VOL. 76, NO. 2 JOURNAL OF GEOPHYSICAL RESEARCH JANUARY 10, 1971
Digital Processing of the Mariner 6 and 7 Pictures
T. C. RINDFLEISC H, J. A. DUNNE, H. ,J. FRIEDEN, W. D. STHOMBEHa, AND R. M. RUIZ Space Sciences D-ivision, J et Propttls'ion Laboratory Pasaaena, California 91103
The Mariner Mars 1969 t.elevision camera system was a vidicon-based digital system and in cluded a complex on-board video encoding and recording scheme. The spacecraft video processing was designed to maximize the volume of data returned and the encoded discriminability of the low-contrast surface detail of Mars. The ground-based photometric reconstruction of the Mariner photographs, as well as the correction of inherent vidicon camera distortion effects necessary to achieve television experiment objectives, required use of a digit.al computer to process the pictllres. The digital techniques developed to reconstl"Uct the spacecraft encoder effects and to correct for camera distortions are described and examples shown of the processed results. Specific distortion corrections that are considered include the removal of structured system noises, the removal of sensor residual image, the correction of photometric sensitivity nonunifonnit,ies and nonlinearities, the correction of geometric distortions, and the correctrn of modulat,ion transfer limitations.
As all physically realizable instruments in_/ and interpretations of the imagery can be based fluence the data they collect, the Mariner Mars on information as representative of the Martian 1969 television cameras left their signatures on surface as possible. the imagery they returned to earth. Analyses The succeeding sections will describe, from of the Mariner photographs must be performed the point of view of image processing, the per with the knowledge that Mars was observed formance characteristics of the vidicon cameras through the spacecraft cameras, and any distor- and data-encoding electronics on board the tions introduced by the camera system processes spacecraft, the over-all flow of the various potentially affect the results. This is particularly Mariner 1969 video-data streams through the important for the Mariner 1969 photographs rectification processing, and the approaches because of the complex video system design that used for the correction of the data distortions . . was chosen to optimally record the low-contrast Specific digital processing techniques wiII be features of Mars and to maximize the quantity discussed fOl' the rectification of: (I) video of data returned by the two flyby missions. system noises, (2) spacecraft video encoding For some photointerpretive uses of the data, nonlinearities, (3) spatial variations in vidicon all that is required is a qualitative understanding sensitivity, (4) system geometric distortions, of the camera-system properties and an optimum and (5) system resolution limitations. contrast display of the data after noise removal, geometric rectification, and edge enhancement MARINER MARS 1969 CAMERA SYSTEM as discussed in the paper by Dunne et al (1971). The Mariner Mars 1969 camera-system design The careful measurement of scene luminance evolved out of television experiment goals, and geometric properties and even the most state-of-the-art hardware capabilities, and careful and intensive photo interpretive evalua Mariner project constraints as discussed by tions of the pictures, however, require the precise Leighton and Murray (1971). The result was a calibration and removal of camera-system effects. vidicon-based digital camera system, similar The purpose of this paper is to describe the to that used on the Mariner Mars 1964 mission, digital-computer techniques used to quantify but with a significantly increased image format. and correct for the systematic video-data dis More important in terms of the image-processing tortions, so that subsequent measurements task, however, was the manner in which the Copyright @ 1971 by the American Geophysical Union. data were processed and digitally encoded for 394 DIGITAL PROCESSING, MARINER PICTURES 395 transmission to earth. In accommodating the raster with each picture element digitized to 6 low visual contrast of the Martian surface, significant bits. Prior to the digital encoding the recovery of photometric information was of the CA V pictures, the amplitude-modulated made much more difficult and indeed impossible camera output signal was processed through without digital processing of the images. an automatic-gain-control amplifier (AGC) and The Mariner 1969 video data were encoded a cubing amplifier to emphasize low-contrast and recorded within the television and data detail, and was recorded on the analog tape storage subsystems (TVS and DSS) on board recorder. The dynamic amplifier gains were not the spacecraft in It complicated manner, as recorded. discussed in detail by Danielson and Montgomery The DV pictures consist of every seventh [1971]. The essen tial characteristics of the various picture element along each scan line linearly types of picture data returned by the spacecraft encoded to 8 significant bits but with the 2 most are summarized here in terms of their effects significant bits (MSn) truncated prior to trans on image processing. A simplified diagram of the mission. In a separate engineering-telemetry flow of data in the Mariner camera system is data stream, statistical information was trans shown in Figure 1. Each camera scanned a basic mitted for every block of 32 television lines raster consisting of 945 picture elements per indicating independently the dominant state line and 704 lines per picture. Pictures were for each of the MSB's. Of the 134 possible DN alternately read out of the low-resolution '\ elements per line, the central 26 were used to camera and the high-resolution n camera at transmit non video science data thus forming a 42-sec intervals. For each such picture prOdUC~ 'data bar' in the middle of the DV pictures. by the cameras, three separate encoded version The ETE pictures consist of every twenty were transmitted to earth: a composite analo eighth picture element along each scan line video (CA V) picture, a digital video (DV linearly encoded effectively to 6 significant bits, picture, and an every twenty-eighth (ETE) but with the two MSB's truncated prior to digital picture. transmission. These ETE data provide coverage The CA V pictures consist of the fully sampled in the 'data bar' of the DV pictures and, together
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MSB -BINARyr,'(lSrSICNI!ICANfBIT lSB . BINARy LEAST SIGNIFICANT Bll DV • (V(Av nH DIGITAl VID£O '\....r- TAP{ R£COROCR PlAYBACK CAY . COMPOsm ANAlOG \I I DlO UE • (VlRY lITH DIGITAL VI[)(O -z.- nUWTRY TRANSMISSION
Fig. 1. A simplified block diagram showing the derivation, processing, and storage of t he three Mariner 1969 video-data st.reams on board the spacecraft.. ]l1:\DFLEI"C'j-[ ET ..II..
Il'ith the DV data, gil'(' information rciatil'(' to recorded at all, and the B-cnmE'ra pictures werc the nonlinear cffects of the AGC and cubinl-( recorded onl.l· el'('ry 20 to GO min. amplifiers on the CA V data, The timc e()n~tants I n the XL modI', sincc thc planet I\'{)ldd c:hange of the AGC amplifier lI'erc cho>'en so thnt the ill brightness from limb to tel'minator, the gain I-(ain would change onl~' l'er,l' sloll'l~' o\'t'r the ('ontl'ols \I'(,I'C n('ce~sary . The :\,E mode had both el'cry-seventh spm'illl-( of the DV sample.". the AGe' and cubing amplificrs operating. lloth A sample of the three spacccraft data streams ;\- and ll-eamcra pietures \1'('1'(' rl'corrlrrl con is ~hOl\'n in Fip;urc 2 alonl-( II'ith thc rcsults of secutil'rl~' in thc near-encounter modc. removing the .-\GC and cubing amplifier cffects on the grollnd. OBJECTI\'"" ;\:\i) STR"TI':GY FOil :'Il"IU.\;Ell Operationally thc camcra sy,tclll had, two hJ..\GI~ l'HOCI·:"; .. W,G mo(k~: f:ll' cnc.ounter (FE) and n(,:1r ('ncountcr The :'.!:Irinrr lOGO telC'l-isioli rxperiment (NE). Because the plallet lI'ol"d 1l0t. fill thc ficld ohjecti\·e,.; included \l'ide-ranging inI'C'stigations of view as the spacecraft. first appruaehcd :'Irars, of :'I[ar;; and requircd proce-;."
