Region of interest in video

Hi ,

Can anyone tell me how to find the region of interest in a video. The actual work is to find out signal blinking in a video . the signal blinking is constant location in the video.it will blink red and green based on the obstacles in front of the object moving.Please explain how can i start the process,whether i have to split videos to images then apply ROI? or in video the blinking is at constant location so i can directly apply the ROI logic?