NASA/TM-2003-212148

Videogrammetry Using Projected Circular

Targets: Proof-of-Concept Test

Jonathan 12 Black

Joint Institute /br Advancement qf Flight Sciencex

George Washil_gtot_ _h_iverxitv

Langley Research Center, Hampton, hTrginia

Richard S. Pappa

Laltgle), Research Celtter, Hampton, Virgiltia

February 2003

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NASA/TM-2003-212148

Videogrammetry Using Projected Circular

Targets: Proof-of-Concept Test

Jonathan T. Black

Joint lnstit_lte /_r Advancement of Flight &_iences

George Washington Universit),

Langley Research Center, Hampton, Virginia

Richard S. Pappa7" 0""Langley Research Center; Hampton, F tlNlnta

N a tional Aeronautk's and

Space Administration

Langley Research CenterHampton, Virginia 236811 2199

February 2003

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Videogrammetry Using Projected Circular Targets:

Proof-of-Concept Test

Jonathan T. Black

Joint Institute for Advmlcement of Flight Sciences

George Washington University

Hampton, Virginia

Richard S. PappaStructural Dynamics Branch

NASA Langley Research (;enter

Hampton, Virginia

ABSTRACT

Videogrammetry is the science of calculating 3D object

coordinates as a function of time fi-om image sequences. It

expands the method of photogrammetry to multiple time

steps eimbling the object to be characterized dynamically.

Photogrammetry achieves the greatest acctu-acy with high

contrast, solid-colored, circular targets. Tile high contrast is

most often eflk_cted using retro-reflective targets attached to

the measurement article. Knowledge of the location of each

target allo_s those points to be tracked in a sequence of

images, thus yielding dynamic characterization of the

overall object_ For ultra-lightweight and inflatable

gossamer structures (e.g. solar sails, inflatable antennae, sun

shields, etc.) where it may be desirable to avoid physically

attaching retro-targets, a high-density grid of projected

circular targets - called dot projection - is a viable

alternative. Over time the object changes shape or position

independently of the dots_ Dynamic behavior, such as

deployment or vibration, can be characterized by tracking

the overall 3D shape of tile object instead of tracking

specific object points. To develop this method, an

oscillating rigid object was measmed using boti_ retro-

reflective targets and dot projection. This paper details

these tests, compares the reslflts, and discusses the overall

accuracy of dot projection videograrmnetry.

INTRODUCTION

The science of videogrammetry expands the methods and

techniques of close-range photogrammetry and applies them

to a sequence of images to generate time history data.

Photogrammetry is defined as the process of making precise

measurements of an object t?om photographs of that object

[Ref. 1]. Using high contrast retro-reflective targets, static

shape characterizations of tfltra-lightweight and inflatable

(gossamer) test articles, such as solar sails and inflatable

antennae, have been achieved in previous _x_rk at the

NASA Langley Research Center (LaRC) [Ref. 2]. Themeasuremerits were used to create three-dimensional models

of ti_ose gossamer structures_ Videograirmletry produces a

time-history of these 3D models. The generated time-

response data call then be used for such tasks as modal

analysis and to track deployment.

The term "gossamer" is applied to a particular class of

spacecraft designed to minimize launch weight and volume

[Ref. 3]. These structures must thereli_re be tightly

packaged for launch, subsequently' deploying or inflating

once in space. Maw, such as solar sails, will use ultra-thinmernbranes- three imcrons in tNc½1ess or less - as main

components to acNeve areal densities of less than 10 grams

per square meter [Ref_ 4]. The combination of the weight

minimization requirement and the inherent fragility of such

structures necessitates that alternatives to physically

attached retro-reflective targets be explored. Projected

circular targets (dot projection) is one of these alternatives.

A target pattern created using dot projection instead of

physically attached retro-reflective targets eliminates the

mass, stifthess, and attachment time of potentially

thousands of targets that would be affixed to the ir0aerently

large - greater than 60 square meters [Ref. 5] - gossamer

structures, while still providing the high contrast required

fi_r quality photob_airmletric and videogrammetric)meastu-ements [Ref. 6]. t rojected fields of dots may also be

customized to any density or size, providing greater

flexibility than attached targets [Ref. 7].

