1. A method for correcting shading effects in a video image using a histogram of gray scale intensity values of picture elements or pels making up the image, which histogram contains a median and black and white color extremes, comprising the steps of: generating a histogram of distribution of gray scale intensity values of the pels in an original image to be corrected: determining an acceptable range of background intensity values by: calculating the median and the black and white color extremes in the histogram: and calculating the distance between the histogram median and the color extreme on the side of the median having the majority of gray scale intensity values and setting said range as twice that distance centered at the median; creating a first background image by sampling the gray scale intensity value of each pel to be corrected and setting all values outside of said acceptable range to that of the color extreme least distant therefrom; creating a duplicate background image from said first background image; sampling the gray scale values of a number of pels in a predetermined region around a first pel in said duplicate background image; changing the value of the pel in said first background image corresponding to said first pel, to the largest value among those of said first pel and the sampled pels; continuing the foregoing sampling and changing steps until all of the pel values in said first background image have been changed accordingly; changing the duplicate background image in accordance with the changes in said first background image; repeating the sampling step over a larger region and a larger number of pels with the corresponding changing in the first background image; and repeating the foregoing changing until a desired corrected background image is obtained which becomes the shading correction for the original image.