Even if the frame is the smallest time unit, one can still spatially segment a video at a sub-frame level, separating the frame image into its constituent objects. This is necessary when performing node segmentation at the object level. Time introduces complexity in this case also, for even after an object is differentiated in one frame, it is usually necessary to follow the same object through a sequence of frames. This process, known as object tracking, is essential to the creation of links from objects in videos. Spatial segmentation of object can be achieved, for example, through the use of intensity gradients to detect edges, color histograms to match regions, motion detection, or a combination of these and other methods.