1. A foreground extracting method comprising: sampling an input image at a plurality of different resolutions to generate a plurality of sampling images; detecting contours respectively from the sampling images to generate a plurality of contour images comprising a current frame and a previous frame that precedes the current frame by a preset interval; comparing first positions of a first plurality of pixels in a first decision-making area of the current frame with second positions of a second plurality of pixels in a second decision-making area of the previous frame; determining directions from the second positions to the first positions and distances between the first positions and the second positions; excluding, from the first decision-making area, at least one pixel of the second plurality of pixels in response to the distance of the at least one pixel being less than or equal to a preset critical magnitude; and setting the first decision-making area having a highest resolution as a foreground area wherein the preset interval decreases as a resolution of the current frame increase.