This paper will detail work at I,aRC using a simple

swinging plate to develop and validate the dot projection

videogrammetrytechniqueby directcomparisonwithtraditionalretro-reflectivetargetvideoodammetry.

TESTSETUP

Figure1showsthesetupusedtovalidatethedotproiectiontechniqueibr dynamicmeasurements.A square24-inchplateofwhite,laminatedpressboardwassuspendedbytwosix-tbot-longstrings,allowingit to swingfreelybetweentwo4x 24inchpressboardpiecesmotmtedonstands.Theplatewasattachedvia anotherstringto a long-strokeelectrodynamicshakersettoapproximatelya10-inchstrokewitha 10secondperiod.TheplateswungbackandforthbetweenthetwostationaD;4x 24inchpieceswiththesame10-secondperiodoftheshaker.TwosynclcuoMzedPulnixTM-1020-15digitalvideocameras(CCD:1008x 1018pixels,9.072x 9.162ram,Removablelens:24- 85ram,f/2.8.-f/22,Upto 15framespersecond)showninFigure2mountedontripodsrecordedthemotion.

(Yaonesideofthe24-inchsquareplmeand4x 24inchsidepieces,stripsofretro-reflectivetape(Fig.3)consistingof0.25inchdiametertargetsspacedtwx)inchesapartwereaffixedtocrea_ea22x 22inchgridof 124dotsontheplateanda2x 22inch_id of24.dotsoneachofthesidepieces(Fig.4(a)).Fiberopticlightsilluminatedthetargets(Fig.3)providingthenecessaryhighcontrasttargetfield. Theoppositesidesoftheplateswereleftblankcreatinganidealdiffusewhitesurt_acefor thedotprojectionpoi_tionof theexperiment,asshownin Figure1. A consumerslideprojector(Fig.1)proiectedagridofdotsthatcoveredthesquareplateandsidepiecesyieldinga22 x 22 inch grid of

169 dots on the square plate and a 2 x 22 inch grid of 26

dots on each of the side pieces (Fig. 4(b)).

DATA ACQUISITION

The test was repeated tbr two separate target conditions:

one using the retro-reflective tape and the other using dot

projection. As stated above, the Pulnix cameras recorded

the oscillating plate system, creating sequences of digital

images used in the videograrrunetric analysis. At a

sampling rate of five frmnes per second, "the data acquisition

yielded a set of 200 images over 40 seconds (ibur cycles)

for each camera position. Figure 5 shows examples of

images taken by the upper right camera. Figure 5(a) was

recorded during the illuminated retro-reflective target test

and Figure 5(b) &1ring the dot projection test. In both cases

high contrast was achieved between the white targets and.

the measured object, creating an optimum conditkm for

photogrammetric and videogrammetric measurement

[Ref. 6].

Around the perimeter of the square plate in Figures 4(b) and

5(b), several of the dots appear cut-off. In the dot projection

portion of the experhnent a stationau projector created the

field of targets, meaning that the plate moved independently

of those targets. As the plate moved, the targets on the

edges slipped in and out of visibility depending on the point

in the cycle, as is "thecase with the top row and right column

in Figure 5(b). Because these points are not visible in all the

images, they were not processed at any time step.

At the time this test was conducted, only two video cameras

were available. Acquiring four synchronized image

sequences using two cameras required allowing all transient

motion to damp out of the oscillating plate system. The

resulting data acquisition recorded only the constant and

continuous (stea@ state) forward and back movement The

combination of the plate moving at a constant 10 second

period and the data acquisition being computer controlled

enabled each capture sequence to be initiated at a precise

time (point in the cycle). The two cmneras were then

moved to different positions and a second synchronized set

of data was gathered, initiated at exactly a multiple of 10

secoMs (cycle time) after the first, yielding synchronized

hnage sequences fi-om four vantage points. The cameras

were spaced approximately 90 degrees apart to provide

equal measurement precision in all three coordinate

directions [Ref. 6].