EVERY 7th DIGITAL VIDEO IOVI EVERY 28lh DIGITAL VIDEO I [TEl
...... COMPOSITE ANALOG VID[o ICAVI 945 PICTURE ELEMENIS/lINE AUTOMATIC GAIN CONTROl. CUBER. ANALOG RECORDED. 6 BIT AI D COfNERSION
• •.•• ,EVERY 71h DIGITAL VIDEO IDVI 134 SAMPLES/ LINE LESS 2!! fOR "DATA BAR'; DIRECT 8 BIT AID COfNERSION - 2 MSB SUPPRESSED COMPOSITE ANALOG VIDEO ICAVI GROUND RECONSTRUCTED CAM[RA OUTPUT ·EVERY 28lh DIGITAL VID[o ([1[1 34 SAMPLES/LINE DIRECT 8 BIT AID COfNERSION - 2 MSB AND 2 LSB SUPPRESSED
x x x NON-lUEVISION SCIENCE "DATA 8AR"
Fig. :!. A diagram alld example,.; of the fmme format, for Ihc three Oil-hoard proce"'ed "p:I('e craft video sll'(,:Im>' '" wcll;1 .... Ihe 1'1',1111 of grolilld I'ccolI .... tl'll(·lion of Ihe photometric refl'I'CII('C for the composite all:t1o~ vidE'o. The picilires ,llO\\'1I are of frame i'\'I:!, alltl the sparseh' "ampled digital vidE'o pil'lllre ...; arc filled oul 10 full frame .... by I'cpcatillg I':ll''' ,ample 7 alld:!S time" respec tively. DIGITAL PROCESSING, MARINEII PICTUREg 397
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QUAN1ITATlVE viOla DATA CAV . COMPOS IT( ANALOG VIDEO DY - hUY nH DIGITAL YIDHi UE - [VlRY l!I1l DICITAL VIDEO Fig. 3. A flow diagram for the grollnd processing of the three spacecmft video streams. major subtasks: reconstruction and rectification. system is shown in Figure 3. Descriptions of Video reconstruction consists of combining the the various functional blocks are presented three data streams (CAV, DV, ETE), restoring in the subsequent sections. the missing most significant bits, and removing The digital computer with its inherent flexi the nonlinear photometric effects of the AGC bility and precision was the only feasible ap and cubing amplifiers. This step generates video proach to solving the Mariner 1969 image data as they existed coming out of the camera reconstruction and rectification problems with heads. Video rectification then takes into account the massive amounts of data involved. Most the various sensor distortions inherent in any of the camera effects were highly nonlinear, television system, correcting them based on and in many ways each frame had unique pro detailed calibration data. In the case of slow-scan blems associated with it, such as in the restoration vidicon systems, such as flew on Mariner, these of most significant bits to the digital video and distortions include system noises, residual image, the removal of automatic-gain-control effects. sensitivity nonuniformities and nonlinearities, In very few instances was it possible to set up geometry distortions, and resolution limitations. the processing in a 'production' mode. Much A discussion of the calibrations performed on human interaction was required along the various the Mariner camera systems is presented by steps. Danielson and Montgomery [1971J. The processing was accomplished in the Image At many points during this processing sequence, Processing Laboratory of the Jet Propulsion images are produced depicting Mars in a variety Laboratory using an IllM 360/44 ' computer of ways useful to photointerpretation. These primarily and a much larger IBM 360/75 for data, displayed with various types of contrast the noise-removal portions requiring large pnhancement, constitute a set of 'maximum Fourier transformations. The Image Processing discriminability' pictures. A collection of near Laboratory 360/44 machine is dedicated to encounter maximum discriminability photo image processing work and has a variety of graphs is presented by Dunne et aI. [1971]. peripheral image-display and scanning equip The over-all flow of the three types of space ment, including an interactive console allowing craft video data through the digital processing direct human control over processing parameters. 308 AL. A special software executive has been developed hancement and restoration processing can be for the machine that simplifies job-control performed on maximum signal-to-noise ratio definition and that provides a highly efficient imagery to achieve optimized result,;. input/ output processor for handling the large Many noise sources ('xist in imaging systems data files. ranging from random, wideband shot and thermal noises to highly structured periodic noises. REMOVAL Furthermore, in situations ,;pace photog- The it is generally to obtain photointerpretive mea,;urements on a multiple imagery in encoded presence of noise. Enhance- techniques. The precise ment processes filtering to the composite frame must image resolution can sharpen features only at be based on one or more quantifiable character the expense of over-all signal-to-noise ratio. istics of the noise signal that rlistingui"h it For these reasons, one of the most important uniquely from the other video component>;. In initial steps in digital image processing is the most real situations one has only statistical suppression of noise, so that subsequent en- information about the various components of
Fig. 4. A digitally comput.ed Fourier power of u IUllar test scene photographed by the .\Iarillcr B in a prelaunch ~I""·."·I·I" The power spectl'llm computed from the sean outpnt of the DJ(;ITAL PROCESSING, l\IAHII'EH PICTllIlES
TWO-D IMENS IONAl FOUR IER FLIGHT PICTURE TRANSFORMATION Fig. ;;. All example of:t two-dimellsiollal FOllrier lr:lIIsformatioll of :\lariller 6 e[l.mera B flight picture liNIS. The ahsolute sPCC'lrllJll amplitudes :LI"C displayed us shades of gray, illneasing f\"C)rn blar·k to while. Z(,1"Il spatiul frequellcy i.< in Ihe cenler of the tra.nsform pictllre. Positive and nel!::1tivc horizontal spatial frequcn,·.ies are displaycd to Ihe right and Icft. of cellter, respectively, :Lllri positive and negative vertical "patinl frequeneies are shoWIl above and below.
