PRINCIPLE AND IMPLEMENT OF MEASURABLE VIRTUAL REALITY (MVR) BASED ON SEAMLESS STEREO-ORTHOIMAGE DATABASE

Transcription

1 RINCILE AND IMLEMENT OF MEASURALE VIRTUAL REALITY (MVR) ASED ON SEAMLESS STEREO-ORTHOIMAGE DATAASE Deren LI, M WANG, Janya GONG Natonal Laboratory for Informaton Engneerng n Surveyng Mappng and Remote Sensng, Wuhan Unversty,.R. Chna dl@wtusm.edu.cn,wangm@rcgs.wtusm.edu.cn, jgong@rcgs.wtusm.edu.cn Commsson IV, WG IV/6 KEY WORDS: Seamless Image Database, Measurable Vrtual Realty, Dgtal Orthophoto Quadrangles (DOQ), Dgtal Stereo-orthophoto artner (DS) ASTRACT: The dea and concept of Measurable Vrtual Realty (MVR) based on seamless stereo-orthomage database s frstly put forward. The meanngs and basc prncple of MVR s dscussed n ths paper. ased on stereovson of human s eyes, the man dea of MVR s to generate a seamless Dgtal Orthophoto Quadrangles (DOQ) database from DEM and odd photos and generate addtonally a dgtal stereo-orthophoto partner (DS) from DEM and even photos, then a large area 3D vrtual landscape envronment can be formed wthout y-parallax. In such envronment, the 3D measurement and analyss can be done under the nterface of normal GIS or CAD systems wthout complex DW. The 3D objects, such as houses, trees, cloverleaf junctons, geologc ruptures, and so on, whch are not acqured durng data acquston at DW, can be measured by end user hmself. The project desgn n such 3D vrtual landscape envronment wll be more ntutonstc and more realstc. The applcaton felds and the prospects of MVR are also presented n ths paper. At last, some experment results of MVR are dscussed. 1. Introducton Vrtual Realty s a rapd development research doman n recent years and has been pad extensve attenton by more and more people (Jnde Lu and Wanjun Jng, 1997). The development of Vrtual Realty reflects the human requrement. Wth the rapd development of computer technology and popularzaton of ts applcaton, the performance and ablty of data processng of computer has been mproved day by day. Durng the nteracton between human and computer, people pursue the way of freedom and realzaton and hope to nteract wth computer n the habts of our daly lfe. y producng vvd vson mage on computer screen or n the other envronments, vrtual Realty can make people feel n the real world and can do some operatons and controls of the system lke n our real lfe. At the same tme, the system can gve real-tme and correct response to the operators. In other words, vrtual realty provdes a vrtual world for people; however, t can gve people the sense lke n the real world. ut for the restrcton of the factor n hardware, software and algorthms, especally the hardware, vrtual realty s not mature and has a long dstance that people have expected (Stephen R Ells, 1994). Vrtual Realty has been appled n many felds. In geographc felds, the smulatons of terran and cyber cty are the aspect of vrtual realty. Terran vsualzaton s a hot topc n GIS researches doman. At present, the methods for terran vsualzaton are mostly based on the acqured terran models or 3D geometrc models (such as DEM, the 3D model of buldngs etc). The 3D geometrc models are bult wth the computers after the observaton pont and drecton are gven, and then a seres of operatons as render, object hdng, llumnaton, texture and projecton have been carred out to create vrtual topographc scenes. ecause t depends on the computer hardware support, t can t afford to buld a lot of 3D models n large area. In addton, 3D geometrc models must be acqured before renderng. So we have to spend a lot of tme and much money on data acquston. At the same tme, there are many unsolved problems, such as constructng complex 3D models, the real texture, and so on. Stereovson model based on human s physologcal parallax s the foundaton of photogrammetry. The means of renderng real terran and ground features at the moment of photography wth stereoscopc equpment s also a style of terran vsualzaton. The advantages of stereo model based mages are only to restore the parallax at the moment of photograph wthout the complex course of constructng models. ut as the mages from orgnal photograph have vertcal parallax, the