1. A method for enhancing the contrast of an image, the image being composed of a multiplicity of pixels each having a specific brightness value, the method comprising the steps of: a) determining a mean brightness valvue of the individual pixels of the image; b) processing the brightness values of those pixels of the image that have a brightness less than or equal to the mean brightness value, in accordance with a first transformation function; c) processing the brightness values of those pixels of the image that have a brightness greater than the mean brightness value, in accordance with a second transformation function; and d) outputting the pixels having the brightness values processed in steps b) and c), respectively, in the form of a transformed output image; wherein a first straight-line function having a first variable slope factor is the first transformation function and the second transformation function is a second straight-line function having a second variable slope factor, the first slope factor of the first straight-line function and the second slope factor of the second straight-line function being determined by an analysis of the transformed output image.