A background image is generated based on a captured image. A data cluster is formed in pixel blocks of the background image using at least one feature of pixels in the pixel blocks. A data cluster formed in each pixel block includes a data distribution having a mean value and a standard deviation from the mean value. After generating the background image, each pixel of a subsequent captured image is compared with the data cluster of a pixel block of the background image to generate a first discrepancy value. A pixel of a subsequent image is compared with a data distribution of another adjacent pixel block of the background image to generate a second discrepancy value. Based on the discrepancy values, the pixels of the subsequent image are regarded as background or foreground pixels in a binary map, in which connected foreground pixels are marked to form a foreground object.