VIDEOGRAMMETRY

The video,_ammetric processing was performed using abeta version of the PhotoModeler Pro 5.0 sottware f?om Eos

Systems [Rel: 6]. The sofiware loads multiple sequences of

images, associating each with the correction parameters

unique to the particular camera with which the images were

recorde& The correction parameters were obtained whenboth cameras used in "the data collection were calibrated

using the Camera Calibrator supplied with the software.

This procedure, described in detail in Reference 8,

calculates the local length, location of the principal point,

the radial lens distortion, and "thedecentering lens distortion

of each camera. Using these parameters, [he software

autonmtically removes any distortions of the images due to

aberrations in the cameras or lenses, enabling accuratemeasurements.

The initial images litr each of the four sequences -

corresponding to the initial time step labeled epoch 0 - were

processedexactlyasastand-aloneprt_iectwouldhavebeen.Thetargetswerenmrkedto sub-pixelaccuracy(seeReference8)inallthephotos,andthecorrespondingpoints(i.e. the calculatedcentersof the targets)referenced(matched)acrosstilephotos.Afterapproximatelyeighttargetshadbeenmarkedandreferenced,thebundleadjustmentalgorithnasmmltaneouslycomputedthecameralocationsandorientationsaswellastilepointlocationandprecisionvalues.Afterthephotographswereoriented,anautomaticmarkingtool markedtheremainingtargets,precedingtheuseof anautomaticreferencingtoolthati_mtchedtheCOiTespondingpointsacrossall ofthephotos.The bundleadjustmentwassubsequentlyrun again,updatinganditeratingon theorientationsandexact3Dlocationsof thei_mrkedandreferencedpoints.Fiimlly,ascaleand.mlaxisweredefined.All ofthestepsdescribedabove,atanypointin theprocess,mayberepeatedoneormoretimestoensurethehighestaccuracyintilefinalresult,whichisa"_pointcloud'_fi_rrningathree-dimensionalmodelofthephotographedobject.Figure6 showsthegenerated3Dviewof thecompletedretro-reflectivetargetproject.Thecameralocationsandorientations(approxinmtely90degreesapartasdiscussedabove)havebeencorrectlycalculatedbythesoftware,ashavethe3Dcoordinatesofthetargetpoints.Reference8describesmmuch_deaterdetailthephotograi_mletricprocessandtheexactfunctioninganduseofthesoftware.

The aboveparagraphdescribesthe photograi_mletryproceduresforanentirestaticproject.Asstatedpreviously,however,this is onlyonestepin thevideogrammetricprocess.Afterthestaticphoto_arnmetriccalculationsofthefirsttimestep(epoch0)aresuccessfullycompleted,it isnecessaryto expmldthosesamecalculationsto theremainingtimesteps.PhotoModelerPro5.0comainsa_Tracking"function"thattracksthemarkedtargetsin thefirst epochthroughall of the subsequentepochs.Essentially,theprogramoverlaysthepointcoordinatest_omthepreviousepochontothecurrentepochin thetrackingprocess.Aroundeachof thepointlocationsfromthepreviousepochit searchesfor anothertargetin a user-specifiedradius._,_enanewtargetislocatedandmarked,the algorithmassignsto the ne_ point the sameidentificationnumberasthepointin thepreviousepoch,therebycreatingmotionofthetargett)omoneepochtothenext. Continuingin thisrnarmer,thetrackingalgorithmsearchesthroughall theepochs,automaticallylocating,marking,andmatcNngtargetswhilemaintainingpointnumbersidenticalto theoriginalepoch.Theresultis asinglesetofpointswiththree-dimensionalcoordinatesforeachoftherecordedtimesteps.Thesecoordinatescanthen

beexportedin ASCIIformatasa list of pointswithcorrespondingX,Y,andZcoordinatesateachtimestep.

