Abstract: In any digital imaging system, the digital input signals must be converted to a visible image on some output material or medium. Because the range of this output medium is usually not the same as the input image range, care must be taken in converting the digital input signals into output signals. This conversion is done via the tone-scale transformation function. The type of tone-scale transformation function used largely determines the quality of the output image. A well designed tone-scale transformation function must ensure good contrast, no clipping and the display of the minimum and maximum densities that are present in the input. This invention deals with a reliable and robust technique of generating the tone-scale transformation function for digital projection X-ray input images. The method is based on the histogram entropy of the input image.