stereo model after a seral of complex orentaton operatons s only set up by a photogrammetrc operators who s specally well traned and the stereoscopc range s only lmted to the small overlap range of one stereo par. The neghbor stereo models are separate and cannot mosac seamless stereo mage database over a large area. The stereo model based eppolar lne mages s free from vertcal parallax, t cannot also be mosac together because of ther dfferent eppolar lne drectons, the stereoscopc range s also lmted a par of mages. T.lachut and S.Collns put forward the concept of stereo orthophoto par n 1968 (Kraus, 1984;Deren L, 1988; Deren L and haobao heng, 1992). The man dea s to construct stereo model usng orthophoto and a specally made photo called the stereo-partner. Orthomage has seamless characterstc and can generate a seamless orthomage database. So the thought of constructng large range seamless stereo modal across stereo pars s put forward. In addton, the stereo model based on stereo orthophoto par has measurement characterstcs. So t s called Measure Vrtual Realty (MVR). 2. What Is MVR? MVR s put forward vs. Vrtual Realty n geographc felds. Its defnton s as follows: MVR ( Measurable Vrtual Realty ) s measurable Vrtual

2 Realty(VR), whch makes use of the prncple of stereovson of human s eyes. It s based on DOQ (Dgtal Orthophoto Quadrangles) and so called Dgtal Stereo-orthophoto artner (DS). DS s a producton of addtonal mage n whch the x-parallaxes are artfcally ntroduced accordng to DEM. A geometrcally correct 3D landscape model wll be obtaned by means of the stereoscopc vew of DOQ and DS wth the stereo observaton equpments such as lqud crystal glasses and anaglyphoscopeto n GIS or CAD platform. We can measure the heght and poston of nterestng features durng vewng stereo model wthout complex operatons, ncludng those features that ddn't be measured durng data acquston at DW. As the orthophoto mages can mosac a very large seamless mage or create a seamless mage database. So every body hmself can seamlessly stereo-vew and measure the whole stereo landscape model n a large area (not only wthn a stereo par). 3. Why MVR? The man features of MVR are the two aspects of Vrtual Realty and Measurable Characterstc. The geometrcally correct landscape modal constructed by DOQ and DS has only x-parallaxes and s free from y-parallaxes. So the stereo model s set up very easly wthout complex orentaton operatons. As the orthomage has good planmetrc geometry accuracy, we also can measure the heght and poston of nterestng features durng vewng MVR model by means of an automatc mage matchng method. The advantages of MVR are at least: 1) The remeasurement of DEM wll provde correcton and refnement of exstng DEM. The blunders of orgnal DEM can be easly detected by usng MVR. 2) Those 3D objects, whch ddn t be or couldn t be measured by the data acquston at DW, such as heght and form of trees, geologcal features, the heght of houses and so on, can be done wth MVR by end users at normal GIS or CAD envronment. 3) The dfferent desgner and engneer who works at 2D GIS or map envronment can drect work at 3D MVR envronment. 4. The asc rncple of MVR In the feld of photogrammetry and RS, dgtal dfferental rectfcaton s a very mature technology at present, whch manly uses for the producton of orthomage. Dgtal orthomage not only has correct planar poston, but also hold abundant mage nformaton. However, t s only twodmenson, not ncludng three-dmenson nformaton. Though that can partly meet the fault by means of overlayng contour on the orthomage, t s mpossble to substtute for stereoscopc vew to obtan stereo sense. Therefore, we can artfcally produce a so-called Dgtal Stereo-orthophoto artner (DS) vs. Dgtal Orthophoto Quadrangles (DOQ). DOQ and DS are all together called Stereo Orthophoto ar (lachut, 1968,1976). The basc prncple of MVR s based on stereo orthophoto par. Fgure 1 summarly shows the basc prncple of MVR(Kraus,1984). As shown n Fgure 1, the coordnate z of the grd Y ponts forms dgtal elevaton model. Fgure 1a shows the dagram of makng orthomage, drectly transformng the elevaton of grd pont to orgnal photo accordng to collnearty equaton