till' total \'ideo signal and thus, e\'clI at the theo for charact.erizillg it is in terms of 11 Fourier retical limit, tileir sl'p:Jr:.ltion is approximate. dccompo"it.ioll. A onc-dimen~ional digital power Thi,; is certainly true in tilc data output of spectrum of a test scene photographed by a. spacl'craft \'idco systems, \\'here the dominant. ~rariner Mars 1969 camera is ~hown in Fig\ll"c 4. signal rl'pre~l'nts a comillex natural s('cne. TIll' The power spectrum clearly show,", a smoothly l's,;('ncc of vidco noisc removal is to i.solate and varyillg background decreasing in amplit.ude l"(' IllO\'C t.he various irien tifial)le and eharaeterizablc with increasing frequency and representing noise cOlllponcnh as rigorously as possible, the average scene ~pectrllm on which is "uppr so as to do :l minimulll of damagc to the actual imposed a number of narrow spikes. These \'ideo elata. In most cases, till' errors int.roduced spikes arc the various components of t.he periodic to tile real signal by the remo\'al process, while noises present in the ?vIariner system and are smull, vary f!"Om IJoint to point. and arc impossible related t.o multiples of the 2400-Hz spacecraft t.o measure meaningfnlly, sinc(' VNy little is power-supply frequency. known in detail about the scene being photo For typical spacecraft syst.ems, the:"e periodic graphed and the dfic:H:Y of rellloval i" data noise" exhibit phase coherence o\"er times that dependent. In t.he following, the remo\'al 01" are long compared to the frame t.ime of t.he thrcc diff('I"(.' nt types of ~tructllred noise will camera. For this reaSOIl, when the two-dimen be c(Psl"I"iiJed, inclll ;.:ional star-likc spikes, The cont.inuum is ir is done wit.hout regard to its \'eltical variat.ion, rcgular in the case of lhe two-dimcnsional the corresponding spect.ral components remoH'c1 transform because there is no a\'rraging, ns in two-dimensional frequency space lie in a in the cnse of t.hr one-dimcnsionnl powcr "Ilcctrum, \'('Itical har. Since the Iloise (,0111110nent is Bccausc the noise ns well as the sccnc cxhibit contained in a restricted region along the bar, t wo-d imensiona I correlat.ion, a more precise a large amount of \'ideo signal is unncce:- Fig, 6, A portiol\ of \l:Irin('J' (j pictllre GN 1:\ (\kritliani ~inlls) with ollIY:l ('Olltr:l"l (,Ilh:ll"'(, mellt applied t(l the r:lw ('oll1po,ile ,tll:llog videt) re('eived frolll the space(,raft. This 'C'gment lI:lS heell enlarged two times to make Ihe syslem noises readily vi,ihle, 401 Fi!-(, I, The re';lIll of rCll\ovillp; 1he pcriodic lI"i,;c,; fmlll 1he ,;c!-(me"t of ():-\ 1:\ ,h",,"l1 ill Fi!-(lIrc G, "ole I h:II 1he ,,;h,,rl horizoll 1a I alia 1"1( tape recorder d ""POll ( lIoi,;e,; a .. e 1I0W clea rl,\' v i,;ihle, t.hl' I'ideo ,;i~II:11. ,\," C:l1I he ,,;eel1 ill the po\l'cr ror determillillg the locnl modulatioll ('ocffici(,lIt, ,pcdrull1 of }ii!l;urc 4, thc ,;pikc,; Ilal"[' ,;kirt, i,; to Illillimize tlir 1':\I'iancr, o\"('r a local rrgioll, e);II'llllill~ h('loII" the ('olitiIlUUIlI, Thr,;r ,;kirl, of I he noi';,I' ,i~lI:d rninu,; n l'ariahlP fractioll rei )re,;rllt a "ulit Ie 101':11 mod UI:11iOIl (~elll'r:d II' of tlir idcalizl'd noi,;(' pat.tel'll, If V" i,,; thr noi;';,I' amplitudc Illoduintioll) o\" th(' lia,;ic lIoi,,!' pattcl"Il illlagr, n" i, thr idealized lIoi,e pat t('l'lI , :1" :\" Illi~ht OITur \\'ith :Implitlld<' (1<'I)('lldrl1cc or i, tile Im'al modulatioll coefficicnt, alld a" , i, "a (1Ir:1l iOIl dT('ch Oil thr l1oi,r ,;i;!;lI:d, Thc,(' tliC' 'clci1l1C'd-up' ,,;igllal l'arinlll'C for tlie local ('aliliot 1)(' r!'nrlily i,olated h,l' rreqUI'IICy-"pac(' rc;!;ion of dill1ell,;ion ,\/, till' definitioll or loc:t1 filll'rill~, ,\ 11101'(' effe('(il'c approach i, to dl't('r ,igllal I'ariallcc mlll(' :I lo('al modulatioll coefficil'lIt for (he J .\1 / '2 .\1 /'2 2 --- 2 i -10:; Fig:, n, The reslill of rcmovillg the a\l"log t:l\Je re('order dropolil \loises from picll\I'c ()\' n "fll'r the periodic \loi"c removal s how\l ill FiV;lIre I, ;\iote Ihal Ihe black :l l1 d while spike lIoi"e,; "",,1 I (,I'ed I hrollv;h I he pi('\lIre are 1I0\\' dC':!rh' visibk, COl1lPlliil1g titc il1n'rsc FOllricr tr:llIsform of or h,' al'('l':Igilig lite :llllplitlitie or the trall, forrn titi, filtcred irn:lgc I r:tll.,form , flillctioll o"er :1 lIill (' h," Ililll' arC':'! abollt e:teh To dcl('J'lIlillc at \\'hieh C'1t'11][,11I, of IhC' trall,- l' iI'll1rllt. Tlti~ t\,'er:lgc i, tlicil "ubtr:1c ll'd frolll f01'l1l 110i,,(' spikl's oc('ur, titc :llllplitlldl' of titt' tlic origillal to yield :t ril'trrllilcd trall,forlll trnll, rOl'ln flllll'tioll i, cOlllplil r ti , :-;ill(,1' Ihi, cOII,i,!illg of Il oi"y ,'pikl" on :1 rela!i"cl.\' uniform fUllclioll COII,i,t,; or itigit-:llnplitllde Iloi,-t' ,-pikl'S :ll1lplitllde backgroulld, ,\ titre,llOld ,'alllC' is sUI)('rimp'hC'd on :t ('oltliltulim titat, i, ('it:lral'ler ~ p ec ifi (' d, :IIHI thl' Fomil'r trall"forlll of the ized 1)\, \'('1'" bigll amplitude, at 10\\' ,p:tlial idra lizI'd 1I0i"r p:11 tel'll i, ohta illcd 1),\' "cit i IIg fr r qllell\'ic, dr('l't'asing to quitt' 101\' :lnlplitud( ' ~ to zero :1 II poin t.-: or t ite i IIl:1 ,!!;e t ra lI,fol'lll for :1 t it igit fl'('lflll'lIeil's. it is Ilos, Fig. II). Thl' rc,,111 of I"ClIlI)vilig I Ill' spike lIoi.,!'s fmlll picllll"(' G\'):l :If tel" Ihe pel"iodic :llId dl"o)loul l1oi,;c J"('lllov:d .,h"\\"11 ill Figurc,; 7 alld 'l. !'\otc ill '·""\p:ll"i.'OIi \\"ilh t hc ra\\" piellll"c 111 Figllre G I h:" terraill rc"llIre, or (:oll ,; idel""bl,· higlH'r r('.'olill iOIl :\IT di,;("cl"lI"hle ill FigllrL' 10. DIGITAL PROCESSING, MARINER PICTURES 4()5 1l1l1111lJ2SSr------ACTUAl. VIO(O SIGNAL ------ENCOo(O VID!O SICNAL WIIM 2 WIST SICNlflCANT BITS TAUNCAT!D ~ IllOOmlll92f------+----'>r----- i ~ I!! a 1~lmf------~f_------~-- i ;;;~ i!l . .. --- ...... _-_ .. -" ~ Fig. 11. An example of the effects of truncat.ing the two most significant bits from an eight-bit binary encoded line of video dat,a. As the true signal crosses t.he quadrant boundarieS of the en coder dynamic range, the reqllired amplitllde informat.ion is lost, forcing all segments of the tl'lll1- cated signal to lie in the lowelt quadrant. was found that the abbreviated technique did for any differences. A multiplicative rather than not introduce a significant amount of image an additive correction is applied because the information into the idealized noise pattern. physical origin of the noise (magnetic tape The results of applying these techniques to dropouts) is multiplicative. Since the correlation remove the periodic noise components from the between points in a picture decrea~s with Mariner 6 picture shown in Figure 6, are ap increasing separation, a linearly decreasing parent in Figure 7. The complicated periodic weight is applied to more distant local lines noises