Contrast enhancement of an image using luminance and RGB statistical metrics
First Claim
1. A method of enhancing an image, said image comprising a plurality of colors, each color having a range of tone levels, said method comprising:
- stretching the range of at least one color in said image, wherein the mean tone level for said at least one color remains approximately the same and the range of tone levels is stretched more at tone levels further away from said mean tone level than at tone levels close to said mean tone level.
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Abstract
An automatic contrast enhancement method improves the quality of an image by increasing the dynamic range of the tone levels in an image without causing an undesirable hue shift. An overall stretch factor that stretches the dynamic range of all the colors is generated based on the standard deviation of the tone levels for the overall luminance of the image. A color weighting factor is used to individually control the amount that each color is stretched. The color weighting factor is based on the difference between the standard deviation of the tone levels for the overall luminance of the image and the standard deviation of the tone levels for each color. An anchor factor is used to preserve the mean tone level for each color while the tone levels far from the mean tone level are changed more dramatically than the tone levels close to the mean tone level, which minimizes hue shifts while maximizing contrast enhancement.
17 Citations
20 Claims
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1. A method of enhancing an image, said image comprising a plurality of colors, each color having a range of tone levels, said method comprising:
stretching the range of at least one color in said image, wherein the mean tone level for said at least one color remains approximately the same and the range of tone levels is stretched more at tone levels further away from said mean tone level than at tone levels close to said mean tone level. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 16, 17, 18, 20)
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15. A method of enhancing a digital image, said image comprising a plurality of colors, each color having a range of tone levels, said method comprising:
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producing histogram information for each color in said image and for the luminance of said image;
stretching the range of each color in said image by a weighting factor based in part on the difference between the standard deviation of the tone levels for the luminance minus the standard deviation of the tone levels for each respective color;
stretching the range of each color in said image by an overall stretch factor determined in part by the standard deviation of the tone levels of the luminance; and
holding the mean tone level for each color at approximately the same tone level, wherein tone levels near said mean tone levels are stretched less than tone levels relatively far from said mean tone level.
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19. A printing system including a printer and a computer, said printing system performing a contrast enhancement method which enhances the printed image by stretching the range of the tone levels for each color in said printed image while maintaining the same mean tone level for each color to prevent hue shift, said method performed by said system comprising:
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stretching the range of each color in said image by a weighting factor based in part on the difference between the standard deviation of the tone levels for the luminance of the image minus the standard deviation of the tone levels for each respective color;
stretching the range of each color in said image by an overall stretch factor determined in part by the standard deviation of the tone levels of the luminance of the image.
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Specification