A frame is obtained from an image sensor. The frame may include a number of zones, each of which in turn includes a number of pixels. Each one of the pixels may be associated with a pixel value. A pixel change value for each one of the pixels in the frame may be determined based on an average pixel value of the pixel over a number of frames. The pixel change value for each pixel may then be normalized based on an average zone change value for the zone in which the pixel is located. The average zone change value may be based on the pixel change value for each one of the pixels in the zone. By using the average zone change value to normalize the pixel change value, an image frame may be analyzed in an efficient and fast manner.