VAL][DATION

TheASCIItimehistorydatafor bothconditionswereanimatedusingtheMh"Scopecommercialsoftwaretosho_tilesquareplateswingingpastthet-vvostationaryrectangnlarpiecesasdisplayedin Figure7. To allowfor bettervisualization,theimageshavebeenrotated90degrees.Thebottomimageshoutstheplateatitsclosestpositiontothecameras,themiddleimageshowstheplateatthemid-pointofitscycle,andthetopimageshowstheplateatitsfarthestpositionl¥omthecameras.TheanimationcapturedinFigure7(a)wasgeneratedusingdatafromtheretro-reflectivetargetcondition.Forcomparison,Figure7(b)showsthesameanimatedsequenceusingprojecteddots.

A grapIficalvalidationofthedotproiectionmethodmaybeaccomplishedby visuallycomparingthetwoanimationsequencesgeneratedfromtiletimeseriesdata.Figures7(a)and.7(b)showthesamethreetimestepsfromthetwoanimations,whichappearvirtLmllyidentical.This issufficientto demonstratethattheoverall3Dshapeof anobject,inthiscaseaflatplate,canbetrackedovertimeasitmovesrelativeto a stationao,fieldof projectedtargetswithoutknowledgeof theexactlocationof anyspecificpointonthatobject.

Figure8providesanotherformofgraphicalvalidation.ThetwoimagesshouTthemovementoftheleft-mostcolumnofpointsplottedovertheentiretest. As expected,theindividualpointsformlines"that trace the paths followed by

tile targets over the cycle. It would also be expected that the

physically attached targets would move with the plate in an

arced manner through the sequence of images, similar to the

path of the tip of a pendulum. Careihl examination of

Figure 8(a) reveals that the paths of the retro-reflective

targets are, in fact, curved u.pwards as expected.

Conversely, it would be expected that, in the case of the

plate oscillating approximately parallel to a field of

stationa,y, proiected targets, the displacement of the points

in the X-Y plane created by the image (image plane) would

be much less than the displacement of the points in the Z-

direction (nornml to the image plane, toward or away from

the projector). Inmgine holding a piece of paper in front of

a projector. The image on the paper will displace in the Z-

direction with the paper as it is moved toward or away from

the projector; however, moving the paper up, down, right or

left in the X-Y plane will not change the location of the

image_ It will remain stationaly " as the paper moves.

Therefore, it would be expected tha_ the plotted paths of the

points m Figure 8(b) would be straight lines. Examination

confirms the fact that each point only moves along the

straight line from the point to the prqiector, not along a

curved path with the pla_e.

Figure 9(a) shows the motion of two points over time in the

retro-reflective test condition. The first graph (left)

describes the lower left point on the swinging plate. Its

motion over the image sequence is as expected: a sine wave

that repeats fbur times, corresponding to the shaker input.

The second graph (right) shown in Figme 9(a) is a

stationary point on the left sidepiece. Figure 9(b) graphs the

same t_o points using dot projection data. The lower left

point on the swinging plate (left) again moves in the same

periodic sine wave as the shaker, and. the stationary point

(right) remains still. The scatter displayed in the graphs of

the stationary points in Figures 9(a) and 9(b) provides an

indication of the random noise arid uricertainty of the

measurement method. In both conditions, the

videogrammetric method is precise to less than one-tenth of

a pixeL

Figures 7, 8, and 9 demonstrate that dot prqiection data

produces results that are qualitatively comparable to retro-

reflective target data. Figtae 10 demonstrates the validity of

dot projection videogrammet_' at a quantitative level. The

swinging plate used for these tests was chosen for its ease of

characterization, not for its flatness. Figure 10 shows that,

in fact, the plate is slightly bowed in the middle by

approximately 0_035 inches. Estimates calculated by the

software indicate the videogran_netric data is precise to

approximately 0.0015 inches; theretbre, a curvature of 0.035

inches is, while not easily visible to the eye, accurately

characterized by both photogrammetry and

videogrammetry. Figure 10(a) was generated using data

from the retro-reflective test while Figure 10(b) used dot

projection data. To enable both test conditions to be run in

succession, opposite sides of [hc same plate were used. It

was simply flipped over after one set of data was completed.