that were extracted are seen in Figure 8. in determining the surrounding average. If Long-line 1Wises. A variety of mechanisms N; j is the noisy picture 0 is the index along the can produce long-line or streak noises in televi direction of the streak noises) and G;; is the sion images such as gain variations, data outages, corrective gain to be applied to the point (i, j), and analog-tape-recorder dropouts. This type the expression of noise, caused by tape-recorder dropouts in the case of Mariner 1969, becomes apparent as k + 1)[(N);+k,j + (N);-u] horizontal streaks in Figure 7 with the removal of the periodic noise. P(P + 1)(N);; The characteristic that distinguishes streak is used where P is the number of adjacent lines noises from the actual scene is their linear used above and below the streak and the average correlation along the scan-line direction and values are defined by the lack of correlation in the perpendicular direction. This distinction is not complete, 1 0/2 however, since linear features are present in (N);; = Q + 1 L N;.j+k k--0/2 some natural scenes, and statistically they will be oriented occasionally along the line direction. In this latter expression, Q is the low-pass-filter The problems of data-dependent noise removal dimension along the direction of the streaks are exemplified by this case since major damage and is determined by the lengthwise correlation to the true video signal may result in particular of the long-line noise. The dimension P is deter localized regions that contain scene components mined by the perpendicular correlation. resembling the noise. A further refinement is used in determining The technique developed to correct for these the numerator of the gain-determining equation. streak noises is to compare the local average The numerator is computed as shown, but then intensity value of lines adjacent and parallel a 'majority rules' logic is applied so that tel·ms to the streaks with the average value of the deviating significantly from the average are streak itself, and to apply a gain factor to account eliminated from determining the average on ll[:\DfLE[";('H ET .11,. a ~econd 1):l~S. This algorithm is a refinemcnt of istic that distinguishcs th r.sc spike noise,.; from thc long-line filt er dc~crihcd by Nathan [19661 · the actual I'idpo i.s the fact that bec:HI~c of The result of correcting for the ~trl' : lk n()i~e ,; resolution lilllit:ltion~ of a. \TYCJu i st-~allll)lpd in Figure 7 is ~holl"n in Figure 9. I t should b(' I-id('o ~y,s t (,lIl, the I'ariation from one pieture (,Illphasized that thi~ correction i., particularly elcmel1 t to :lIl()t her or t.11(, t 1'11(' ~igna I i,.; Ii III i tcd , data dependent in ih I,ffl,.,t :llld althougll cl()~('r Thc logic u~ c d to r(,IIlO\"(~ thcsc .spikc lI o i~('~ to the truth in the large, llIay introduce artifact~ i,-; very ~iml)I('. Eaeh picturc ('lclHcnt i.-; cX:lmined in (he small. and if it is ,.;ignifirHlltly abol"l' c[tch of it~ n('i!-(liI)()I"~ Isolated spike noises, Tile occulTcnce of a or ~i!-(llifical1t1.\" heloll' its n('igltl.lor~, it i~ l'('pl:I('('d bit CITor in tclell1l'tering digital I' ideo data or hy the a\"(~r:lge ncighboring illl('llsity. TIl(' 1"I'''; lIlh the presellce of tel11por:III.I· ~harl) ~I)ikes of Ilois(' of :lppl.I'ing thi ...; COITPelioll to tll(, remainillg from th(' analog cl(,etronics can cause i,-;olatcd "pikp lIoi"cs in Figlll'C' !) an' ShOII'1l in Figur(' 10. picturc e\cnlC'n[s to d('viat" ~ignifi e antly 1'1'0111 III this ('asl' , a" with thc IOllg-linc Ilois!', 1111' the ~urroullding data. The rel11Ol'al of the removal proec ...;, i~ not I)crfect alld r(,,,tor('s onl.l" p('riodic and streak noiscs illu~tratcd in Figure 9 :III approxim:ttion to thc truc I'idco data tha t r(,I'cal, this additional noise COll1poncnt ('on was maskcd by thr II 0 i...;c. Thc lhrc ...;hold pa si~ting of black and II"hite ~pike~. The charactcr- rameter,.; lI"ere cho,.;cn ";0 that oilly the rrlatil'cly RAW DV-2 MS8 MIS SI NG PROCESSED DV -2 MS8 RE STOR ED RAW m -2 MS8 MISSING PRO CES SED m -1 MSB RESTORED DV~EVER Y 71h DICIfAL VIDEO 1-'1 EVERY 28 lh DIGI JA L VI DEO Fig;. 12. The rcsllils of res torillg; the two 1110,,1· SiP;lIific,"iI, bih to Ihe two digital video frames received for .\iarillcr 7 camera A pictl\l"c 7N I;';. The fr:lnll's h~v(' be'<'11 cxpallded 10 the filii I"amera forillat. by l'('Jll'af.inp; e:tch samplo 7 '"Id 21'; linH's, re,.:pe('tivl'iy. The diap;onal strcaks :\1'0 callsed by the canlC'ra-systel1l lIoi ...;es :tlin"cd 1>('1":111.";(, of Ihe sparse s:tlllplillp;. 407 RAW CAV-CUBER & AUTOMATIC RECONSTRUCTED CAV·CUBER & AUTOMATIC CAIN CONTROL mECTS PRESENT GA IN CONTROL EffECT 5 REMOVED CAV-COMPOSITE ANALOG VID£O Fig. 13. The resll]t of removing the effects of t.he cllbing Iwd alltomatic-gain-contro] ampli fiers from pietllre 7N 1:3 lI~ing the re.~II]h of the most. signifi(,:lnt. bit. re.~t()mt ion shown in Figllre 12. Note that. the Oil-board processing hau t.he uesireu effeel. of enhancing ]ow-colltm~t smface uetai] bot h Oil alld ofT t he polar cap in the mw l'omposi te :In:lloj!; viueo. few points obviously in error were corrected, three data "treams and its application to re and any reasonably probable signal variation construct the \'ideo ~ignal at t.he ~ensor output, for was left unaltercd. subsequent correction of calibrated vidicon A comparison of Figures (j and 10 shows a distortions. dramatic improvement in signal-to-noise ratio The ~teps required for this video reconstruction by using the digital computer to isolate and are "hown in Figure 3. The two sparsely sampled remove various st.ruct.ured noise eOll1ponents data "treams (every sevrnth, DV, and every from the raw vidco. This type of processing, twenty-eighth, ETE) must have the tl"llncated although preliminary to further restorative two mo~t significant bits restored. The fully steps, itself produces an enhancclllent t.hat sampled analog data (CA V) must have the allows analysis of surface detail closer to the cubing-amplifier effects removed. The final resolution limits of the camera system. Tech step then is to combine the three processed niques for removing noises from video are of streams to reconstruct the dynamic gain applied widely varying charact.er and in some cases by the automatic-gain-control amplifier (AGC) strongly video-system :lIlel elata dependent. and to proeluce a fully sampled image as it appeared direct.ly at the camera sensor output.. N O:-iLII\EAR EI\COl>!I\G Rt:CO:-lSTRUCTlO~ Restoration of the most significant bits. The The th ree type;.; of encoeled v ideo da ta tra ns binary encoding of a picture element represents mitted from t.he ;Vlariner 1969 spacecraft to a series approximation to the ca,mera ~ignal earth have a complex relationship to the video output at that point with the most significant data