Patent EP0798919A2 successfully refreshed from Google Patents.
EP0798919A2
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Document image compression system and method
Assignee: Unisys Corp
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Filing: 1992-11-19
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Granted: —
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Expires: 2012-11-19
Abstract
Abstract A discrete transform image data compression system (Figure 2) in which frequency transform coefficients are modified in accordance with a matrix of quantizer values employs a predefined plurality of quantization matrices to adaptively select, on a document-by-document basis, an approximate memory packet size for each document's compressed image data storage by selecting one of the plurality of quantization matrices in accordance with the packet size estimate obtained for each document image. Additionally, the system employs generation of contrast reduction and gray level stretch remapping curves as a function of global image data characteristics, such as a gray level histogram of the document image data. The remapping curves are utilized to preprocess the image data for more effective data compression.
Inventors
- 👤 Michael John Higgins-Luthmann
- 👤 Robert Charles Kidd
- 👤 Robert Dennis Klein
- 👤 Robert Chihsung Yen
- 👤 Charles Martin Zalinski
Bibliographic Details
Application Number
—
Jurisdiction
EP
Data Source
google_patents
Last Synchronized
2026-11-09 12:09
CPC Classifications
H; H04; H04N; H04N1/00; H04N1/40; H04N1/407; H04N1/4072; H04N1/4074; H04N19/00; H04N19/10; H04N19/102; H04N19/124; H04N19/126; H04N19/134; H04N19/136; H04N19/14; H04N19/146; H04N19/149; H04N19/169; H04N19/17; H04N19/172; H04N19/18; H04N19/60; H04N19/85
Claims Analysis
Total Claims: 15
Claim 1
Independent Claim
182 words
Apparatus for pre-processing document image data prior to data compression, said data being arranged as a plurality of pixels, each representing one of a plurality of gray levels, the apparatus comprising: means (201, 203) for generating document image data comprising highly visible pixel data and nominally visible background pixel data, histogram generation means (205, 207) for generating a histogram having a highly visible pixel data portion and a nominally visible background pixel data portion, histogram processing means (207) for generating a contrast reduction pixel conversion on said highly visible gray level pixel data, said histogram processing means (207) further performing a stretch pixel conversion function of the converted highly visible gray level pixel data and the nominally visible gray level pixel data in accordance with preselected characteristics of said histogram generating a contrast increase pixel conversion of said pixel data, and means (207, 211) for combining the contrast reduction and the gray level stretch conversion functions and for applying a combined conversion function to the document image data prior to data compression to minimize loss of said nominally visible background pixel data.
Claim 2
Dependent Claim
121 words
Apparatus pre-processing document image data prior to data compression of claim 1, characterized in that the contrast reduction pixel conversion function comprises a piece-wise linear function (307, 305) wherein a converted output pixel gray level value is related to a corresponding input pixel gray level value by a linear function (307) having a slope less than unity for the highly visible input pixel gray level values less than a predetermined contrast reduction protection value (301) selected as a function of characteristics of the histogram, and wherein a converted output pixel gray level value is equal to a corresponding input pixel gray level value for input pixel gray level values (305) equal to or greater than the contrast reduction protection value (301).
Claim 3
Dependent Claim
153 words
Apparatus pre-processing document image data prior to data compression of claim 1, characterized in that the gray level stretch conversion function comprises a piece-wise linear function (407, 405) wherein a converted output pixel gray level value is a blackest value (409) for all input pixel gray level values less than or equal to a predetermined black clip value selected as a function of characteristics of the histogram, a converted output pixel gray level value is related to a corresponding input pixel gray level value by a linear function (407) having a slope greater than unity for input pixel gray level values greater than the black clip value and less than a predetermined white clip value selected as a function of characteristics of the histogram, and a converted output pixel gray level (405) value is a whitest value for all input pixel gray level values greater than or equal to the white clip value.
Claim 4
Dependent Claim
153 words
Apparatus pre-processing document image data prior to data compression of claim 1, characterized in that the gray level stretch conversion function comprises a piece-wise linear function (407, 405) wherein a converted output pixel gray level value is a blackest value (409) for all input pixel gray level values less than or equal to a predetermined black clip value selected as a function of characteristics of the histogram, a converted output pixel gray level value is related to a corresponding input pixel gray level value by a linear function (407) having a slope greater than unity for input pixel gray level values greater than the black clip value and less than a predetermined white clip value selected as a function of characteristics of the histogram, and a converted output pixel gray level value (405) is a whitest value for all input pixel gray level values greater than or equal to the white clip value.
