Facsimile and static presentation processing – Static presentation processing – Attribute control
Reexamination Certificate
1998-07-01
2001-04-24
Lee, Thomas D. (Department: 2724)
Facsimile and static presentation processing
Static presentation processing
Attribute control
C382S237000
Reexamination Certificate
active
06222641
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field
The invention relates to image processing. More particularly, the invention relates to the descreening of halftoned images.
2. Description of the Prior Art
Halftone techniques have long been used to produce color images using a small number of inks and a restricted set of densities (typically 100%/0%, or ink
o ink). In digital imaging halftoning usually involves a threshold array, which is tiled to cover the whole page, or a dynamic error-diffusion procedure that averages the errors due to the device limits throughout the page.
When such images (that were originally printed using a halftone technique, either digital or analog), are digitally captured by a scanning device, it is often desirable to restore the original color intensities (or ink densities) and obtain a contone equivalent of the halftoned image. This process is often referred to as descreening because it removes the screen that was applied while halftoning. Failure to remove the low frequencies that were introduced through the halftoning results in artifacts when the scanned image is either displayed or reproduced in print.
Various descreening approaches have been described in the art. See, for example, J. Stoffel, Half Tome Encoder/Decoder, U.S. Pat. No. 4,193,096 (Mar. 11, 1980); P. Roetling, Unscreening of Stored Digital Halftone Images, U.S. Pat. No. 4,630,125 (Dec. 16, 1986); H.-T. Tai, Image Processing Method To Remove Halftone Screens, U.S. Pat. No. 5,239,390 (Aug. 24, 1993); D. Seidner, D. Eylon, Apparatus and Method For Descreening, U.S. Pat. No. 5,384,648 (Jan. 24, 1995); P. Lavelle, J. Stoffel, Multi-Resolution Image Signal Processing Apparatus and Method, European Patent No. 041400 (Jun. 6, 1984); R. Eschbach, Image-Dependent Exposure Enhancement, European Patent No 648040 (Nov. 2, 1995); Z. Xie, M. Rodriguez, Electronic High-Fidelity Screenless Conversion System and Method Using A Separate Filter, European Patent No. 581415 (Feb. 23, 1994); and P. Stansfield, A. Reed, Image Processing, European Patent Application No. 301786 (Feb. 1, 1989).
Unfortunately, such prior art techniques are either concerned with such issues as compression/decompression or descreening only in circumstances where the actual screen pattern is known prior to descreening.
It would be advantageous to provide an improved descreening technique. It would be further advantageous to provide an improved technique for descreening halftone images, where the halftone screen pattern is not known prior to descreening.
SUMMARY OF THE INVENTION
The invention provides a descreening technique that descreens halftone image information without prior knowledge of the halftone screen that was applied to the image. The preferred embodiment of the invention provides a descreening process that comprises the following steps (independent of resolution):
Smooth the image, where smoothing is accomplished by applying a convolution with a low pass filter (LPF) kernel, (the kernel size typically depends on the image resolution) which is a parameter to the descreening function (the kernel size typically depends on the image resolution). This is typically a small size kernel, e.g. 3×3, 5×5 up to 9×9, depending on the resolution of the original image and possibly extra information about the scanner and the scanned screen. Note: The kernel need not be square, e.g. a rectangular kernel, such as 5×9, may also be used.
Using the smoothed image, determine for each pixel which pixels around it should participate in the final convolution. For the current pixel, consider a window of the size of the kernel, with the current pixel at the center. A threshold T1 is given as a parameter which is used to mark the pixels in the current window. Consider a pixel in the window: If for all color components, the difference between this pixel value to the center pixel value is less than T1 (in absolute value) the pixel is marked with a 1, otherwise it is marked with a 0.
Apply the descreening filter. If the number of pixels marked with a 1 in the window is less than a Factor f of the kernel size (a value of f=⅓ was found to give good results), the original pixel value is restored. Additionally, for a color component for which there is a small change in values within the original (non-smoothed) window (i.e. the difference between the maximal value to the minimal value in this component is less than another threshold T2), restore the value of this color component. If both of these conditions do not hold, compute a new value for each component according to the following formula:
new
—
value
=
∑
l
,
j
mark
(
i
,
j
)
·
pixel
(
i
,
j
)
·
lpf
(
i
,
j
)
where:
mark(i,j)=the mark associated with the pixel at location (i,j) in the current window (0/1: according to whether pixel (i,j) is within the threshold T1 from the center pixel (1) or not (0));
pixel(i,j)=the value of that pixel; and
lpf(i,j)=the value of the LPF kernel at the corresponding location.
To enhance performance, both in time and memory, the first two steps above, i.e. the smoothing and marking steps, can be applied only to the intensity component of the image information, with no change to the third step. In addition, descreening is only applied to those pixels that are marked as halftone pixels by a previous halftone detection step.
REFERENCES:
patent: 4193096 (1980-03-01), Stoffel
patent: 4630125 (1986-12-01), Roetling
patent: 4941185 (1990-07-01), Reed
patent: 5239390 (1993-08-01), Tai
patent: 5327257 (1994-07-01), Hrytzak et al.
patent: 5333064 (1994-07-01), Seidner et al.
patent: 5384648 (1995-01-01), Seidner et al.
patent: 5462841 (1995-10-01), Carleer
patent: 0041400B1 (1984-06-01), None
patent: 0301786A1 (1989-02-01), None
patent: 0511754A2 (1992-11-01), None
patent: 0581415A2 (1994-02-01), None
patent: 0622949A1 (1994-11-01), None
patent: 0648040A2 (1995-04-01), None
patent: 098004/2 (1997-02-01), None
Brinich Stephen
Electronics For Imaging, Inc.
Glenn Michael A.
Lee Thomas D.
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