output from the camera sensors. Whereas (or high-order) bits giving coarse amplit.ude the distortion characteristics of the sensors information and the lower-oreler bits progressively are constant with t.ime and are calibratablr, refining the approximation. The truncation of the properties of the encoding system are dyuamic, the two most significant bits (i\ISB) from such and they adapt to the unique luminance varia a binary code introduces an ambiguity into the tions of the scene being photographed. A major gross amplitude of the resulting number. As aspect of the Mariner image-processing task shown in Figure II, each tnlllcatrd data point was t.he extraction of the implicit information could lie in any of four equal quaelrants of the about the encoding-system effects from the dynamic range of the digitizer. From the conlext -iOS HI:\DFLt-:I,.;CH t-:'I' AI,. of the ~Iowly I':nying signal ~hol\'n in Figure II, lJeen possible, with vcry fell' except.ions, to 110\Vl'l'cr, it i" intuitively evident that relatil'e rcstore unambiguously the m issi ng sign ific:ln t :\[SI\ transitions occurrcd at thl' abrupt dis bits to thc .\[ariner flight digital "trearns, The continuitics or thc :\ISB trullcated da"hcd tUrI'C, algorithrn that was uscr! searchcs the rail' digital ,\s tllc adil'ity or t.he "ignal incrcascs, i,e" as data for abrupt transition", "uch as appear ill 1:\I'ge amplitudc change:; in the true sign:)1 take Figure II, alld introduces rno"t significant bits place 01'(')' shorter and shorter di.~tanccs, it so as to minimize t.he discontinuities, 'fhi" "carch I)('comes more and 1l10rc rlifficult to reassign is donc for each column indepelldently, since unambiguou"ly most significant bits to the the picturc elernC'nts along the columns arr truncated dat:l. The choice of t.his bit-truncation contiguous and the column~ arc spaccd 7 and da ta-com pression scheme IV;)S ba.sed Oil the 28 picturc clemcnt,,,; apart, rr,spcet.ively, for the low-tontr:\st character of :\Iars and the cor DV and ETE data "treallls. Clearly, if the ah relation from pidul'l' clcmcllt to pictlll'c clemC'lIt soillt.r di/ferrllcc betwccn adjaccnt ra\\' illtC'lI"itiC'" imposcd on the camcl';l outPlit by the resolutioll is rithcr 1:'lrge 01' small, OIlC call sa ,I' II'ith COII limitatiolls of the "~,,,tCI\l. For pictures in whitll ficicncc th:\t a :'IISn tran"ition has or has not the I'idco signal is co nfined ol'cr-all to fell'cr occlIl'red, If til(, diffcrC'IICC i" of illtrrrneC\iatc than fOllr quadrants, i.e., for I'N.'· flat SCCI1CS , magllitlldc, the OCClllTl'lICC of a tr:1n ~iti on is there remains an amplitude ambigllity for the alllhiguou", Thc locatioll, of "uch [\rniJigllili('s whok pietlll'e eH'n :lftN the rl'!ati1'(' :'IISB an' logged for l';wh column for later 1I.SC in trall"ition dis('.on tinllilics arc repairrd, :-llith I'l'soh'illg thc :'I[SIl patt('\'11 for tilc entirc fral1lc, \Il1certainti c.~ arc rl'"oll'cd Oil thc ha.si" of thc :\fter procl'".sillg all thc columll" of t.he pictllre statistical s:lInpling of thc statc of (':\eh of lite in thi ,,,; II'ay, :1J)proximatr illtell.sit,I' contilluit,l' illlag('-r1ata .\ISB's lhat IH'I'l' ret II riled ill thl' lJetlrl'l'n adjacent columll" i" il\lpo~cd to attempt l'ngillcl'rillg telemetry. Thc"l' additional data to rl"oll-c 1'l'llIaillillg :llllhiguiti('", .\ "tart ill,!.!; III('aslire indl'pclldcnti,l' 11'I1('thcr each :'I[SI\ IV:!" eolullln i~ ('I\("'l'1I eilhrr Illanuall~' or by :1l1to prcdominantly on or off for ('ach 32-lill(, hlock ll1atieally 1(lcaling that column with th<' most. ill the pictur('~ . quadrallts prl'~C'nt (but 1I0 t. ('xc('l'ding; rom) allli '\llllOlIr.;h the hir.;h-frcq ucne,I' 1I0isc~ ill till' II'ith till' 1'l.'II·e~t :1lllbigllitil'~. 'l'ili, ('olull1ll is flight ima.t!:(>s sigllificantly ('omplicntrrl thc :'Irsn ('lllllp:\I'l'd with thosc adjacl'nt, and :'11:-:>1)'" :II'C rc"tora t ion pracC'".s and nc('(',,~i ta ted ('oll ...;idrra hie chall~('d hetlH'cII a rn biglloll~ t I'an~i t ions to hliinan illtN:)ctioll II'ith thl' compulin!-(, it. Il:Is prod UCl' tile ,.;mooti1cst r('sul t. Th is process RESIDUAL IMAGE PRESENT RES I DUAL IMAGE REMOVED Fi~. 14. Tit" 1'(',1 Ii I (If rE'Ill()Villl!; Ilw lilld) rC',idll:l1 il1l:ll-(lJ frolll \I:!riller 7 r.:lmer:.l 13 pict lire 7FG2, ~ill('e "I('Cl'""iv(' (':1111(')':\ ('Xp""'lre.'; II'Ne 1101 I rall,nl iII ed iII I hC' fa r-CII ('(Ill II t cr "e'l'I('II('C", I he pi('lllre :-;howlI wa:-; lI:-ied 10 11I1)(Jei il:-o:. OWIJ rc:-;idllai illlagp, -100 100 FOOT -LAMBERTS 400 FOOT -LAMBERTS 1600 FOOT -LAMBERTS 800 FOOT -LAMBERTS Fif!;. ];;. A '(' BEFORE PHOTOMETR IC CORRECT ION AFTER PHOTOMETRIC CORRECTION Fig,1G, A cornplIler-colilollred eompari~on of Ihe average of all the \Inriller 7 camem B Highl piclure" beforc alld after ('UIT('('lioli of Ihe vidicull :'f:'lhilivil,\' lIolilillc:1I'ilics alld ~palial IIOlllllli- formi( ie", COIIi(Hlrillg: wa:, achicvcd :1, ill Figllre J ;;, The I'e:,idual ,hadillg al'l e\' phol olllci ric I'O\' rcctioll i" " 1Ii>jl'('livei,\' (,oll:,i~telll ",il h cxpecled ph:hf'-:llIgle ('!'fed:', ill thl' c('utC'r of thc pidure, Thc dark band ill Complex gray-scale targets \\'err photogra pill'" this arC':.I i" all artifact that rC'flcct>; thc dyuamic in olle orientatioll three times in sLlcce"sioll , thell rC'spouse of the AGe, The rC'con"truetC'd data rotated and photographed thrce Illore times, rcprC'''C'lIt vidco at the output of thc camC'ras The targcts \\'ere drsignl'd ill sueh a \\'ay tha t \\'ith nil t.he noulillear e-l1codill/2: effect" r('I\1O\'(,(!. a rotatioll of 00° r('sulted in all ('xpoSlll'e chall)!;e eit.hcr up 01' down at each point in th(' irna)!;e HI';SjIlUAL hUG!:: HE~(OV,\L plnnc, By measuring camcra output \':tluc's ill The \'idicoll tubes employed ill the \larillcr 'GO s('\'l'ral of the pictures in the scries d('s('rillCd TV camera sy"tems \\'('I'e high-"cllsiti\'ity storage ahove, allll!'()ximnte rctellti\'ity cocfficicnts \\'('re tubes and thcrefore exhibite-d sigllifi('ant frame determincd ['01' the entire ,scanncd arm 0[' the to-frame image rctention, I t was determined t.ubl', ,\lIother t.