Real-time image enhancement techniques

Claim 1

A method of enhancing a color image using a video processing system equipped to represent a dynamic range of intensity values and a dynamic range of saturation values, the method comprising the following steps: (a) representing the color image by a plurality of pixels in YC r C b (luminance and chrominance) color space, the color image having a finite range of luminance values and a finite range of chrominance values; (b) applying a first transformation function to the finite range of luminance values and a second transformation function to the finite range of chrominance values; the first transformation function transforming the first finite range of luminance values to correspond to the entire dynamic range of luminance values; and the second transformation function transforming the finite range of chrominance values to correspond to the entire dynamic range of chrominance values; the first and second transformation functions providing an enhanced image having fully optimized luminance and chrominance values without converting a color image from YC r C b color space to another color space, wherein the first transformation function is a piece wise linear mapping function in two dimensions having three substantially linear regions defined by four intensity value points; the first dimension representing the finite range of luminance values, and the second dimension representing the dynamic range of luminance values; (c) defining a first luminance value point as being represented by the minimum luminance value in the image; (d) defining a second luminance value point as being represented by the maximum luminance value in the image; (e) selecting third and fourth intensity value points having values in the first dimension between the first and second luminance value points, wherein the piece wise linear mapping function consists of first, second, and third line segments, the first line segment being defined by points vmin and vlower, the second line segment being defined by points vlower and vupper, and the third line segment being defined by points vupper and vmax, such that vmin is the minimum luminance value in the image, vmax is the maximum luminance value in the image; (f) selecting vlower and vupper based upon the relative overexposure or underexposure of the image such that, if the image is relatively under-exposed, vlower is selected to be relatively close to vmin and vupper is selected to be relatively distant from vmax, thereby compressing high luminance values and expanding low luminance levels; and if the image is relatively over-exposed, vupper is selected to be relatively close to vmax, and vlower is selected to be relatively distant from vmin, thereby compressing lower luminance values and expanding higher luminance values; wherein vupper and vlower are calculated by performing the steps of: (i) developing a luminance value histogram for the image to be enhanced by dividing the range from vmin and vmax into a plurality of sub-ranges such that, for each of the plurality of sub-ranges, the number of pixels in the image having luminance values in this sub-range is calculated; the center of gravity of the luminance value histogram being defined as the first moment and being indicative as to the relative over- or under-exposure of the image, such that the first moment is relatively closer to vmin than to vmax for an underexposed image and relatively closer to vmax than to vmin for an overexposed image; and (ii) selecting vlower and vupper such that, as the first moment becomes increasingly closer to vmin, vupper increases and vlower decreases, and, as the first moment becomes increasingly closer to vmax, vupper decreases and vlower increases.

Claim 2

A method of enhancing a color image using a video processing system equipped to represent a dynamic range of intensity values and a dynamic range of saturation values, the method comprising the following steps: (a) representing the color image by a plurality of pixels in YC r C b (luminance and chrominance) color space, the color image having a finite range of luminance values and a finite range of chrominance values; (b) applying a first transformation function to the finite range of luminance values and a second transformation function to the finite range of chrominance values; the first transformation function transforming the first finite range of luminance values to correspond to the entire dynamic range of luminance values; and the second transformation function transforming the finite range of chrominance values to correspond to the entire dynamic range of chrominance values; the first and second transformation functions providing an enhanced image having fully optimized luminance and chrominance values without converting a color image from YC r C b color space to another color space, wherein the first transformation function is a piece wise linear mapping function in two dimensions having three substantially linear regions defined by four intensity value points; the first dimension representing the finite range of luminance values, and the second dimension representing the dynamic range of luminance values; (c) defining a first luminance value point as being represented by the minimum luminance value in the image; (d) defining a second luminance value point as being represented by the maximum luminance value in the image; (e) selecting third and fourth intensity value points having values in the first dimension between the first and second luminance value points, wherein the piece wise linear mapping function consists of first, second, and third line segments, the first line segment being defined by points vmin and vlower, the second line segment being defined by points vlower and vupper, and the third line segment being defined by points vupper and vmax, such that vmin is the minimum luminance value in the image, vmax is the maximum luminance value in the image; (f) selecting vlower and vupper based upon the relative overexposure or underexposure of the image such that, if the image is relatively under-exposed, vlower is selected to be relatively close to vmin and vupper is selected to be relatively distant from vmax, thereby compressing high luminance values and expanding low luminance levels; and if the image is relatively over-exposed, vupper is selected to be relatively close to vmax, and vlower is selected to be relatively distant from vmin, thereby compressing lower luminance values and expanding higher luminance values; wherein vupper and vlower are calculated by performing the steps of: (i) developing a luminance value histogram for the image to be enhanced by dividing the range from vmin and vmax into a plurality of sub-ranges such that, for each of the plurality of sub-ranges, the number of pixels in the image having luminance values in this sub-range is calculated; the center of gravity of the luminance value histogram being defined as the first moment and being indicative as to the relative over- or under-exposure of the image, such that the first moment is relatively closer to vmin than to vmax for an underexposed image and relatively closer to vmax than to vmin for an overexposed image; and (ii) selecting vlower and vupper such that, as the first moment becomes increasingly closer to vmin, vupper increases and vlower decreases, and, as the first moment becomes increasingly closer to vmax, vupper decreases and vlower increases.