Real-time image enhancement techniques

Claim 2

A method of enhancing a color image as set forth in Claim 1 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 intensity values, and the second dimension representing the dynamic range of intensity values, the method further comprising the steps of: defining a first intensity value point as being represented by the minimum intensity value in the image; defining a second intensity value point as being represented by the maximum intensity value in the image; and selecting third and fourth intensity value points having values in the first dimension between the first and second intensity value points.

Claim 3

A method of enhancing a color image as set forth in Claim 2 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 intensity value in the image, vmax is the maximum intensity value in the image; the method further comprising the step of: 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 intensity values and expanding low intensity 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 intensity values and expanding higher intensity values.

Claim 4

A method of enhancing a color image as set forth in Claim 3 wherein vupper and vlower are calculated by performing the steps of: developing an intensity 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 intensity values in this sub-range is calculated; the center of gravity of the intensity 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 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 5

A method of enhancing a color image as set forth in Claim 4 wherein vupper and vlower are calculated by performing the steps of: defining the full dynamic range of intensity values of the video processing system as [VMIN, VMAX], wherein VMIN corresponds substantially to the minimum intensity value capable of being represented in the video processing system, and VMAX corresponds substantially to the maximum intensity value capable of being represented in the video processing system; linearly mapping intensity values in the range of vmin and vlower in the first dimension to intensity values in the range of VMIN and VLOWER in the second dimension, where VLOWER = VMIN + {( vlower - vmin )/ m} , where m is a real number in the approximate range of 2 to 4; linearly mapping intensity values in the range of vlower and vupper in the first dimension to intensity values in the range of VLOWER and VUPPER in the second dimension, where VUPPER = VMAX - {( vmax - vupper )/k}, where k is a real number in the approximate range of 2 to 4; and linearly mapping intensity values in the range of vupper and vmax in the first dimension to the intensity values in the range of VUPPER and VMAX in the second dimension.

Claim 6

A method of enhancing a color image as set forth in Claim 5 wherein the video processing system consists of a video display device and VMIN and VMAX are selected to correspond to the minimum and maximum intensity values capable of being displayed on the video display device.

Claim 7

A method of enhancing a color image as set forth in Claim 5 wherein a second transformation function is used to transform the saturation components of the original image into saturation components for the enhanced image.

Claim 8

A method of enhancing a color image as set forth in Claim 7 wherein the second transformation function is determined by applying a histogram stretching procedure to a saturation histogram representing the saturation components of the color image.

Claim 9

A method of enhancing a color image as set forth in Claim 8 wherein the histogram stretching procedure includes the steps of selecting a first saturation value point, smin , representing the minimum saturation value in the image, and a second saturation value point, smax , representing the maximum saturation value in the image.

Claim 10

A method of enhancing a color image as set forth in Claim 9 wherein an upper limit of saturation, supper , is defined as that saturation level where the area under the saturation histogram from that level to the maximum level of saturation, smax , is equal to a small percentage of the total number of pixels in the image.

Claim 11

A method of enhancing a color image as set forth in Claim 10 wherein the small percentage is approximately equal to 0.5% of the total number of pixels in the image.

Claim 12

A method of enhancing a color image as set forth in Claim 10 wherein saturation components in the range of smin to supper are linearly mapped to the entire dynamic range of saturation values [SMIN, SMAX] available in the video processing system, whereby small perturbations in the saturation values of pixels are ignored without compromising the integrity of the enhanced image.

Claim 13

A method of enhancing a color image as set forth in Claim 10 wherein the intensity values of the image are modified using the first transformation function to provide enhanced image intensity values, and the saturation values of the image are modified using the second transformation function to provide enhanced image saturation values.

Claim 14

A method of enhancing a color image as set forth in Claim 13 wherein the hue components of the image to be enhanced constitute the hue components of the enhanced image.