\']'e of (lata uscd to detcrmillc' the e-:lrly ill the calihration data Hllal,\',is phasc that ~p:ttial \'arintioll of retenti\'ity within t.hr sC For a sequence of pictures taken by a camera, crepancies between pictures corrected by means the appropriate map is applied as a modulation of the flat and sloping version of the ylariner 7 matrix to picture N - 1 of the sequence, so that camera A retentivity maps amount to 2-3%. an appropriate fraction of its intensity valucs The removal of residual images from far can be subtracted from picture N at each image encounter (FE) data involves an additional elcment to effect residual image removal. complication because successive camera exposures This approach has been used in the removal arc not available. Only widely spaced FE pictures of residual images from preflight light-tran4er were recorded so that the Hnalog-tape-recorder calibration data and has been used in the removal capacit.y (33 pictures) would be optimally of residual images frolll all flight data, In all utilized as the planet image scale slowly changed except the ;\[ariner 7 camera A, the preflight during approach. Residual removal, then, re retentivity maps produccd an effective removal quires a 'hootstrap' technique wherein each of visible residual imagcs from the flight pictures, recorded frame is used to approximate the im with only an over-all amplitude-scale adjustment mediately preceding camera exposure and thence required in the retentivity maps for several t.o remove the residual image from itself. Planet cn meras, image changes caused by rotation during the However, preflight calibration analysis in 84 spc hptween camera pxposures are Ipss than dicated a gently sloping, nearly planar retentivity two picture elements for the closest far-encounter map for the yIariner 7 camera A \·idicon. When pictures and were ignored within the accuracy thi,.; map was used in an attempt to remove of this approximation. Because of the size of residual images from \briner 7 flight data, it. the residual effect, the visible residual limb of was found to be inappropriate, and the spatial the previous exposure provided a p;uide to trans retentivity coefficient ,.;Iope seemed no 10nl1;er late llnd rep;ister each image with the residual to exist. Although the difference between the of the preceding frame (the spacecraft guidance flat-field and the sloping calibration retentivity system caused slight shifts in the planet image surface was small, the difference between pictures from frame to frame). At thi,; point the appro 'conected' u,;ing the two were visually distin priate retentivity coefficient Illap was u,;ed to guishable. The necessity to abandon the preflight subtract the necessary fraction of the simulated ?\Iariner 7 camera A retentivity map is disturbing, preceding frame to remove the residual. suggesting as it docs that either a measured An example of the removal of the residual property of the vidicon had changed during image frum FE frame 7F62 is shown in Figure flight or, alternatively, that the preflight mea 14. The residual removal technique described surements were inadequate. Photometric dis- greatly reduced the visibility of the residual I • • • f- I- -t- .-1- t- BEFORE GEOMETRIC CORRECTION AFTER GEOMETRIC CORRECTION Fig. 17. A comparison of the preflight geometric calibration grid target. a.~ photographed by the :'.[uriner 6 camera A before and aftor ('O ITeet ion. BEFORE GEOMETRIC CORRECTION ArTER GEOMETR IC CORRE CTI ON Fil!; , JR, A cOlll]larisoll or the reslllb before alld :lrtpl' ('orrectin)!; the c:lrnera-s,v,tern !!:l'ollletri(' distortions for \fariner 7 canH"ra A pictllre 7\11:l, Note thc lar!!:c relative' ('orrec'lions IIc('es"ary in the Ilpper left and lower riJ!:h t ('orner", imaf);e componen t. I n genera I, I'esiciurl I rell1o\':\ I d~'Il:lInie rallge of thr system. Contourcd cxamplrs was possible to approximately ;3% unecl'taint,\' of the rrsultant pictures for a :'Ifnrillt'r () Right III the correctcd intensity valur, e:1m('1':1 arr shown in FigurC' I ;j . The gradients evidcnt in these picturcs (8 intellsity levrb out l'HoTO~mTrtlC Dr.cALInHATlo\' of 255 corrrsponci to one blnck-to-\\'hite cycle) AftI'I' residual image rClllo\':t1 is cOlllplrtrd, result from spatial srllsiti\'ity \'ariatiolls in the the pictul'es ~till retain thr 'signature' of non vidicon target. (input firlds wcrc Rat to withill linear and spatiall,v \'aryinf); light-srnsiti\'ity 2-;3 rqui\'aiPnt len,b). Dat:l of this typc were proprrtics of the vidicon, Each of thc \'idicons collrrtrd in a Ilumber of calihrations for each utilized in the ;'I[arinrl' missions was calibrntrd C:lllwr:l filter and gain state, and 0\'1'1' a range of rxtrnsivr\y to detrrminr, as a function of position Ol)rratin),( trmperatures c\('sigIlNI to include in thc framr, thr rrlationship hrt\\'ern input r\:pectcd rllcounter t.rmperat.ul'cs, Examinatioll luminancr and call1rl':1 output signal. Thi,; of the lhC'l'mal data sho\\'cd that the shading includru Illrasllring thr "ystem light-transfl'r chamctrristics \'aricd ill .~uch a way that no singlc. propcrties for each of the camera filt.er" and as temprrature correction eorfficiellt could he :l function of calliI'm tempernturr, In effect each applic'd O\'er titc elltirC' \'idieon surfacc, but elrment of the 94.5 salllpic- h~' 704 line raster is rather each image' elrmellt l'l'quircd its o\l'n cor treated as a tiny photometer with unique light rec tioll factor. Aceordillgly, ell(,oullt.rl' trm transfrr propertirs. Thrse photolllc1 ric en libra prrature-cnlibratioll filcs wrre gellC'l'atcd h.v com tion dat:! arc org:Jnizrd and stored in such a way billing cold and room-trmpC'l'aturc calibration that when each picture elemrnt in a Right fralllr data on a picturr-ciemellt b.\' picturc-r\rment is to bc cOlTrctrd, t.he a pproprin te en libra tion basis, with a different interpolation factor :lpplied cUI'vr for thaI rlrment is loeatcd, the recordrc\ t.o caeh image elelllcnt. camera-output signal is cOllverted to the aetual Preliminary rxamination of thc corl'ccted Right scene IUlllillallcr, nlld thr result is storcd ill the cia tn ind ica tcs Iha t this approach has been output eorrectrd image. successful, to a first ol'der at Irast, ill removing; Danielson and ,1/ on/gomcry [19711 descri\)r vidicon ~hadin!4 in thc phot.omet.rically corrected the colleetion of thr calibratioll data Oil whieh the pictures. As an example, Figure 15 is a cont.oured photomet.ric. c.orrrction of the :'Ifarincr picturcs Vrl'SiOll of HI(' average of all :\[al'iner 7 e:lJllera is based. Th('sr con~ist of a ;;erirs of 'Rat-firld' I~ pictllrrs that \\'ere recoll;;tnletcd by applica pictur e ~ generatrd by exposing each camera to tion of thr AGC correctioll (again, Ollr cyele cor a spatially uniform stimulus at a nlriety of rrsponds to 8 ON). A similarly cont.oured vcrsion accurately measlIl'rd luminance \',tlues o\'cr the of the~e samr pictures aftcr photometric cor- 414 RI:-IDFLEU,CH ET AL. rection is also given in Figure 16. Similar results severe. Electronic geometric distortions have the have been obtained with the other flight cameras. additional complication of varying in time be Residual shading in all cases is coincidental with cause of internal electronic fluctuations and expected photometric gradients due to phase gross changes in the magnetic environment of angle effects, although analysis is not yet com the cameras, such as removal from the earth's plete with regard to a quantitive match and magnetic field. The