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UC Irvine UC Irvine Previously Published Works UC Irvine UC Irvine Previously Published Works Title REDUCTION OF DISPLAY ARTIFACTS BY RANDOM SAMPLING Permalink https://escholarship.org/uc/item/7r713132 Journal PROCEEDINGS OF THE SOCIETY OF PHOTO-OPTICAL INSTRUMENTATION ENGINEERS, 432 ISSN 0361-0748 Authors AHUMADA, AJ NAGEL, DC WATSON, AB et al. Publication Date 1983 License https://creativecommons.org/licenses/by/4.0/ 4.0 Peer reviewed eScholarship.org Powered by the California Digital Library University of California Reduction of Display Artifacts byby RandomRandom SamplingSampling A. J. Ahumaaa,Ahumada, Jr., D. C. Nagel, A. B. Watson, NASA Ames Research Center, Moffett Field,Field, CACA 94035^4035 and J. l.1. Yellott, Jr. School of Social Sciences, University of California, IrvineIrvine CACA 9271792717 Abstract The discrete, sequential scanning (sampling)(sampling) procedures used inin electronic displays generate disturbing artifactsartifacts suchsuch asas flicker,flicker, MoireMoire-type -type patterns,patterns, and paradoxical motion. Application of the theory of random scanning procedures toto displays cancan reducereduce thesethese artifacts. The human retinaretina provides anan exampleexample ofof aa systemsystem thatthat usesuses randomnessrandomness toto avoidavoid sampling artifacts.artifacts. Though the retina performs a discrete spatial samplingsampling ofof thethe stimulus,stimulus, artifacts are notnot evident.evident. For example, we do not seesee Moire patterns when we looklook atat finefine gratings. This isis because thethe retinalretinal receptorsreceptors areare positionedpositioned randomly,randomly, ratherrather thanthan inin aa regular array.array. Random sampling avoids conventional aliasing, butbut introducesintroduces noise.noise. Constraints on the random sampling can banish most of the noise toto thethe regionregion above thethe Nyquist frequency,frequency, wherewhere itit isis easilyeasily removed removed by by post post-sampling -sampling lowlow-pass -pass filtering.filtering. For visual displays this means that the sampling artifacts can be tradedtraded forfor noise,noise, and thisthis noise can be placed inin regionsregions ofof spacespace-time -time frequencyfrequency forfor whichwhich the human visual system has little sensitivity. Here we report that artifacts, especially thosethose associatedassociated with motion, are reduced by constrained random sequencing of horizontal scan lines. Three scanscan lineline sequencing procedures are compared: a sequential one, a single randomrandom sequencesequence repeated,repeated, andand a new random sequence on eacheach update.update. The single random sequence flickered leastleast andand was .,imostalmost asas good good asas thethe newnew randomrandom sequencesequence procedureprocedure atat minimizingminimizing motionmotion artifacts forfor thethe display of a vertical lineline movingmoving horizontally.horizontally. Introductionintroduction Sequential scanning artifacts The usual sequential scanningscanning proceduresprocedures inin televisiontelevision-type -type displaysdisplays generate a number of disturbing artifacts. Consider the simplest common scanning procedure: pixelspixels withinwithin aa lineline are shown from left to right and lines are shown from top to bottom. This sequencesequence isis repeated at thethe updateupdate rate.rate. If the update rate is below the temporal cutoff frequencyfrequency of the visual system, flicker will bebe aa perceivedperceived artifact.artifact. Unfortunately, the temporaltemporal cutoff frequency of the visual systemsystem cancan bebe asas highhigh asas 60dO Hz.Hz. In addition, keepingkeeping thethe updateupdate rate above the temporal cutoff frequencyfrequency doesdoes notnot ensureensure anan artifactartifact-free -free display. Movement of either the display content or the eyes of the observer can generate artifacts eveneven when the update rate is wellwell above the visual temporal cutoff frequency because motion can transform temporal artifacts intointo spatialspatial artifacts.artifacts. The window of visibility Watsonrtatson and colleagues usedused thethe conceptconcept ofof aa "window"window ofof visibility"visibility" inin spatiospatio-temporal -temporal frequency spacespace toto explainexplain howhow oothmoth spatialspatial andand temporaltempora} resolution;eolution combinecombine toto requirerequire a nighhigh update rate in sampled displaysdisplays ofof movingmoving objects.objects. ''' ' The "window""window" isis aa rectangularrectangular region in spatio-temporalspatio -temporal frequencyfrequency spacespace whichwhich isis boundedbounded byby thethe highesthighest spatial and temporal frequencies which areare visible.visible. The spatial frequency limit S isis a measure of visual acuity in