Patent US 4,231,065

A local area contrast enhancement scheme for a video display includes a two dimensional recursive filter to obtain a moving average of the intensity variations in all areas of the display. This moving average is subtracted from the original signal to produce a signal which contains only local variations and this latter signal is expanded to increase the display contrast. The local area contrast enhancement scheme allows low contrast details to be perceived and permits hands-off operation of the display under varying scene conditions. The recursive filter approach results in a simple implementation of the enhancement scheme with both analog and digital circuitry.