Vision is a big subject, still poorly understood, so our discussion in this section has been somewhat sketchy and disconnected. There are a whole bundle of techniques which may be used, depending on the properties of the images you want to recognise. Some, but not all, have been shown to correspond closely to processing in the human visual system. In any case, visual processing proceeds in stages, from edge detection to model-based recognition, obtaining representations which correspond more and more closely to real physical objects.