cycles per degree of visual angle, and the temporal limitlimit T isis equal to the critical flicker frequency measured inin Hz.Hz. The observer is insensitive to signal energy outside of thethe window. Consider the case of a point of light moving along a straight line at a velocity V, in degrees of visual angle per second.second. The left side of Figure 1 shows thethe distribution of light energy in space and timetime coordinates.coordinates. It is a line whose slope isis thethe velocityvelocity ofof thethe point. The right side of Figure 1 shows the distribution of energy inin temporaltemporal frequencyfrequency and spatial frequency coordinates, that is, the Fourier transform of the left side. The frequency domain distribution isis justjust aa lineline perpendicularperpendicular toto thethe spacespace-time -time domain line. Actually, the representations areare threethree-dimensional; -dimensional; thethe darknessdarkness ofof the lines can be thought of as representing thethe amplitudes,amplitudes, whichwhich inin thesethese casescases areare lineline impulseimpulse functions.functions. If the image of the moving point is sampled at a constant rate of R Hz, the distributions in the two domains are as illustratedillustrated inin FigureFigure 2.2. The process of regularregular sampling inin the spacespace-time -time domaindomain introducesintroduces thethe usualusual spectralspectral replicatesreplicates atat multiplesmultiples of R Hz along the temporal frequency axis.axis. 216 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 10/09/2015 Terms of Use: http://spiedigitallibrary.org/ss/TermsOfUse.aspx LU u LU cr UJ LU o o:LU UJ o Q UJa: ui u. Q TIME (SECONDS)(SECONDS) TEMPORAL FREQUENCYFREQUENCY (HZ)CHZ) Figure 1.1. Light distributionsdistributions forfor aa movingmoving pointpoint in in space space-time -time (left)(left) andand spatio spatio-temporal -temporal frequency (right).(right). LU LU a O U) Q LU CD -J TIME (SECONDS)(SECONDS) TEMPORAL FREQUENCYFREQUENCY (HZ)<HZ) Figure 2.2. The spacespace-time -time (left) andand spatio-temporalspatio -temporal frequencyfrequency (right) distributionsdistributions for a sampled movingmoving point with the "window of visibility" indicated with dashed lineslines inin thethe frequency domain. If the spectral replicas lielie outsideoutside thethe rectangularrectangular "window"window ofof visibilityvisibility", ", then the distribution insideinside thethe window isis thethe samesame asas inin thethe unsampledunsampled case.case. In this case, nono artifacts will bebe seen.seen. It is easy to show that there will be no visible artifacts (all(all. spectral replicas are outside thethe rectangularrectangular window)window) if the update rate isis greater thanthan a value Rmin given by thethe formula,formula, Rmin = T + V SS (1) where T is the temporal frequency limit c:cT the visual system inin Hz and S isis thethe spatialspatial frequency limit in cycles per degree.degree. Psychophysical experexperimyns. iments,.havehave verified thatthat thethe required update rate does increaseincrease accordingaccording toto EquationEquation (1).(1). ' '*j Random sampling The standard method of avoiding sampling artifacts without increasingincreasing thethe samplingsampling raterate is to lowlow-pass -pass filterfilter thethe signalsignal before sampling. If the input isis fromfrom aa devicedevice likelike aa television camera, the lowlow-pass -pass characteristicscharacteristics ofof thethe devicedevice itselfitself cancan bebe matchedmatched to the 217 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 10/09/2015 Terms of Use: http://spiedigitallibrary.org/ss/TermsOfUse.aspx update rate.rate. This can removeremove most ofof thethe more obviousobvious samplingsampling artifacts,artifacts, butbut atat thethe costcost (at practicalpractical update rates) ofof lessless spatiospatio-temporal -temporal resolution than thatthat ofof thethe visualvisual system. In addition, prepre-filtering -filtering cannot removeremove otherother artifactsartifacts suchsuch asas thethe line line-slant -slant effect to be described below.below. And inin thethe casecase ofof computercomputer-generated -generated imagery,imagery, antianti-aliasing -aliasing filtering becomes an expensive, delaydelay-producing -producing proposition.proposition. Information theory tells us that we are bound to lose information if a transmission system with a lower channel capacitycapacity isis interposedinterposed inin aa transmissiontransmission system.system. Since thethe visual world and the visual system have very large channel bandwidths, affordable video systems are bound to generate artifactsartifacts becausebecause ofof thethe limitedlimited numbernumber ofof possiblepossible outputsoutputs per time period. Merely counting possibilities
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