DIGITAL STEREOPLOTTING USING THE PC
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1. 50 of the pixel size But there is a great possibility to enhance the mea surement accuracy by the computer supported mea surement If the measured point it is not a dot imaged by one pixel but it is defined rather by the crossection of lines or by a target of the known shape the mathematical analysis of chosen digital image portion can be applied There are already excellent results published eg Streilein amp Beyer 1991 or Jachimski amp Trocha 1992 which show that in the very favorable conditions the point determination accuracy can be 0 1 or even 0 05 of the pixel size This are very interesting prospects for the development of the plotting techniques on the screen of a monitor 3 THE AGH DIGITAL SCREEN STEREOPLOTTER As it was mentioned at the beginning of this paper the program and hardware for the stereo plotting on the screen was developed by the authors at The University laboratories Our Digi tal Screen Stereoplotter was originally designed as a stereocompiler Blachut 1971 which can fa cilitate the use of the stereoorthophotographs by the LIS customers It comprises the following functions the stereoscopic selection of the fragment of the halftone image on the sterecorthophotogram displayed on the screen at the reduced scale selection of the thematic maps which are to be displayed on the monitor from the numerical map files the stereoscopic survey and plotting of chosen features from the stereoo2. must be erased for that from the screen me mory and they can be regenerated two ways by afresh visualization of the full containing vectors and halftone image by recording in the computer memory the pixels masked by line segments to use them again if necessary The first method have been used in our programs when the existing line segment had to be erased while the second method was very convenient for the line segment animation design Zooming it is quite useful function of pro gram when plotting on the screen It can be used to enhance the operator confidence when selecting pointing by cursor pixels to measure the half tone image details or to have a general look on the scene The provided by the manufacturer ZOOM function harware is not always convenient That hardware image magnification technique doesn t distinguish between the halftone and vec screen 130 torial contest and blows up all the image As the result the vectors are not only longer but also thicker what is not very convenient There fore it is better to use specially programmed procedures for image magnification The advantage of the hardware ZOOM function it is its high speed though The potential accuracy of the surveying on the screen is limited by the pixel original size referred to the original scanned picture or to the object Proper selection of pixels during the plotting procedure can not result in the ac curacy greater than
3. of digital images using the popular equipment could be introduced to the secondary school programs geography lessons to popularize the potentials of photogrammetry and remote sensing techniques among children This way proper training could be provided anda very hard obstacle on the way to the mass use of stereoscopic survey methods by the grown ups could be removed The authors hope that the digital screen stereoplotter type equipment will be used as a regular peripheral to the LIS es and stereoplot ting will not be any more the exclusive skill of the very narrow group of operators We are also of the opinion that for such mass use by the non photogrammetrists the stereoorthophotographs pro vide so great advantages that the stereoortho rather than natural stereopares should be stored in the LIS files The results gained during experiments with our Digital Screen Stereoplotter show that that type of plotting is very promising and experi ments should be continued 131 Address for mail Prof It J zef Jachimski amp Mgr Eng Janusz Zielinski Photogrammetric Research The Faculty of Mining Geodesy and Environmental Engineering University of Mining amp Metallurgy AGH Al Mickiewicza 30 C4 30059 Krak w Poland Tel 04812 338100 ext 3826 or 2272 Fax 0481 2 331014 BIBLIOGRAPHY Agnard J P Gagnon P A Nolette C Bouilianne M 1990 A computer based general photogramme tric system Photogrammetric Engineerin
4. than followed digital ste reoorthophoto 1989 and programs to visualize and edit the vectorial numerical map on the PC monitor with the halftone digital raster image in the background First we developed programs for EGA graphics and than for SVGA 800x600 Finally programs for stereoscopic survey of photopairs on SVGA 800x600 was produced It is our hope that soon for better resolution SVGA 1024x768 programs will also be accomplished All this programs are 127 to be applied as an user oriented peripherals to the LIS User should be able to examine his the matic vectorial map eg cadastral on the PC mo nitor with the halftone terrain image in the background and to produce a new vectorial thema tic map if necessary using an accurate stereo survey on the screen 2 CONDITIONS OF THE METRICAL USE OF GRAPHICS ON PC MONITORS The initial configuration of the PC AT com puter for which the Digital Screen Plotter was programmed included the FGA graphics EGA card allows to generate only 4 steps of luminosity for each of 3 RGB colours and for each of 640x350 points on the screen To get a grey colour of point one must generate equal luminosity for 3 RGB colours To broaden the 4 steps grey scale to show better the image details the 4 steps exten sion scale in the colour close to neutral grey black green was designed Such 8 steps halftone scale was very contrast and gave unnaturally coloured but readable halftone images unfortun
