Digital method of modifying an image contrast so as to retain imaging of small objects

Claim 1

A digital method of modifying an image to obtain an output image on the basis of an input image, each image being defined by a set of picture elements, denoted pixels hereinafter, each pixel containing at least a digital value defining its grey level among a set of possible values, said method comprising: a first stage in which a histogram is established which is representative of the distribution in a percent figure of the grey levels of the pixels of said input image, a second stage in which, on the basis of the distribution characteristics of said histogram a correspondence function is established between the input and output grey levels, a third stage in which the output image is formed on the basis of the pixels of the input image and of said correspondence function, said correspondence function consisting of clipping the pixels having extreme grey levels and of linearly re-distributing the grey levels of the remaining pixels, wherein clipping of the extreme grey level pixels is effected on the basis of an input grey level value which is specific for each image and is determined during the second stage, said input grey level value being chosen so as to provide that the grey level corresponds to a change in the significant slope of the curve of the established histogram, which change of slope is located between a predetermined first and a second accrued percentage figure.