to obtan mage data for orthomage. In order to obtan stereo landscape model, the x-parallaxes are artfcally ntroduced accordng to DEM and rght mage.the smplest method for parallax ntroducton s to use parallel ray wth a certan angle between -axs (Fgure 2b) to make a projecton to DEM. The artfcal parallaxes reflect the feature of terran and form an auxlary mage, whch can construct a stereo model wth orthomage. Fgure 2 s a vertcal plane dagram of projecton; take the ground pont as an example, the heght dfference relatve to the plane of projecton s, p 0 s the ortho projecton pont of and p 1 s the slope parallel projecton pont of. Orthophoto s obtaned by ortho projecton and Stereo Ortho-photo artner s obtaned by slope parallel projecton. The parallax s obtaned by stereo measurement, so t s obvous that = tan α = k (1) S rojecton Center S rojecton Center Left Image y x rojecton Drecton Rght Image y x rojecton Drecton DEM DEM Y Y a. Ortho rojecton b. Slope arallel rojecton Fgure 1 The asc rncple of MVR Y Y

3 roject Drecton a. Orthophoto roject Drecton Fgure 2 The arallel rojecton Method As the drecton of slope parallel projecton parallel wth plane, there are only x-parallaxes between orthophoto and stereo orthophoto partner. ecause they are generated from left and rght photos of a stereo par separately, t meets the prerequste condton of stereo measurement and can provde not only planmetrc correct poston but also the heght of ground pont. In order to make measurement, the angle α had better agree wth the parallaxes of orgnal stereo model, generally t s taken tan α =, s the base lne of stereo model, H s the H flght heght. The heght dfference relatve to the reference plane can be calculated by the x-parallaxes measured from the stereo orthophoto par dvded by coeffcent k and multpled the denomnator of photo scale. At last, the heght s get after the heght of startng pont s added. The formula s as follows, = M + 0 (2) k 5. The Several Methods for Introducton of Artfcal arallax An mportant advantage of MVR s the measurement of heght nformaton, therefore the correct ntroducton of artfcal parallax should be a very mportant ssue. Slope parallel projecton above dscussed s the smplest but not the best method; the other two more rgorous methods wll be dscussed n the followng. α 1 0 b. Stereo Orthophoto artner The parallax functon of logarthm projecton s: H = ln H Where: s base lne of photograph H s flght heght s the heght of ground pont s the parallax correspondng wth the heght of In logarthm projecton method, the heght of ground pont can be calculated by the followng formula, = H 1 exp + 0 (4) 5.2 Nonlnear rojecton Method Wth Changng Angle Though logarthm projecton method can ntroduce the parallax n accord wth the orgnal parallax, the pont of ntersecton between logarthm functon and DEM must be solved by successve approxmaton. So t s consdered that usng other method to replace logarthm projecton. As shown n Fgure 4, Fgure 4a shows the parallaxes produced by the dfferent heght dfference of ground pont relatve to reference plane durng photography. Fgure 4b shows two projecton rays wth the angle α 1 and α 2 for producng the same parallax wth the parallax of orgnal photograph. From the geometry relaton n Fgure 4, we can obtan, 1 tg α 1 = tg α 2 = 2 1 α 1 α 2 2 (3) (5) 5.1 Logarthm rojecton Method The parallaxes ntroduced by slope parallel projecton are lnear functon of heght dfference, whch s not n accord wth the parallaxes of orgnal stereo model and can cause measurement problems. In order to overcome the measurement problems, logarthm projecton s used to ntroduce parallaxes. The prncple of logarthm projecton s as shown n fgure 3. roject Drecton roject Drecton x1 x2 a The Orgnal arallaxes Fgure 4 The Orgnal arallaxes and Introduced arallaxes Therefore, n order to accord wth the orgnal parallaxes durng photography, Nonlnear rojecton Method Wth Changng Angle s put forward for ntroducng parallaxes. Fgure 5 shows the basc prncple of ths method; the angle of ray s changng wth the heght of ground pont. x1 x2 b The Introduced arallax Identcal wth Orgnal arallaxes a. Orthophoto b. Stereo Orthophoto artner Fgure 3 The Logarthm rojecton Method

