US 6,393,148 B1 · Contrast enhancement of an image using luminance and RGB statistical metrics

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.

Claims17

Dependent claims

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, wherein said stretching the range comprises:
generating one overall stretch factor for all of said plurality of colors;
generating a color weighting factor for each respective color;
generating an anchor factor based on the mean tone level for each respective color, wherein said anchor factor holds the mean tone level at approximately the same tone level;
and
using said overall stretch factor, said color weighting factor, and said anchor factor to stretch the range for at least one color.

3 more elements

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:

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.

2 more elements

Dependencies are inferred from the claim text.

Contrast enhancement of an image using luminance and RGB statistical metrics

Life of this patent

Contrast enhancement of an image using luminance and RGB statistical metrics

Life of this patent

Abstract

Claims17