5. DIGITAL STEREOPLOTTING USING THE PC SVGA MONITOR J zef Jachimski and Janusz Zieli ski Faculty of Mining Geodesy and Environmental Engineering University of Mining amp Metallurgy AGH Cracow Poland ABSTRACT For the general purpose PC SVGA computer a program package has been elaborated which enables the visualization and stereoscopic measurement of pairs of digital images In the paper the most vital in formation concerning software and hardware design is discussed and the plotter characteristic is given KEY WORDS Computer Graphics Analytical Plotter Stereoscopic Digital Systems 1 INTRODUCTION Land Information Systems LIS comprise va riety of descriptive information but on the first place they comprise geometrical data in a form of vectorial thematic maps The number of thematic layers certifies not only the universa lity of a data base but also the amount of money needed to keep the data base up to date In the data base there is cadastral informa tion which is of legal type and must be kept actual with the high geometrical accuracy But in the data base can be several other thematic la yers which comprise information about land use in the form of vectorial maps Such information ho wever often could be read easily directly from the aerial pictures It would be less expensive to keep such geometrical information in the pas sive form of digital raster images scanned from aerial photographs than to r
6. User Manual Autodesk LTD 1989
7. a tely it was not possible to broaden the scale further for example to 12 steps Remaining 8 of the 16 colours of EGA palette capacity could be used to visualize simultaneously variety of 8 di fferent thematic layers of LIS vectorial numeri cal map at that stage of development only 4 bright colours to show vectors red blue green yellow were used in our program The development of graphics hardware for PC computers resulted in VGA and SVGA cards The available popular SVGA cards allow for the maxi mum resolution of 1024x768 pixels and palette of 256 colours available simultaneously on the screen of analogue monitor The luminosity of each of the basic RGB colours can vary within the range of 64 steps scale what gives 64 256 K of various colours available The grey halftone sca le can be produced within the maximum of 64 steps scale For each colour including grey available Simultaneously on the screen a palette code num ber of 0 255 is assigned and one byte of memory on the graphic card is used to remember the code number for each pixel of the halftone image pro duced on the monitor screen The maximum resolution requires 1024x768 xlByte 768Kb memory available on the graphic card There are relatively popular cards which offer 1Kb memory and can easily handle the 256 colours palette for the top resolution but many of them offer only 512kb memory and therefore can produce only palette of 16 colours for the top resolutio
8. be carried out fully automati cally using automatic correlation procedure and this procedure doesn t require any additional in formation but just properly oriented natural ste reomodel Automatic correlation is not advanta geous however for production ortho stereopares in large scales or medium scales for urbanized ter rains For such cases it is safer to use a pre processed digital elevation model DIM to cont rol also automatically the differential rectifi cation process Differential rectification of na tural analogue pictures may be performed with a very high accuracy using a slit differential rectifier eg Avioplan OR1 Wild or Orthocomp Z2 zeiss The pre scanned halftone analogue pic tures in the raster digital form can be also dif ferentially rectified using adequate software package and even only PC type computers Here also rectification can be accomplished using au tomatic correlation rules eg Agnard et al 1990 or using preprocesed digital elevation model feg Jachimski et al 1988 Fig 1 Computer PC 386 with the colour 14 gra phical monitor The mirror stereoscope in special mount allows for the comfortable operator position when plotting here the stereoscope is in the resting position Considering the real feasibility to include the digital halftone raster images to the LIS da ta base we started some programing experiments in 1982 First the program to produce digital ortho photo was made 1985
9. corner of the screen within one dimensional image table image vector existing in the card memory The natural image scrolling on the screen is obtained by successive selection of images shifted by the number of pixels which compose one full image line on the screen image line length Naturally certain line of a subse quently shifted image begins with the pixel num ber greater smaller by the number equal to the line length The natural scrolling is limited to the num ber of additional picture lines lines exceeding the screen capacity existing in the card memory Selection of the pixel address greater smaller than that really existing in the card memory cau ses the address overflow which is reduced by sub traction from that unusual address the number equal to the full capacity of the card memory So e g the address next to the greatest existing on the card memory is the address of the first pixel in the image vector Therefore the scrolling which exceeds the number of additional lines in the card memory gives in the cyclic visualization the repetition of that image portions which were removed from the screen at the beginning of scro lling The effect of undisturbed cyclic scrolling in boundary conditions is possible only in case when the full card memory capacity can be divi ded by the image line length without remainder Otherwise the shifted image details within each subsequent line of overflown numbers appear on the upper
