1. A method for computer-aided object classification, soft segmentation and layer extraction in computed tomographic colonography, said method comprising the steps of: providing a contrast enhanced computed tomography (CT) digital image of the colon, said image comprising a plurality of intensities associated with a 3-dimensional (3D) grid of voxels; finding a foreground region of voxels with an intensity higher than a pre-defined foreground threshold; creating a 3D trimap of said colon wherein said image is segmented into said foreground region, a background region, and an unknown region between the foreground and background; starting from said background, extracting successive layers of said unknown region until said foreground region is reached, and classifying each extracted layer as background or foreground; and generating a foreground matte, a background matte, and an alpha matte, wherein alpha indicates a mixing ration of foreground and background voxels.