Method for sharpening a digital image

1. A method of sharpening a digital image having image pixels values, comprising the steps of:

determining a plurality of D min values, each said D min value representing a respective pixel value of a reference dark scene image corresponding to a minimum possible exposure for a system that produced the digital image;

averaging said determined D min values to remove fluctuations due to noise in the pixel values of the reference dark scene image and provide an average D min value;

for each pixel value in the digital image providing a gain factor that is dependent on the difference between the pixel value and the average D min value, such that pixel values nearer the average D min value have smaller gain factors, said gain factors are provided by a lookup table that models a monotonic function of pixel value, said monotonic function increasing from the average D min value to a predetermined pixel value and being constant for pixel values that exceed the predetermined pixel value;

applying an image processing path including one or more image transforms to the original digital image resulting in a rendered digital image;

applying the image processing path to the gain factors to form rendered gain factors; and

using the rendered gain factors to sharpen the rendered digital image.

2. A computer program product stored on computer readable medium for sharpening a digital image having image pixels values, comprising the steps of:

determining a plurality of D min values, each said D min value representing a respective pixel value of a reference dark scene image corresponding to a minimum possible exposure for a system that produced the digital image;

averaging said determined D min values to remove fluctuations due to noise in the pixel values of the reference dark scene image and provide an average D min value;

for each pixel value in the digital image providing a gain factor that is dependent on the difference between the pixel value and the average D min value, such that pixel values nearer the average D min value have smaller gain factors, said gain factors are provided by a lookup table that models a monotonic function of pixel value, said monotonic function increasing from the average D min value to a predetermined pixel value and being constant for pixel values that exceed the predetermined pixel value;

applying an image processing path including one or more image transforms to the original digital image resulting in a rendered digital image;

applying the image processing path to the gain factors to form rendered gain factors; and

using the rendered gain factors to sharpen the rendered digital image.

3. A system of sharpening a digital image having image pixels values, comprising:

means for determining a plurality of D min values, each said D min value representing a respective pixel value of a reference dark scene image corresponding to a minimum possible exposure for a system that produced the digital image;

means for averaging said determined D min values to remove fluctuations due to noise in the pixel values of the reference dark scene image and provide an average D min ;

means for providing a gain factor for each pixel value in the digital image, said gain factor each being dependent on the difference between the pixel value and the average D min , value, such that pixel values nearer the average D min value have smaller gain factors, said gain factors are provided by a lookup table that models a monotonic function of pixel value, said monotonic function increasing from the average D min value to a predetermined pixel value and being constant for pixel values that exceed the predetermined pixel value;

means for applying an image processing path including one or more image transforms to the original digital image resulting in a rendered digital image;

means for applying the image processing path to the gain factors to form rendered gain factors; and

means for using the rendered gain factors to sharpen the rendered digital image.