10. ddle and blinking pixel pointing out the cen ter The colour of cursor can be assigned freely but the best colour of the blinking white dark pixel is similar to the colour of the halftone image exposed on the screen to be measured The pixel blinking is controlled by the computer in terrupt so the blinking period does not depend on the programmed operation of editing the vecto rial map The technique of operation using the chosen block of pixels is used to create and animate the cursor on the screen The shapes of cursors are remembered in blocks matrices in the computer memory The cursor matrix is written to the screen memory in the chosen cursor position while the image position masked by the cursor is temporally kept in computer To change the cursor position the stored image portion must be rewri tten to the present cursor position while the cu rsor will replace the neighboring portion of ima ge which is again stored in the computer etc One of the important features of the screen plotting it is simultaneous display of the vecto rial map with the halftone image in the back ground In the vector graphics the objects defined as the line segments are visualized on the screen as a sets of pixels The coexistence of the vec tors superimposed on the halftone image on the screen is possible by the visualization of the raster halftone full image which is then masked by pixels which create a vector The masked pi xels
11. ead in advance from them the coordinates of various contours etc for thematic geometrical vectorial information Geometrisation of content of aerial photo graphs taken from the relatively low altitude always must consider also the differentiation of the land topographical surface The stereoscopic images must be used to ensure proper perception and proper survey of details There are two types of precisely measurable continuous tone images of the land surface stereogram and stereoorthopho togram A stereogram is a purely natural product of the photogrammetric flight and it would requi re only scanning to be stored in the digital data base of LIS Stereoorthophotogram it is a prepro cessed stereogram It consists of orthophotogram and its stereopartner Blachut 1971 Jachimski 1978 Both of them they are preprocessed aerial photographs therefore must be more expensive than natural stereopare but they offer great ad vantages It is much easier to superimpose vecto rial map on orthophotoplan than on a photograph because neither elevations of points of numerical vectorial map nor orientation elements of photo graphs are needed It is also easier to store and select proper fragment of orthophctogram and its stereopartner than to play with many frame pictures in the perspective projection which ir regularly cover the land Ortho stereopares are produced in the pro cess of differential rectification Differential rectification can
12. g and Remote Sensing 5 1990 p 623 625 Blachut T J 1971 Mapping and Photointerpreta tion System Based on Stereo Orthophotos Mitteilungen aus dem Institut fur Geodesie und Photogrammetrie auch der Eidgen ssischen Technischen Hochschule Zurich 14 1971 Jachimski J 1978 Problem stereoskopii w orto fotografii The Stereoscopy Problem in Ortho photography Zeszyty Naukowe AGH Krak w Geodez ja 54 1978 s 3 121 Jachimski J Trocha W 1992 Determination of the position of crosses with the subpixel accuracy on the image taken with the CCD camera Internat Arch Photogrammetry amp Remote Sensing Commission V Washington 1992 Jachimski J Mierzwa W Pyka K Boro A Zieli ski J 1988 Digital Image Rectification on Microcomputers for Orthophoto Production Internat Arch Photogrammetry amp Remote Sensing Vol 27 part B9 p T1 135 I1 144 Kyoto 1988 Streilein A Beyer H A 1991 Development of a Digital System for Architectural Photogramme try Proceedings of the XIV CIPA Internatio nal Symposium 2 5 Octber 1991 Delphi Greece Sutty G Blair St 1988 Advanced Programmer s Guide to the EGA VGA Brady Books New York 1988 Sutty G Blair St 1990 Advanced Programmer s Guide to the Super VGA Brady Books New York 1990 PEGA II 480 Card Enhanced Graphics Adapter User Manual VGA ULTRA Users Manual PURETEK AUTOCAD version 2 17 ADE 3D Reference Manual AUTOCAD wersja 10 Podr cznik u ytkownika The
13. lization or number of colours used for the vectorial map visualization must be limited to make the observation easy and to reduce the human operator confusion It should be mentioned here that there are unexpected difficulties in programming of gra phics The producers of the graphic cards openly supply usually only limited routines using the BIOS commands which provide exclusively the fun ctions for writing or reading the single pixel in the graphic card memory This doesn t facilitate the programming of the effective and fast graphic procedures such as procedure of a block visuali gation needed for the image animation or the pro cedure of memorization of the chosen fragment of image on the screen so to program before the G Sutty s 1990 publication was available we had to disassembly the BIOS procedures to grab the necessary information The computer monitors of 19 or 17 size are often in use but most popular and most economi cal is monitor 14 image size is smaller app rox 240x180mm The monitor screen in the most cases is shaped like rectangle 4 3 From the point of view of proper image perception by the human operator it is quite important to form on the computer screen an image geometrically simi lar to the original Let us assume that a scanner records the digital image using circular or square pixels arranged in the regular square grid To reproduce such regular digital image on the screen without affine deforma