Therefore, if the plate is bowed 0.035 inches during the

retro-reflective test, the same bow shotfld be visible in the

opposite direction when it is flipped ibr the dot projection

test. The dot projection data in Figure 10(b) does show an

inverse curve to the retro-reflective data in Figure 10(a) of

the same magnitude. This indicates ttmt the dot projection

results are in excellent quantitative agreement with retro-

reflective target results. Given the accuracy and the

capacity of dot projection videogrannnetry to define and

model the overall 3D shape at a particular time step (Figure

10(b)), those 3D shapes can then be ammated over all of the

time steps to produce the desired time-response data

[Ref. 9].

CONCLUSIONS

Videogrammetry is an important tool in the dynamic

characterization of ultra-lightweight and inflatable space

structures. Traditional videogrammetry relies on physically

attached retro-reflective targets that may not be suitable m

gossamer applications due to the added mass, stiffness, and

attachment time of the targets. The experiment detailed

above validates videogranmletry using projected circular

targets (dot proiectiol _) as an alternative to retro-reflective

targets by directly comparing the twa) methods. Dot

projection measured the same 0D35 inch curvature of an

oscillating plate as was measured using retro-reflective

targets, and proved capable of tracking the three

dimensional shape of that plate over time. The resulting

dyimmic data was identical to [hat generated using retro-

reflective targets. Specific, but certainly not limited, to

gossamer structures with predominantly out-of-plm_e

dynamics, dot projection videograrmnetry proved a robust

and flexible alternative to traditional retro-reflective target

videogrammetry.

ACKNOWLEDGEMENTS

The authors would like to acknowledge Tom Jones and Kara

Slade of NASA LaRC, Jack Leifer of the University of

Kentucky, and Joe Blandino of James Madison University

for all of their gtfidance.

REFERENCES

[l] Mikhail, E. M., Bethel, J. S., m_d McGlone, J. C.,

lntroducaoll to M()dem PhotogrammetJ?', Jolm Wiley &

Sons, New York, NY, 2001.

[2] Pappa, R. S., Jones, T. W., Black, J. T., Walford, A.,

Robson, S., and Shortis, M. R., "Photogrammetrv

Methodology Development for Gossamer Spacecraft

Structures, '_ NASA/TM-2002-211739, June 2002.

[3] Jenkins, C. H. M. (editor), Gossamer _S_acecr@:

Membrane and b_/latable &rucmres 7_dmology for 5)race

Applica#ons, Vol. 191, Progress in Astronautics and

Aeronautics, AIAA, Reston, VA, 2001.

[4] Virgin, L., arid Holland, D., "_Post-Buckling Dynanfic

Structural Modeling and Analysis of Square Solar Sail

Configurations," Seminar given at NASA Langley Research

Center, 08/21/01.

[5] Mclnnes, C. R., _Sblar SailiHg Fechnology, Dynamics

and ffdission Applicatiot_s, Praxis Publishing I,td,

Chichester, UK, 1999.

[6] Eos Systems, Inc., Photo_:[odeler Pro User's :_Iat_ual,

Version 4, Vmlcouver, B_C, Caimda, 2000.

[7] Ganci, G., and Bro_, J., "Developments in Non-

Contact Measurement Using Videogrammetry," Boeing

Large Scale Metrology Seminar, 2000.

[8] Pappa, R S, Giersch, L_ R., and Quagliaroli, J. M.,

"Photogrammctry of a 5m Inflatable Space Antemm Wffh

Consumer Digital Cameras," NASA/TM-2000-210627,Dec. 2000.

[9] Wong, W. O., "Vibration-Mode Shape Visualization

with a Time Average TV Holo._aphy System," Tempus

Publications, 1998.

LettCamera

Proiector

RightCamera

Shaker

Figure1-TestSetup

Figure2 - PulrfixTM-1020-15monochromedigitalvideocamera

i_----_--_¸ ¸¸ ¸

....mi_ '<lii

mm

Figure 3 - Retro tape and targets an-

illuminated (top) and illuminated (bottom)

(a) Retro-reflective targets

Figure ,-1.- 24 inch plate and side pieces

(b) Prelected dots

(a) Retro-reflective dot pattern (b) Projected dot pattern

Figure 5 - Upper right images used in processing

Figure 6 - 3D vievx of camera stations and marked points

_?:?:?:?:i

(a) Retro-reflective target data (b) Dot prqjection data

Figure 7 - Sequence of three images from animation of s_Gnging plate

(a) Retro-reflective target data (curved paths) b) Dot projection data (straight-line paths)