4 roject Drecton roject Drecton matchng software. Fgure 5 Nonlnear rojecton Method Wth Changng Angle The parallax functon of Nonlnear rojecton Method Wth Changng Angle s: = H The heght of ground pont can be calculated by the followng formula, 6. How To Generate MVR? l H = + The prncple of MVR has been descrbled above. As the stereo orthophoto par s lmtted only a par of photos.however, MVR requres seamless roamng and vewng the stereo model n a large area.so t should be consdered the whole workng area and meet the requrement of DOQ and DS comng from left and rght mage whle producng MVR. For ths, the rule for producng MVR must be observed. Now, take three trpes wth ten photos of each as a exapmle to llusate the rule. Frstly, the photos s encoded along strpe drecton as shown follows Fgure 6 The Data Organzaton of MVR Accordng to the above way of data orgnaton, the steps of generatng MVR are as follows. Step1: Dgtal Orthophoto Quadrangles (DOQ) generated from left mage (1,3,5) and DEM Denotes makng DOQ usng odd number photo Denotes makng DS usng even number photo Step2:Image database generated from DOQ to construct a large seamless area through dgtal mosac. Step3: Dgtal Stereo-orthophoto artner (DS) generated from rght mage (2,4,6) and DEM usng above method Step4: Automatc retreve of stereo orthophoto par from DOQ and DS mage databases Step5: Usng lqud crystal glasses /Anaglyphoscopeto to observe and measure contnuous real 3D object by usng mage а1 a2 r a. Orthophoto b. Stereo Orthophoto artner (6) (7) In order to measure the heght of ground detals, the DS should be produced by the methods of logarthm projecton or nonlnear projecton wth changng angle that can acheve better measurement precson. In the case of the area wth lttle ground features, the slope parallel projecton may be used, however, the slope angle should be the tangent of base lne and flght heght and the flght heght should equal the average heght of the stereo model. If the heght doesn t be care about, DS can be produced by smple slope parallel projecton, whch may be used for mage stereoscopcal nterpretaton. 7. The Applcaton Felds and rospects of MVR 7.1 The Advantages of MVR Smply compared wth the product of orthomage, there are many advantages of the stereo model formed by MVR. 1. The convenence of orentaton and measurement. As DOQ and DS have the same scale and no y-parallaxes, t s convenent for the operaton of model orentaton. The heght calculaton can be accurately solved accordng to formula of parallax ntroducton. So stereoscopc vew and measurement can be performed wthout knowng a lot of knowledge of photogrammetry 2. The whole process of observaton and measurement doesn t need professonal sklls. The non-photogrammetry experts and engneers can easly learn and use MVR to solve ther own problems. 3. The Measurement of MVR can be at any GIS or CAD envronment. At present, the advanced C can equp wth correspondng stereo dsplay and measurement nstrument. 4. The stereo model formed by MVR s the best medum for photogrammetry to cooperate wth other dscplnes, such as engneerng desgn, plannng, forest, geology etc. Non-photogrammetry operators can acqure ther wanted nformaton by themselves wth very smple stereo measurement equpment through MVR. 7.2 The Applcaton Felds and rospects of MVR At present, MVR can be appled but not lmted n the followng aspects: 1. The map of forest and vegetaton dseases and nsect pests. The tree and vegetaton can be precsely classfed n the stereo observaton usng the MVR formed by color nfrared mage, ncludng drectly measurng the heght of vegetaton. 2. The area evaluaton of used farmland. MVR can help dentty the classfed edge of land and drectly measure the area n stereo envronment. It s very good for researches on the shape of the earth's surface and water loss and sol eroson. 3. The prelmnary plan of traffc lne. Ortho photo s the foundaton of traffc lne plannng, whch can spend short tme and less money. MVR can really reflect the condton on the spot and form a large-scale seamless stereo model, so t s very convenent for nterpretaton, selectng lne and measurng vertcal plane. 4. Selectng lnes for electrc power system and makng the map