14. lower part of the screen Luckily in such case exclusively the position of pixels se lected by the overflown numbers is incorrect what causes practically the ruin of only the uppest or lowest margin of the image on the screen leaving other parts of the image undisturbed In the case of the resolution 1024x768 with the 1Mb memory in the card the undisturbed cycle of scrolling is assured the lines with the overflow numbers will create on the top margin of the screen the repetition of the lowest part of the image exis ting in the card memory or vice versa But using the 800x600 resolution mode we are getting only 55 lines of undisturbed vertical scrolling 512x1024 800 600 55 36 and afterwards the shift of image details by 288 pixels along subse quent lines appears on the upper or lower margin of the screen 0 36x800 288 Fortunately the re maining part of the screen remains undisturbed what makes the plotting possible even in such not very comfortable conditions but using the popu lar and not expensive graphical mode hardware The analysis of the value of y parallax can show the cases when such less favorable condi tions appear Let us assume that an aerial ste reopare was scanned using 25um pixels This means that on one monitor screen simultaneously can be displayed portion of the left and right picture of the size 400x600 pixels or 1024x768 pixels what after multiplication by pixel size gives the aerial picture area 10x15
15. mm for 800x600 graphical mode or 12 8mmx19 2mm for the 1024x768 graphical mode Assuming that the usable photo gram area does not exceed 220x220mm we can cal culate the y parallaxes changeability at the cor ner of the photogram For the 1024x768 mode the natural scrolling will not exceed 55 lines x 25um 1 375mm Assuming that this would be the maxi mum of allowable y parallax which will not cause the display irregularities and assuming that va lues of elements of angular relative orientation are equal to each other one can calculate that this angular elements should not exceed 1 8 what is not very little When greater y parallaxes in crement will appear on the screen the plotting will still be possible but with the above men tioned sensations or with the intermediate re writing of the card memory to conveniently dis play the adequate portions of both images For the 1024x768 mode the natural scrolling allows for changes of y parallaxes not greater than 256 lines x 254m 6 4mm within one screen image por tion This would be sufficient even for the very badly oriented aerial pictures There are several possible shapes of cursors which could be used to point out the chosen pixel in the screen We selected the most convenient the arrow with blinking pixel in its fore part the square thin frame with blinking pixel in the middle seems to be the best for measurements and the cross with the arm lines broken in the mi
16. n of 1024x768 the code number for one pixel requires only half of a byte In the case of 512Kb memory only lower resolution 800x600 pixels image can be produced with the use of 256 colours palette Among the SVGA cards available on the market quite interesting are cards employing graphic processor ET 3000 or ET 4000 produced by TSENG LABORATORIES The card producer supplies a card controller program which enables the use of such functions of graphic processor as image magnifi cation ZOOM or image shifting SCROLL in the chosen window on the screen Admittedly the card controller is open to fulfill such functions only in the standard lower graphics modes but we found experimentally that they are available also in the high resolution modes with some limita tion however at the 800x600 pixel resolution The broad 256 colours palette creates a chance to distinguish on the screen several thematic areas by the use not only the grey scale but also the coloured scales of relative 128 densities for various image areas Aguard et al 1990 Considering the 30 steps halftone scale as sufficient for halftone image visualization we can theoretically use 8 various colours halftone scales simultaneously on the screen having additionally approach to 16 bright colours to show various thematic vectorial maps 8x30 16 256 In practice however for the human physiology reasons the number of colours used Simultaneously for half tone image visua
17. nt and given scale factor size of the pixel The present program was developed for the PC AT SVGA 800x600 256 card and 14 screen Fur ther development is planned for the card 1024x768 256 and not only for the stereoorthopho tography but for the natural stereopares also We hope for the liquid crystal screen and for the optical discs which should facilitate ve ry much the system versatility and comfort for the users The combination of computer controlled liquid crystal glasses and the liquid crystal screen allows to get the stereo effect without mirror stereoscope what makes the operator work much more comfortable Considering the mass use of the system though we must keep in mind also the possibilities of the popular and not expen Sive hardware 4 CONCLUSIONS The Digital Screen Stereoplotter developed at the AGH laboratories was designed as a tool for plotting of digital stereopares and pairs of continuous tone photogrammetric images in the di gital form At present system accepts stereo orthophotopairs and allows to plot simultaneously 6 different thematic layers of numerical map The development continues and will provide for com fortable stereo interpretation and surveying also natural stereopares in digital form Systen which uses only PC typical monitors and other pe ripherals can be considered as a program package for the mass applications not only by professio nalists but also by amateurs The stereoplotting
