An MPEG-Processor-based Robot Vision System for Real-Time De(2)

This paper describes a PC-based vision system that can be used to detect moving objects from a mobile robot. An image processing board equipped with an MPEG motion estimation processor calculates a sparse but robust optic flow in real-time. An algorithm to

2Real-timeoptic owsensor

TheMPEGcompressionstandardsusethespatio-temporalredundancyinanimagesequenceforband-widthreduction.Ifpossible,onlydisplacementvectorsaretransmittedinsteadofthecompletepixelinformation.Al-thoughtheMPEGstandardsdonotregulatehowthesevec-torshavetobecomputed,thestate-of-the-arttechniqueiscorrelation.Ascorrelationiscomputationallyexpensive,specializedprocessors,socalledMEPs(MotionEstimationProcessors)haveevolvedfromthisarea.Areferenceblock(RB,16x16pel)fromimageiscomparedwithasearchwindow(SW,32x32pel)intheimage.Forallpossi-bleoffsets

,acorrelation-likevaluecalledSAD(SumofAbsoluteDifferences)iscalculated;themini-mumdesignatesthebestmatch,anditspositionde nesthedisplacementvector:

(2)

(3)

Ifconsecutiveimagesarecompared(i.e.

),theresultingvector eldcanberegardedasoptic ow.

TheideatouseoneofthoseextremelyoptimizedMEPsforthegenerationofoptic owisnotnew.InparticularInoueetal.describetheintegrationofaMEPintotheirim-ageprocessingtransputernetwork[5].Resultingfromtheirwork,acommercialversionisavailable,andmeanwhileisusedinvariousresearchprojects[3,4].

Aproblemthatseveralresearchersreportisthattheoptic owgeneratedbysuchacorrelationprocessorcanbecomeverynoisy.ThishappenswhentheimagestructureoftheRBsorSWsisambiguousorcompletelymissing.Thenthedetectionofasigni cantminimumaccordingtoEqn.3fails.

Fig.2aillustratestheproblem.The owwasgeneratedbyalinearforwardmovementofthecamera,soallvectorsshouldintersectinasinglepoint,theFOE(FocusOfEx-pansion).Duetolocallackofstructure,mostvectorspointtocompletelydifferentdirections.Unfortunately,thiseffectdominatesinmostof ce-typeenvironments.Furtherevalu-ationofsucha ow eldisvirtuallyimpossible.

Tosolvethisproblem,weaugmentedaMEPwithexter-nalcircuitrythatcalculatesanadditionalcon dencevalueforeachvector[7].Inthesimplestcase,thiscon dencevaluecanbetestedagainsta xedthresholdtosiftoutthenoisy owvectors.Adrawbackisthesparsenessofthere-maining eld(seeFig.2b).

OurprototypesystemconsistsofanISAimageprocess-ingboardcontainingtheMEP,andaLINUXhostPC(seeFig.

3).

Figure2.Optic ow,generatedbyaMEP.a)complete,b)sifted.

Figure3.Optic owsensor:Systemstruc-ture.

Threeframememoriesallowthecomparisonoftwoim-agesandsimultaneousacquisitionofthenextimageintothethirdmemory.DependingontheutilizationoftheMEPinternalpipeline,upto525vectorscanbecalculatedperframe(PAL,25Hz).Forapplicationsliketrackingmulti-