Local adaptive contrast enhancement
First Claim
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1. A method for enhancing contrast of pixels of an image having foreground and background portions comprising the steps of,scanning the image;
- dividing the scanned image into a plurality of pixels;
generating a series of data signals, each signal having an intensity magnitude proportional to the amount of foreground and background in each pixel;
selecting a first number of pixel data signals defining a first window of pixels and a corresponding center pixel in said window;
storing in a memory values for offset and stretch corresponding to predetermined ranges of standard deviation and average intensity;
computing the average intensity and standard deviation of the selected pixel data signals in the window;
accessing said memory to retrieve a stored value of stretch corresponding to a computed standard deviation in a range between zero and approximately zero and corresponding to a computed average intensity in a range much greater than zero;
accessing said memory to retrieve a stored value of offset corresponding to a computed standard deviation in a range between zero and approximately zero and corresponding to a computed average intensity in a range between zero and approximately zero; and
selectively changing the intensity magnitude of a pixel data signal for a pixel disposed near the center pixel of said window in accordance with accessed values of offset and stretch.
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Abstract
In a mail sorting system 10, output pixel intensities of an optical scanner 18 have their contrast locally enhanced. A contrast enhancer 24 uses statistical methods (averaging, standard deviation) coupled with empirical stretch and offset data stored in a ROM 25, to enhance pixel contrast in a pipelined processing operation.
71 Citations
27 Claims
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1. A method for enhancing contrast of pixels of an image having foreground and background portions comprising the steps of,
scanning the image; -
dividing the scanned image into a plurality of pixels; generating a series of data signals, each signal having an intensity magnitude proportional to the amount of foreground and background in each pixel; selecting a first number of pixel data signals defining a first window of pixels and a corresponding center pixel in said window; storing in a memory values for offset and stretch corresponding to predetermined ranges of standard deviation and average intensity; computing the average intensity and standard deviation of the selected pixel data signals in the window; accessing said memory to retrieve a stored value of stretch corresponding to a computed standard deviation in a range between zero and approximately zero and corresponding to a computed average intensity in a range much greater than zero; accessing said memory to retrieve a stored value of offset corresponding to a computed standard deviation in a range between zero and approximately zero and corresponding to a computed average intensity in a range between zero and approximately zero; and selectively changing the intensity magnitude of a pixel data signal for a pixel disposed near the center pixel of said window in accordance with accessed values of offset and stretch. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12)
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13. A system for enhancing contrast of pixels of an image having foreground and background portions comprising,
an optical image scanner for scanning an image and generating a series of pixel data signals having magnitudes representative of the intensity of each pixel in the image, means the selecting a first number of pixel data signals to define a first window of pixel signals and a corresponding center pixel signal in said window, means for computing the average intensity and standard deviation of the pixel signals of said window, a read-only memory for storing values of offset and stretch corresponding to a predetermined range of standard deviation between zero and approximately zero and predetermined ranges of average intensity, including a first average intensity range between zero and approximately zero and a second average intensity range of much greater than zero, and means for enhancing contrast by selectively changing the intensity of a pixel data signal of a pixel near the center pixel in accordance with the accessed values of offset and stretch.
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25. In a system for contrast enhancement of pixel data, a chip set, including a first integrated circuit comprising,
an input for receiving data signals representative of pixel intensity, a first circuit for squaring the value of pixel intensity data signals, a second circuit for summing the squared values of pixel intensity data signals, a third circuit for summing the input pixel intensity data signals, a fourth circuit for squaring the sum of input pixel intensity data signals, a plurality of shift registers connected to said second, third and fourth circuit means for receiving data representative of the sum of the input pixel intensity data, the sum of the squares of input pixel intensity data, and the square of the sum of input pixel intensity data, an averaging and standard deviation calculating circuit coupled to the shift registers for computing the average and standard deviation of input pixel intensity data, a second integrated circuit comprising a memory having stored values of offset and stretch, said stored values of offset and stretch being stored in accordance with a range of standard deviation and average intensity, and a third integrated circuit comprising a pixel intensity data alteration circuit coupled to the first and second integrated circuits, for altering the magnitude of the input pixel intensity data signals in accordance with the standard deviation and the average intensity to generate output signals representative of locally contrast enhanced pixel intensity data.
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27. A method for enhancing contrast of pixels of an image having foreground and background portions comprising the steps of,
scanning the image; -
dividing the scanned image into a plurality of pixels; generating a series of data signals, each signal having an intensity magnitude proportional to the amount of foreground and background in each pixel; selecting a first number of pixel data signals defining a first window of pixels and a corresponding center pixel in said window; storing in a memory values for offset and stretch corresponding to predetermined ranges of standard deviation and average intensity; computing the average intensity and standard deviation of the selected pixel data signals in the window; accessing said memory when the computed standard deviation is in a range between zero and approximately zero; retrieving from said memory a stored offset value when said computed average intensity is in a range between zero and approximately zero; retrieving from said memory a stored stretch value when said computed average intensity is in a range much greater than zero; and selectively changing the intensity magnitude of a pixel data signal for a pixel disposed near the center pixel of said window in accordance with retrieved value.
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Specification