1. An image processing device comprising: image input means for capturing a scene including a moving object and generating a series of images, each image comprising a plurality of pixels, each of the images being generated at respective, different times; image storage means for storing the series of images generated by said image input means; difference image generating means for selecting at least two images generated at different times from among the series of images stored in said image storage means, computing a difference between the selected images by subtracting respective values of corresponding pixels of the selected images from each other to generate a difference image including a plurality of difference pixels having respective difference values that may be positive, negative, or zero, and storing the difference image; direction estimating means for estimating a direction of motion of the moving object along a plurality of axes for respective pixels of the difference image from the values of the difference pixels of the difference image and generating at least one vector representing the direction of motion of the moving object along the axes for the respective pixels, each vector having a direction along an axis determined by whether the corresponding difference pixel is positive or negative and whether the pixels on opposite sides of and adjacent to the difference pixel along the respective axis have different values; means for computing the sum of the vectors estimated by said direction estimating means for the respective difference pixels and generating an initially-estimated motion vector for the respective difference pixels; and means for averaging, for a respective difference pixel, the initially-estimated motion vector for the respective difference pixel and for difference pixels adjacent the respective difference pixel, thereby generating a motion vector for the respective difference pixel.