Claim 5
Dependent Claim
37 words
Apparatus pre-processing document image data prior to data compression of claim 1, characterized in that the contrast reduction pixel conversion function and the gray level stretch conversion functions are combined and implemented as a look-up table (221).
Claim 6
Dependent Claim
38 words
Apparatus pre-processing document image data prior to data compression of claim 2, characterized in that the contrast reduction protection value is a predetermined percentage of a gray level value corresponding to a preselected peak value of the histogram.
Claim 7
Dependent Claim
56 words
Apparatus pre-processing document image data prior to data compression of claim 3, characterized in that the black clip value is determined by comparing a cumulative sum of histogram frequencies to a first preselected threshold value and a white clip value is determined by comparing a cumulative sum of histogram frequencies to a second preselected threshold value.
Claim 8
Dependent Claim
114 words
Method for pre-processing document image data arranged as a plurality of pixels, each representing one of a plurality of gray levels, the method comprising the steps of: a) generating a histogram of all pixels in the document image data, b) generating a contrast reduction pixel conversion function of highly visible gray level values of said histogram, c) generating a gray level stretch conversion function of gray level values of said histogram to effect an increase contrast pixel conversion function in accordance with preselected characteristics of said histogram, d) combining the contrast reduction and gray level stretch conversion functions, and e) applying a combined conversion function to the document image data prior to data compression.
Claim 9
Dependent Claim
120 words
Method for pre-processing document image data arranged as a plurality of pixels of claim 8, characterized in that the contrast reduction pixel conversion function comprises a piece-wise linear function (307, 305) wherein a converted output pixel gray level value is related to a corresponding input pixel gray level value by a linear function (307) having a slope less than unity for input pixel gray level values less than a predetermined contrast reduction protection value (301) selected as a function of characteristics of the histogram, and wherein a converted output pixel gray level value (305) is equal to a corresponding input pixel gray level value for input pixel gray level values equal to or greater than the contrast reduction protection value.
Claim 10
Dependent Claim
156 words
Method for pre-processing document image data arranged as a plurality of pixels of claim 8, characterized in that the gray level stretch conversion function comprises a piece-wise linear function (409, 407, 405) wherein a converted output pixel gray level value is a blackest value (409) for all input pixel gray levels less than or equal to a predetermined black clip value selected as a function of characteristics of the histogram, a converted output pixel gray level value is related to a corresponding input pixel gray level value by a linear function (407) having a slope greater than unity for input pixel gray level values greater than the black clip value and less than a predetermined white clip value selected as a function of characteristics of the histogram, and a converted output pixel gray level value is a whitest value (405) for all input pixel gray level values greater than or equal to the white clip value.
Claim 11
Dependent Claim
158 words
Method for pre-processing document image data arranged as a plurality of pixels of claim 9, characterized in that the gray level stretch conversion function comprises a piece-wise linear function (409, 407, 405) wherein a converted output pixel gray level value is a blackest value (409) for all input pixel gray level values less than or equal to a predetermined black clip value selected as a function of characteristics of the histogram, a converted output pixel gray level value is related to a corresponding input pixel gray level value by a linear function (407) having a slope greater than unity for input pixel gray level values greater than the black clip value and less than a predetermined white clip value selected as a function of characteristics of the histogram (Fig 6), and a converted output pixel gray level value is a whitest value for all input pixel gray level values greater than or equal to the white clip value.
Claim 12
Dependent Claim
39 words
Method for pre-processing document image data arranged as a plurality of pixels of claim 11, characterized in that the contrast reduction pixel conversion function and the gray level stretch conversion function are combined and implemented as a look-up table.
Claim 13
Dependent Claim
41 words
Method for pre-processing document image data arranged as a plurality of pixels of claim 9, characterized in that the contrast reduction protection value is a predetermined percentage of a gray level value corresponding to a preselected peak value of the histogram.
Claim 14
Dependent Claim
59 words
Method for pre-processing document image data arranged as a plurality of pixels of claim 10, characterized in thatthe black clip value (401) is determined by comparing a cumulative sum of histogram frequencies to a first preselected threshold value and the white clip value is determined by comparing a cumulative sum of histogram frequencies to a second preselected threshold value.
Claim 15
Dependent Claim
56 words
Method for pre-processing document image data arranged as a plurality of pixels of claim 13, characterized in that the slope (307) of the linear function (307, 305) for pixel gray level values less than the contrast reduction protection value is empirically determined for a test set of documents of a class of documents to be compressed.
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