Edge detection of monitoring image is basic work for advanced image processing and pattern recognition.Theory of directional filter is introduced into edge detection of monitoring image.Process of edge detection based directional filter is elaborated by taking Roberts operator as an example.Petrou-Kittler filter is introduced as basic filter

aiming to ramp edge in monitoring image.Petrou-Kittler filter is reconstructed by directional filter theory

and tri-directional Petrou-Kittler filter came into beings.Experimental results show that edge information is complete

thin and correct by processing of tri-directional Petrou-Kittler filter.