5 of vertcal plane MVR provde a vrtual stereo seamless geometry model, whch can precsely make measurement on the ground surface and are very convenent for selectng lnes for electrc power system and makng vertcal plane. 5. Map Revson. MVR can be mmedately produced as long as there s DEM database. It s avalablty for revsng the ground features and the lttle change n heght. It s proved through experments that ortho photo s more convenent for map revson then orgnal photo. However, MVR can make out more 50 percent ground features by stereo observaton (Kraus, 1984). MVR can be regard as a brdge to communcate between photogrammetry and other dscplnes. The terran surface of nature s complex and changeable. So t s very arduous tasks for provdng servce only by photogrammetry workers, and furthermore, the requrements are dfferent for dfferent felds. For example, forest branch may be nterested n the heght of trees; plannng branch may be nterested n the heght of houses; geologc branch may be nterested n the heght of geologc ruptures. If the stereo model formed by MVR s taken as a new dgtal product, the geologst, the forester, the planner, the engneer or any other user of aeral photograph can easly use the accurate stereo model by hmself over the whole project area. 8. The Experment Results and Conclusons Accordng to above prncple, two test area wth dfferent mage scale are taken. The parameters of the photo are shown n Table 1. The stereo model can be observed wth crystal glasses or anaglyphoscopeto. The followng two experment results are a part of the whole stereo model and the stereo model can be drectly observed wth anaglyphoscopeto.all knds of ground features can be really vewed durng stereoscopc observaton. At the same tme we can roam the whole stereo model across stereo par and measure the heght of nterested ground features wth stereo mark. It s suffcently proven the feasblty and prospect of MVR. As the content of ths paper s lmted, the measurement accuracy of MVR wll be dscussed n another paper. Table 1 The arameters of hotography Test I Test II rncple Dstance mm mm hoto Scale 1: :8000 Format 23CM 23CM 23CM 23CM hoto Type Grey True Color xel Sze 25um 50um Average Flght Heght 4225m 2090m Resoluton of DEM 12.50m 5m Resoluton of orthomage 1m 0.5m Data Range Two Strps and Three Stereo Model of Each Strp Fve Strps and Fve Stereo Model of Each Strp Fgure 7 System Interface of MVR

6 Fgure 8 a Test Area I: Guangx rovnce n south Chna Fgure 8 b Test Area II: Three Gorges roject Area Fgure 8 The Experment Results References lachut T.J., Further Extenson of the Orthophoto Technque. The Canadan Surveyor,22(1), pp lachut T.J. and M.C.van Wjk, roducton and Accuracy of Smultaneously Scanned Sterro-orthophoto. hotogrammetrc Engneerng,42(12), pp lachut T.J., The Stereo-Orthophoto Technque n Cadastral and General Mappng. hotogrammetrc Engneerng and Remote Sensng,42(12), pp huxun hang and Janqng hang,1996. Dgtal hotogrammetry. ress of Wuhan Techncal Unversty of Surveyng and Mappng, Wuhan, Chna. Acknowledgment The research descrbed n ths paper was funded by the Excellent Key Laboratory Foundaton of Natonal Nature Scence Commttee. (No ) Ths support s valuable. Collns S.H., The Accuracy of Optcally rojected Orthophotos and Stereo-Orthophotos.The Canadan Surveyor, 24(5), pp Collns S.H., 1970.The Ideal Mechancal arallax for Sterro-orthophoto.The Canadan Surveyor, 24(5), pp Collns S.H., The Stereo-orthophoto ar. hotogrammetrc Engneerng,38(12), pp Collns S.H.Stereoscopc Orthophoto Maps.The Canadan Surveyor,1968,22(1), pp Kraus, K.1984.HOTOGRAMMETRIE and2, 1984 Duemmlers Verlag, 5300 onn1. Deren L and haobao heng,1992. Analytcal hotogrammetry. ublshng House of Surveyng and Mappng, ejng, Chna. Deren L,1988. New Technologes of hotogrammetry. ress of Wuhan Techncal Unversty of Surveyng and Mappng, Wuhan, Chna. Jnde Lu and Wanjun Jng,1997. What Is Vrtual Realty?--Essental Concepts And Workng Defnton. Computer Applcatons, 17(3), pp. 1-4 Stephen R Ells, What are Vrtual Envronments. CG&A, 3(1), pp hzhuo Wang, rncples of hotogrammetry (Wth Remote Sensng). ublshng House of Surveyng and Mappng, ejng, Chna.