optical distortions are stable residual errors. Cross correlation between photo in time, within the resolution limits of the metrically decalibrated data obtained from the cameras, depending only on a stable mechanical various cameras has, however, indicated the mounting. presence of large (20-30%), unexplained incon Calibration measurements were made to sistencies, particularly at low light levels. These determine both the gross system distortions of comparisons have been made between A and U combined optical and electronic origin, and the cameras on the same spacecraft in areas of over small time-varying electronic distortion changes. lap, and between Mariner 6 and 7 I3 camera The composite distortion characteristics were (far-encounter) views of the same surface features measured before flight and were assumed to viewed under approximately the same conditions. remain stable. The time-varying electronic The resolution of these anomalies, if accomplished, distortions were measured for each frame in situ will likely provide valuable information 110t by means of an array of 63 black reseau points only about Martian photometry but for the deposited directly on the vidicon target. The design of future space camera systems and their small changes in reseau positions from frame to calibration. frame describe the small changes to the over-all electronic distortion. A two-step geometric cor GEo~n;TRlC DISTORTIO:-'- CORRECTIO:-'-S rection is thus made to the flight Ilictures. First, In addition to projection distortions caused the I'eseau locations determined for each flight by oblique viewing, geometric distortions are picture are corrected so as to coincide with the introduced into television pictures by optical reseau locations in the composite distortion and electronic imaging aberrations. In the measurement. Then the composite correction is Mariner cameras both optical and electronic made to restore over-all geometric fidelity. distortions exist, but the electronic are the more A calibration picture used to determine the 5 4 3 RELATIVE FREQUENCY RESPONSE 2 o~------~~~o 0.5 SPATIAL FREQUENCY (CYCLES/PIXEU A. TYPICAL VIDICON SYSTEM MTF B. I DEAL CORRECTION FILTER (NO NOISE LIMITATIONS) -- ARTIFIC IAL ENHANCEMENT GAIN TRUNCATIONS TO OFFSET DEGRADING NOISE EFFECTS Fig. 19. A plot. of the preflight calibrat.ed horizontal modulation t.ransfer function for the Mar iner 6 camera Band t.he theoretical recipl'Ocal correct.ion funct.ion. The truncated correction filter actually used applied a maximum gain of 1.!'i and was ba.~ed on observed camera-noise levels after noise removaL DI(;I'LIL PIlOCESSI)\;(;. l\IARINEIl PICTl'IlES 415 AFTER MODULATION TRANS FIR CORRECTION Fig. 20. A comparison of the reslilts before lUll] after applying the t w()-dimen~ional rnodulat iOIl t.r:lll~fer filter correct ion to .\Jariller 7 camera A pici ure 7N 17. The vcr."ion ~h()wn ha~ not had I he automat ie-gaill-coill rol and cubillg-amplif'ier correct ion~ applied. compo~ite geometric p:lI'ameter,; for a .\[ariner that i.~ a piecewise, linearly interpolated to correct 6 camera is shown in Figure 17. The pict.ure is the elect.ron ic distortion cha nge for f'ach picture the result of photographing an accurately scribed element of the frame. The resultant picture.~ grid tarp:et with the camera. The orthogonality are then subjrcted to the compo,;ite correct.ion . and regularity of the test grid were verified by ;\n rxample of the fully correeted flight frame t.heodolite measmements to be better than half 7NI3 is shown in Figurc 18. The cu,;p-like ap the resolution limits of the camera "ystem. '\[e:1- pearance of thr upper left. and 1001'rr right COrll('l"S surements of the positions of the grid intrr"ec in the corre'ctcd frallle indicatr the ,;everit..,· of tions in the camera output and their \"ector the electronic distortions in tlH'~r arcas. Dis cI isplacemen ts from the idea I di,;tortionless placemrnts of up to 2.5 pictlll"e elemenh are locations provide a two-dimensional map of the common neal' the cOl"llrrs. picture-element. po~ition corrections. The cor For the bulk of the far-eneountpr data, esti rection is applied by a piecewise, linear inter mates of reseallX lorations had to IlP u;:ed for polat.ion of t.he grid-target-interscction data blark space, since t.he black rrseiHlX were not and determination of a posi tion corrrction for visible in those port.ions of the pidures not el'ery picture element in the frame. This single containing the planet. These estimates were correction removes both opt.ica I a nd electron ic ~enerated by mrans of averaged positions for distortions based on the elect.ronic distortiOIl those reseaux basrd on subsequent data whcre characteristics at. the time the calibration frame the whole field was illuminated and the rese:lux was t.aken . The geometrically corrrcted ver;;ion were visiblr. 13eC:111Se of the piecewise, linear of the calibration grid target is also shown in intrrpolation algorithm used, errors in rC'seau Figure 17. locations far from the limb contrihute negligibly The removal of t.he small frame-to-frame to the geometric fid elity of the corrected Illanet elect.ronic distortions in the flight. dabt is ac (lata. complished before this compo~ite correction by Ih;,;oLUTlOX EXH,-\XCI':~IEXT cOl'I'ectin~ the reseau locations in the flight framcs in such a way t.hat the reseaux fall exactly upon '1'h(' scale size of object-scC'nC' detail \'isiblc those in the calibration picture from which the through a t.rle\"ision can1E'ra is limited, in a com posi te correct ion para meters were deri\·ed. perfect s.l·stem, by diffraction effects in lhe uptic,;. This is accomplished in much the same way as This, of course, ignore~ ~uch effects as atmospheric the composite correction. The \'ector displace turbulence. For most s~·~tl'ms including the ment of each reseal! in a flight frame from the .