as the pixels are sufficiently small, the individual dark pixels will not be apparent to a viewer. Instead, the viewer will see smooth areas having different shades of grayscale, which are directly related to the density of dark pixels.
A common type of halftoning technique is known as “an error diffusion halftoning technique.” In error diffusion halftoning, a halftoning error associated with a generated halftone signal for each pixel of a digital image is distributed among neighboring pixels in order to determine which pixel should be deposited with ink or toner. The halftone signal is derived by comparing a given value, which is a combined value of a grayscale value of a pixel and a cumulative error from previously processed pixels, with a predefined threshold value. The difference between the given value and the halftone value is the halftoning error. By distributing the halftoning errors, the density of dark pixels will be determined by the grayscale values from a number of surrounding pixels. Consequently, regions of the digital image having lighter shades of grayscale will yield lower densities of dark pixels, while regions having darker shades of grayscale will yield higher densities of dark pixels. In this fashion, digital images having more than two shades of grayscale can be printed using a binary printer.
In
FIG. 1
, a conventional system
10
that executes error diffusion halftoning is shown. The system includes an input device
12
, an error diffusion halftoning (EDH) device
14
, and a binary output device
16
. The input device provides a digital image that is to be printed by the binary output device. The input device may be a scanner that can capture the digital image from a photograph, a digital camera that can capture the digital image from an actual scene, or a storage device that can receive the digital image from an external source. The error diffusion device includes a summing unit
18
, a thresholding module
20
, a subtraction unit
22
, an error diffuser
24
, and an error buffer module
26
.
In operation, the system
10
processes the digital image by sequentially operating on the image pixels of the digital image in a raster scan order, which is a left-to-right, top-to-bottom sequence. For each pixel of the image, a grayscale value g
i,j
of that pixel is transmitted from the input device
12
to the summing unit
18
of the EDH device
14
, where g
i,j
&egr;[0,255] for 256 grayscale. The values i and j identify the row and column, respectively, of the current pixel being processed. The summing unit
18
combines the grayscale value g
i,j
with a final error e
i,j
and outputs a summed value s
i,j
. The final error e
i,j
is derived from halftoning errors associated with the previous pixels that were processed by the EDH device. The summed value s
i,j
is then transmitted to the thresholding module
20
and the subtraction unit
22
. The thresholding module compares the summed value s
i,j
to a threshold value, e.g., 127 for 256 grayscale. The comparison produces an output halftone value h
i,j
, which is one of two values, e.g., 0 or 255. If the summed value s
i,j
is less than the threshold value, the output halftone value h
i,j
equates to a first value, e.g., 0, that directs the output device
16
to refrain from depositing ink or toner. However, if the summed value s
i,j
is equal to or greater than the threshold value, the output halftone value h
i,j
equates to a second value, e.g., 255, that directs the output device to deposit the ink or toner.
The output halftone value h
i,j
is also transmitted to the subtraction unit
22
to derive a halftoning error that results from converting the summed value s
i,j
into one of two halftone values. The subtraction unit subtracts the halftone value h
i,j
from the summed value s
i,j
. The result of this operation is a halftoning error n
i,j
, which is transmitted to the error diffuser
24
. The error diffuser then divides the halftoning error n
i,j
using a known distribution process, such as the Floyd-Steinberg error diffusing process. The divided halftoning errors are transmitted to the error buffer module
26
. The error buffer module processes the divided halftoning errors, such that each divided halftoning error can be diffused into a neighboring pixel of the current pixel. These divided halftoning errors are combined with other divided halftoning errors from previously processed pixels to form final errors that are to be diffused into subsequently processed pixels. The final errors are temporarily stored in an error buffer (not shown) within the error buffer module. The error buffer has a capacity to store the final errors for an entire row of image pixels. For 256 grayscale, each bin of the primary error buffer is an 8-bit bin. When the grayscale value g
i,j+1
of the next pixel is processed, a final error e
i,j+1
stored in the bin of the error buffer associated with that pixel is transmitted to the summing unit
18
. This final error is combined with the grayscale value g
i,j+1
and the above-described process is repeated. In this fashion, the halftoning errors from the pixels of the digital image are distributed to reproduce the digital image as a halftone image using the binary output device
16
.
Although conventional error diffusion halftoning systems, such as the system
10
, operate well for their intended purpose, what is needed is a cost-efficient error diffusion system and a method of managing errors in such a system.
SUMMARY OF THE INVENTION
An error diffusion halftoning system and a method of managing halftoning errors utilize a quantization technique to reduce the required size of a primary error buffer that is needed to diffuse the halftoning errors. By implementing the quantization technique, the primary error buffer can be reduced from an 8-bits-per-bin error buffer to a 2-bits-per-bin error buffer for 256 grayscale. The reduction in bin size decreases the cost of the primary error buffer and, consequently, the cost of an error diffusion halftoning (EDH) device of the system that generates halftone signals from grayscale values of a digital image.
The error diffusion halftoning system includes an input device, the EDH device, and a binary output device. The input device may be a digital scanner, a digital camera, or a storage device that can acquire digital images. The binary output device may be a conventional inkjet or laser printer. The EDH device is operatively connected to the input device and the binary output device to process grayscale pixel values of a giv
Kresch Renato
Shaked Doron
LandOfFree
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