page1titlesVision by Man and Machine Tomaso Poggio imagesimage1page2page3imagesimage1page4page5imagesimage1tablestable1page6titlesa imagesimage1image2page7imagesimage1image2image3page8imagesimage1page9titlesc a' b' c' imagesimage1image2image3image4page10titles1< ,I imagesimage1image2image3page11imagesimage1image2image3image4page12imagesimage1image2image3image4page13page14page15titles~ ••••• - •• ~ imagesimage1image2tablestable1table2page16titlesa c .' , • , d""""'- imagesimage1image2image3image4image5image6image7image8image9VisionbyMan and MachineHow does an animal see? How might a computer doit?A study of stereo visionguides research on both these questions. Brain science suggests computer programs;the computer suggests what to look for in the brain.Tomaso PoggioApril, 1984The development of computers of increasingpower and sophistication often stimulatescomparisons between them and the humanbrain, and these comparisons are becoming moreearnest as computers are applied more and more totasks formerly associated with essentially humanactivities and capabilities. Indeed,itis widely ex-pected that a coming generation of computers androbots willhave sensory, motor and even "intellec-tual" skillscloselyresemblingour own. How mightsuch machines be designed?Can our rapidly grow-ing knowledgeof the human brain be a guide?Andat the same time can our advances in "artificialintelligence" help us to understand the brain?At the level of their hardware (the brain's or acomputer's) the differencesare great. The neurons,or nerve cells, in a brain are small, delicate struc-tures bound by a complex membrane and closelypacked in a medium of supporting cellsthat controla complexand probably quite variable chemicalen-vironment. They are very unlike the wires andetched crystals of semiconducting materials onwhich computers are based. In the organization ofthe hardware the differences also are great. Theconnections between neurons are very numerous(anyone neuron may receive many thousands ofinputs) and are distributed in three dimensions. In acomputer the wires linking circuit components arelimited by present-day solid-state technology to arelatively small number arranged more or lesstwo-dimensionally.In the transmission of signals the differencesagain are great. The binary (on-off) electric pulsesof the computer are mirrored to some extent in theall-or-nothing signal conducted along nerve fibers,but in addition the brain employs graded electricalsignals, chemical messenger substances and thetransport of ions. In temporal organization the dif-ferences are immense. Computers process infonna-tion serially (one step at a time) but at a very fastrate. The time course of their operation is governedby a computer-wide clock. What is known of thebrain suggests that it functions much slower butthat it analyzes information along millions of chan-nels concurrently without need of clock-drivenoperation.How, then, are brains and computers alike?Clearly there must be a level at which any twomechanisms can be compared. One can comparethe tasks they do. "To bring the good.news fromGhent to Aix" is a description of a task that can bedone by satellite,telegraph, horsebackmessengeror82 • TOMASO POGGIOpigeon post equally well (unless other constraintssuch as time are specified). If, therefore, we assertthat brains and computers function as information-processing systems, we can develop descriptions ofthe tasks they perform that will be equally applica-ble to either. We shall have a common language inwhich to discuss them: the language of informationprocessing. Note that in this language descriptionsof tasks are decoupled from descriptions of thehardware that perform them. This separability is atthe foundation of the science of artificial intelli-gence. Its goals are to make computers more usefulby endowing them with "intelligent" capabilities,and beyond that to understand the principles thatmake intelligence possible.In no field have the descriptions of information-processing tasks been more precisely formulatedthan in the study of vision. On the one hand it is thedominant sensory modality of human beings. If wewant to create robots capable of performing com-plex manipulative tasks in a changing environment,we must surely endow them with adequate visualpowers. Yet vision remains elusive. It is somethingwe are good at; the brain does it rapidly and easily.It is nonetheless a mammoth information-process-ing task. If it required a conscious effort, like addingnumbers in our head.. we would not undervalue itsdifficulty. Instead we are easily lured into oversim-ple, noncomputational preconceptions of what vi-sion really entails.Ultimately, of course, one wants to know howvision is performed by the biological hardwareof neurons and their synaptic interconnections. Butvision is not exclusively a problem in anatomy andphysiology: how nerve cells are interconnected andhow they act. From the perspective of informationprocessing (by the brain or by a computer) it is aproblem at many levels: the level of computation(What computational tasks must a visual systemperform?), the level of algorithm (What sequence ofsteps completes the task?) and then the level ofhardware (How might neurons or electronic circuitsexecute the algorithm?). Thus an attack on the prob-lem of vision requires a variety of aids, includingpsychophysical evidence (that is, knowledge of howwell people can see) and neurophysiological data(knowledge of what neurons can do). Finding work-able algorithms is the most critical part of the proj-ect, because algorithms are constrained both by thecomputation and by the available hardware.Figure 5.1 STEREO VISION BY A COMPUTER is shownin aerial photogI"aphs (provided by RobertJ.Woodham).They were made from different angln !IOthat objects ineach have slighUy different positions. The images weremade by a mosaic of microelectronic 8e1UlOl'8,each of whichmeasurn the intensity of light along a particular line ofsight, as do the photoreceptor cells of the eye. The map illthe bottom wal generated by a computer programmed 10follow a procedure devised by David Marr and the authorand further developed by W. Eric L Grimson. The com-puter <ered the images 10 emphasiu spatial changes inintensity. Then it pedormed stereopsis: it matched featarnfrom one image to the other, determined the disparitybetween tbeir positions and calculated their relativedepths in the three-dimensional world. Increasing eleva-tions in the map
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