18. nt with the large y observation parallax is not possible Therefore a certain software function have been designed which allows to clear such parallax by the renewed display of right and left images around cursors in similar position on the right and left portions of the screen This function is designed to clear big y parallaxes only during the preparatory stage of relative orientation After the natural stereopare has been rela tively oriented the operator should be getting the y parallax free stereo vision To keep the cursor y parallax free during the observation of natural stereopare the vertical scrolling of at least one of the pictures composing stereopare must be used And here certain problem of that scrolling range for the popular SVGA cards appears On the card equipped with the memory of 312Kb TSENG ET 3000 only 469Kb of memory are 129 used to remember one full screen when the resolu tion 800x600 is employed 800x600x1Byte 469Kb Similarly on the card with the memory 1Kb used at the 1024x768 resolution mode only 768kKb of memory is occupied by the image needed for one screen So in the both cases a portion of memory reminds for writing in the card memory an image greater than needed for one screen visualization which enables the natural vertical image scrolling on the screen An image projected onto the monitor screen is selected from the memory in the card by the address of the first pixel of that image the upper left
19. rthophotogram using polygon or trajectory in the selected colour The colours are assigned to the max 6 thematic layers zooming the image displayed by the integer factor The image in the original scale and enlargement can be replaced by each other on the screen and plotting can continue in both scales net desired details of the vectorial map can be erased from the screen and then from the numerical map file database the line segments can be animated on the screen to facilitate designing the inscriptions can be introduced All the functions connected directly with point measurement or plotting of polygon or tra jectory are mouse controlled The mouse of PC AT has 3 programmable buttons which can be used to move cursor up or down in the space to register the point or beginning end of the line In the preparatory mode the mouse can move cursors separately on the left or right picture of stereopare When the left and right cursors points out identical details a command can be used to shift both cursors together with the ima ges so that both cursors are placed just in the center of the left and right screen parts For the use of stereoorthophotographs which are in the local coordinate system the functions were designed to use them with the data bank co ordinate system New coordinate system can be de termined by transformation using two or more chc sen points or approximately by just one point azimuth of given line segme
20. signed almost in the moment of its registration scanning It is enough to apply a transformation to standard pi xel coordinates to get the coordinates of each pixel in the fiducial coordinate system So the measurement of digital image displayed on the mo nitor lies in the selection of the No of the pi xel assigned to certain detail of interest The affinism of the image display on the screen does not influence the accuracy of measurement but only disturbs the human operator imagination It is one of the features of plotting on the screen that there should not exist instrumental adjust ment errors Fig 2 Stereoscope in front of the monitor equip ment ready for the stereoplotting Similar rules of measurement are applied for the stereopare survey There are two stereo pictures displayed on the two halves of the screen On each of them the separate cursor of equal shape is superimposed Both cursors can he moved simultaneously and additionally the right curser can be shifted only in the x direction to measure the x parallax When measuring stereo orthophetogram such shifts of cursor are quite sufficient But to prepare for survey on stereo orthophotogram when the components of the stereo pare are badly correlated or in case of prepa ring for survey of natural stereogram survey for relative orientation there is need for indepen dent shift of the right cursor also in the y direction The stereoscopic measureme
21. tion a certain graphical mode should be employed such that proportion of the number of pixels in the line to the number of lines on the screen should be 4 3 1 33 alike as proportion of the screen di mensions The best shape of the image displayed on the screen is at the graphical mode 1024x768 1024 768 1 33 and 800x600 1 33 The EGA 640x350 1 9 mode gives great affine image de formation 43 Even the high resolution 1280x1024 1 25 used with the bigger monitors gives some but rather small affine scale defor mation 6 which in many cases could be cleared by certain electronic adjustment of monitor though From the point of view of image geometry the modes 1024x768 and 800x600 are excellent On the 14 monitors in the first case digital image is displayed using pixel 0 23mm in the se cond case pixel 0 30mm Pixels of that size assures easy perception of a halftone image on the screen The image granularity is noticeable but doesn t make difficult neither the bare eyed observation nor the stereoscopic observation with the use of mirror stereoscope For the stereoscopic observation the two stereoscopic images are displayed at once on the left and right halves of the screen The stereo scope is placed horizontally in front of the mo nitor using special mount fig 1 and 2 to allow for comfortable operator position From the natu re of the digital image appears that to each pi xel the image coordinates are as
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