\[arinrr 1969 camera~, thi~ limit is not realized reference calibration location provides a net :1:; the eamera ~ensor systC'm it~elf im poses 416 HINDFLEISCH ET AL. resolution limitations that are dominant. The sine-wave transmittance targets of various fre result is an apparent loss of contrast in small quencies and extracting the sine wave-amplitude scene detail (small relative to the resolution of degradation along the scan line and perpendicular the system) or, equivalently, a blurring of the to the scan-line directions [Danielson and Mont transitions between adjacent intensities in the gorn.ery, 19711. These curves are then used to camera output. model the two-dimensional MTF function by If the camera system is linear or, as is the case assuming the contours of equal response are for the Mariner 1969 mission, if the input scene ellipses. The phase component of the MTF is is sufficiently low in contrast, the degradation ignored, not so much because the omission is in image resolution is described in terms of an justified as because the measurement of phase amplitude-independent system point-spread properties has not been possible. The resulting function. If the point-spread function is further MTF is then used to generate the correction more independent of position in the image plane, filter. such as for narrow-angle systems, then the The correction filter is theoretically the re camera output O(x, y) can be written in terms ciprocal of the MTF function. For typical systems of the object scene intensity distribution /(x, however, the response at high spatial frequencies y) and the system point-spread function S(x, y) becomes quite low, so that gains in excess of by a simple convolution relation ten at these frequencies are necessary. The addition of wideband noises within the system O(x, y) = du dv S(u, v)l(x + u, y + v) makes this theoretical correction undesirable. J J Since the scene spectral components decrease This relation can be inverted by using Fourier in amplitude at high frequencies and since the transformations to express the desired input noise is essentially white, any filter correction scene in terms of the camera output degrades the gross signal-to-noise ratio of the resulting image. Thus some criterion for sup pressing the large high-spatial-frequency gains = dv l(x, y) Jdu J F(u, v)O(x + u, y + v) is necessary on the basis of the competition be tween small-feature and edge-transition fidelity F(x, y) is a correction filter related to the point on the one hand and degraded signal-to-noise spread function through the relation ratio on the other. There are quantitative criteria for this trade F(x, y) = ~-{~[S(!, y)J] off such as the Wiener-Hopf filtering technique. For many purposes, however, these are unac where ~ is the Fourier transform operator. ceptable, sincc they are based on large-scale This sequence of equations has a discrete statistical measures of error and, in the small, counterpart appropriate to digital computation. produce less than optimum results. The additional condition of Nyquist sampling For many purposes a simple trial and error must be imposed in the discrete case for validity. solution to the problem is adequate. In Figure 19 The correction filter function F is a function a typical MTF curve for the Mariner cameras only of the uniform camera point-spread function is shown and labelled 'A.' The theoretical cor and thus may be calibrated independently of rection filter based on this MTF is shown as the object scene. This is done for the Mariner curve 'B.' By introducing an artificial truncation 1969 experiment by measuring the modulation to the theoretical curve, as shown in the dashed transfer function (MTF) or the Fourier trans curves, the emphasis of high-spatial-frequency form of the point-spread function rather than noise amplification can be controlled. A filter the point-spread function directly. The reason or series of filters consistent with the camera is that for vidicon systems, in order to produce signal-to-noise properties, the typical scene a sufficient output point-spread amplitude to spatial-frequency composition, and eX[Jeriment make reliable measurements for all spatial measurement goals can be derived. frequencies, the input must be so large as to drive For Mariner 1969, primarily because of the the system into nonlinear response. The MTF severe noise problems, the modulation-transfer is measured by exposing the camera to spatial correction had to be truncated with a high- DIGITAL PROCESSING, MARINER PICTURES 417 spatial-frequency gain of 1.5. Even so, a signi this most difficult task has produced a set of ficant sharpening of surface detail was produced, data significantly extending man's knowledge of as is seen in the comparison of the Mariner 7 Mars and has demonstrated the power of digital photographs of Figure 20. techniques as an adjunct to the interpretation of These corrections were applied as a convolution video data. filter with the kernel generated from the recipro Acknowledgmenls. The task of calibrating and cal MTF function rather than as a direct processing the Mariner Mars 1969 photographs was frequency space filter. The reason is strictly very large and included contributions from many one of economy. The convolution kernel matrix people at the Jet Propulsion Laboratory in addition for these corrections needs to be only on the to the authors. The contributions of the following are hereby gratefully acknowledged: F. C. order of 15 picture elements square, and increased Billingsley, D. Brown, S. A. Collins, G. E. Danielson, calculation speed is realized by matrix con E. F. Dobies, A. R. Gillespie, E. T. Johnson, volution rather than transformation. This trade D. Mathews, J. M. Soha, K. R. Taylor, G. M. off is in contrast to that of the previously dis Tet.suka, T. E. Thorpe, and V. W. Tllk. In addition the authors gratefully acknowledge the cooperation, cussed periodic noise filtering. In order to obtain patience, and advice of the Mariner Mars 1009 sufficiently narrow frequency space bandpass Television Experiment Team members and in characteristics, the convolution kernel would particular the help of Profes.'IOr R. B. Leighton. be prohibitively large, thereby making direct This paper presents the results of one phase of transform space filtering the more economical research conducted at the Jet Propulsion Labora tory, California Institute of Technology, under approach. contract NAS 7-100, sponsored by the National Aeronautics and Space Administration. SUMMARY The extended coverage and high surface REFERENCES feature discriminability goals of the Mariner Danielwn, G. E., and D. R. Montgomery, Calibra Mars 1969 television experiment resulted in tion of the Mariner Mars 1969 television cameras, the design of a highly complex video system. This J. Grophys. Res., 76, this issue, 1971. Dunne, J. A., W. D. Stromberg, R. M. Ruiz, design included as a prerequisite to quantitative S. A. Collins, and T. E. Thorpe, Maximum image interpretation the extensive restorative discriminability versions of the near-encounter and corrective processing of the Mars photo Mariner pictures, J. Grophys. Res., 76, this graphs. Because of the nonlinear distortions issue, 1971. Leighton, R. B., and B. C. Murray, One year's inherent in vidicon and in fact most television processing and interpretation-an overview, sensors, and because of the complex nonlinear J. Grophys. Res., 76, this issue, 1971. encoding scheme evolved, the flexibility of the Nathan, R., Digital video-data handling, Jet digital computer was required to produce the Propui. Lab. TR .'12-877, 1966. desired images portraying Mars as it actually (Received August 10, 1970; appeared. The successful accomplishment of revised September 15, 1970.)