Image analysis – Histogram processing – With a gray-level transformation
Reexamination Certificate
1997-07-01
2001-03-06
Boudreau, Leo H. (Department: 2721)
Image analysis
Histogram processing
With a gray-level transformation
C382S167000, C382S260000, C382S274000, C382S275000, C345S182000, C358S451000
Reexamination Certificate
active
06198845
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention pertains to methods and apparatus for determining background content of an image of a scanned document. More particularly, this invention relates to a method for determining the document background and suppressing its effects on document copies. This invention also relates to methods for adjusting the dynamic range of the image.
2. Description of Related Art
In copier systems, copying a document (or more generally, an original) while suppressing the substrate of the original is often required, such as when the original is printed on colored paper. Background detection can be performed on just the leading edge of the document or the whole document. However, whole page background detection generally requires pre-scanning the entire original. The detected background can be removed by adjusting the gain of the scanned image and clipping the values that exceeds the system processing range.
One approach to estimating the original's background value is by performing a running average in an area of the original where there is just background. This approach also blocks using any video below a selectable threshold just in case non-background material is included in the selected area. This running average approach works well with pure background, but performs poorly when non-background material is present, especially when the non-background material occurs at the edge of the window where the final average is most affected.
Automatic background suppression senses the background and automatically suppresses the background before final printing. Conventional automatic background suppression systems generate a histogram of the document using standard methods and then calculate the mean and standard deviation. This often involves significant amounts of calculation to determine the gain needed to eliminate the background noise.
To further prevent gross loss of image detail of an input document, the dynamic range should also be adjusted.
FIG. 11A
shows an image of a newspaper photograph with no background noise elimination or dynamic range adjustment.
SUMMARY OF THE INVENTION
This invention provides an improved method for estimating the background grey-level of an image of a scanned document.
This invention further provides an improved method for adjusting the dynamic range of an image via a tone reproduction curve map.
This invention also provides a method that is simple but which meets the requirements for determining the background value. Together with estimating the mean value of the background, one method of this invention estimates the standard deviation of the background so that background suppression can be optimized.
The method of this invention produces a single number called gain. A histogram of the image values in a selected area of the document is inputted. Alternately, the image values are subsampled before generating the histogram. The sub-sampling approach does not affect the calculation of the gain number and may be used to reduce the hardware requirements. The histogram is then smoothed. In general, histogram data tends to be noisy and smoothing of the data is desirable.
Smoothing the histogram is accomplished by adding the occurrence frequencies in four adjacent bins of the histogram, placing the sum in a new bin and dividing the sum by four. An original histogram with 256 bins is therefore compressed into a histogram of 64 bins.
Next, the approximate shape of the histogram is determined, where the bin frequency is a function of the bin number. Although a number of methods for solving for the coefficients in the preferred embodiment of this equation could be used, a second order polynomial is used that is fit through three points of the compressed histogram.
The selected points include the frequency value in the bin with the highest occurrence frequency and the frequency values in the bins on each side of the bin having the highest occurrence frequency. Then, the standard deviation of the distribution curve of the compressed histogram is determined. Finally, the standard deviation is used to determine the gain factor.
An alternative method estimates the statistics of the document background. A small sampling window is applied to the lead edge of a document to generate a 64-bin histogram. The peak occurrence frequency value of the histogram and its neighboring points are indicated by the grey-level values and the corresponding histogram frequencies are determined. The background mean grey level is determined using a weighted average of the three grey-level values thus obtained. The three sets of Cartesian coordinates and the determined mean are directly used in the normal distribution equation to determine the standard deviation. The standard deviation is then used to determine the gain factor for the document. The gain factor is used to estimate the background grey level of the image of the scanned document.
The determined background grey-level is then used to adjust the dynamic range of the image via a tone reproduction curve map. Instead of mapping the entire input grey-level space, only the input grey-level space from the image reflectance value to the established background level of the image is mapped. Instead of limiting the output grey-level space, the input grey-levels are mapped to a range extending from zero to the calibrated “whitest white” of the system, using the full available dynamic range. This increases the resolving power and reduces the line edge noise to improve the shadow detail rendition in halftone pictorials rendered by a single threshold.
These and other features and advantages of this invention are described in or apparent from the following detailed description of the preferred embodiments.
REFERENCES:
patent: 5046118 (1991-09-01), Ajewole et al.
patent: 5164993 (1992-11-01), Capozzi et al.
patent: 5544258 (1996-08-01), Levien
patent: 5832105 (1998-11-01), Morimoto et al.
patent: 5848181 (1998-12-01), Ogata
Blaakman André M.
Farrell Barbara L.
Fox Richard S.
Lahue George W.
Nagarajan Ramesh
Boudreau Leo H.
Mariam Daniel G.
Oliff & Berridg,e PLC
Xerox Corporation
LandOfFree
Method for determining document background for adjusting the... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method for determining document background for adjusting the..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method for determining document background for adjusting the... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2438493