3Figure 8 - t oint movement plotted over entire dataset in the upper left image

_- ii _ "_ _! _'J_ /:_ 200L_.............................:...........................,_L..........................................................J

0 10 20 30 40

900 i .... • - - r...... T

g 85o!\ _\

>-

81300:: 10 20 30 40

Time(s)

5&85 / ]

L /53.5Cj.............................:...........................................................:............................__i 0 20 30 40

_4a_% _ :5? .... 571 :20Time (s)

(a) Retro-reflecdve target data

=

O

X

•"G 840_- - -,_ - - , .... _ _ _

g: 82o0

>- 780 L , , _ j0 10 20 30 40

Time (s)

_°°t-_-p-<- :_,.... ,,4 :,1 7°"°I.... _- - 2_ .... "....

0 10 20 30 40 0 10 20 30 40

247 5% ..........................i-'O-........................2JO 30 a/Orime (s)

(b) Dot pro2 ection data

Figure 9 - Position of lower left swinging plate target vs. time (left) and position of

stationary sidepiece target vs. time (right)

i" -i iiJ:J:J _ii i ¸i¸i̧i¸i¸ii¸i¸/¸¸¸¸/¸iiiiiii_!!!i!i!i!_TT¸_:

(a) Retro-re[]ective target data (b) Dot projection data

Figure l O - Contom- surfhces of swinging plate showing slight bow in the middle

:::::::

;i.-o.oes_,,

iiiiil._o,o_o_.

REPORT DOCUMENTATION PAGE Form ApprovedOAdBNo. 0704-0188

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February 2003 Technical Memorandum4. TITLE AND SUBTITLE 5. FUNDING NUMBERS

Videgrammetry Using Projected Circular Targets: Proof-of-Concept Test

6. AUTHOR(S)

Jonathan T. Black, and Richard S. Pappa

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

NASA Langley Research Center

Hampton, VA 23681-2199

9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)

National Aeronautics and Space Administration

Washington, DC 20546-0001

WU-755-06-00-21

8. PERFORMING ORGANIZATIONREPORT NUMBER

L-18257

10. SPONSORING/MONITORINGAGENCY REPORT NUMBER

NASA/TM-2003-212148

11. SUPPLEMENTARY NOTES

Presented at the 21 st International Modal Analysis Conference (IMAC); Kissimmee, Florida; February 3-6, 2003.

12a. DISTRIBUTION/AVAILABILITY STATEMENT

Unclassified-Unlimited

Subject Category 39 Distribution: Standard

Availability: NASA CASI (301) 621-0390

12b. DISTRIBUTION CODE

13. ABSTRACT (Max/mum 200 words)

Videogrammetry is the science of calculating 3D object coordinates as a function of time from image sequences.

It expands the method of photogrammetry to multiple time steps enabling the object to be characterized

dynamically. Photogrammetry achieves the greatest accuracy with high contrast, solid-colored, circular targets.

The high contrast is most often effected using retro-reflective targets attached to the measurement article.

Knowledge of the location of each target allows those points to be tracked in a sequence of images, thus yielding

dynamic characterization of the overall object. For ultra-lightweight and inflatable gossamer structures (e.g.

solar sails, inflatable antennae, sun shields, etc.) where it may be desirable to avoid physically attaching retro-

targets, a high-density grid of projected circular targets called dot projection is a viable alternative. Over

time the object changes shape or position independently of the dots. Dynamic behavior, such as deployment or

vibration, can be characterized by tracking the overall 3D shape of the object instead of tracking specific object

points. To develop this method, an oscillating rigid object was measured using both retro-reflective targets and

dot projection. This paper details these tests, compares the results, and discusses the overall accuracy of dot

projection videogrammetry.

14. SUBJECT TERMS

Videogrammetry, Photogrammetry, dot projection, gossamer, ultra-lightweight,

inflatable, retro-reflective, projected circular targets

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