Basics of StimuliBasics of StimuliJohn H. KrantzJohn H. Krantz

Hanover CollegeHanover College

OutlineOutline

The nature of the beastThe nature of the beast Basic Perceptual IssuesBasic Perceptual Issues

SpatialSpatial TemporalTemporal ColorColor LuminanceLuminance

The General BeastThe General Beast

Screen Mosaic Triad Screen Mosaic Triad ArrangementArrangement

Screen Mosaic Color Strip Screen Mosaic Color Strip ArrangementArrangement

Interlaced ProjectionInterlaced Projection

Progressive ProjectionProgressive Projection

The Electron BeamThe Electron Beam

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Seeing the FlickerSeeing the Flicker

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Spatial IssuesSpatial IssuesSpatial InhomogeneitySpatial Inhomogeneity

AliasingAliasing

Edge Effects and ContrastEdge Effects and Contrast

Spatial InhomogeneitySpatial Inhomogeneity

The same output value leads to different The same output value leads to different luminance outputs at different screen luminance outputs at different screen locations.locations.

Thus, you get different outputs at different Thus, you get different outputs at different screen locations.screen locations.

In general, draw to center if criticalIn general, draw to center if critical

CRT % Change from MinimumCRT % Change from Minimum

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How to Handle Spatial How to Handle Spatial InhomogeneityInhomogeneity

Keep stimuli in central regionKeep stimuli in central region

AliasingAliasing

Technical definition:Technical definition: When an image contains frequencies beyond the When an image contains frequencies beyond the

range of the sampling matrix, the wrap-around and range of the sampling matrix, the wrap-around and occur as lower frequencies, distorting the imageoccur as lower frequencies, distorting the image

AliasingAliasing

Description of aliasingDescription of aliasing Generally try to draw and image that has Generally try to draw and image that has

too fine of detail or sharp edgestoo fine of detail or sharp edges Causes “jaggies”Causes “jaggies” And pixel artifactsAnd pixel artifacts

That is you can notice the pixels That is you can notice the pixels

Aliasing and Pixel Aliasing and Pixel EffectsEffects

Craik-Cornsweet

Craik-Cornsweet: Described

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The figure above is an exaggerated map indicating the light levels across the image on the previous slide. Note how the center and edges have identical luminance. That can be seen by sitting far enough away from the screen

Minimal Contours

Minimal Contours DescribedThere are two circles below. Both circles have the same luminance(intensity level) at the center. Click on your mouse andThis one changes abruptly watch as the edges are blurredto the level at the center. and the circle disappears.

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Anti-aliasingAnti-aliasing

Solution to aliasingSolution to aliasing Bandlimiting or anti-aliasingBandlimiting or anti-aliasing

Essentially blur the edges or image so fine detail is lostEssentially blur the edges or image so fine detail is lost In a lot of SoftwareIn a lot of Software

Ideally use a gaussian filterIdeally use a gaussian filter This is the shape of CRT electron beam which is why dots on This is the shape of CRT electron beam which is why dots on

screen never show upscreen never show up Infante (1985); Silverstein et al. (1990)Infante (1985); Silverstein et al. (1990)

Many graphics software and even some languages Many graphics software and even some languages anti-alias (Java2D)anti-alias (Java2D)

Temporal IssuesTemporal IssuesMotion reproductionMotion reproduction

FlickerFlicker

Stimulus TimingStimulus Timing

Motion ReproductionMotion Reproduction

Frame rates of monitors far exceed what is Frame rates of monitors far exceed what is needed for movement updateneeded for movement update NTSC video: 30 updates per secondNTSC video: 30 updates per second Movies: 24 updates per secondMovies: 24 updates per second Most web video 10 to 15 frames/secMost web video 10 to 15 frames/sec

Computer VideoComputer Video

Most Update rates are 10-15 frames/secMost Update rates are 10-15 frames/sec

5 fps5 fps 10 fps 10 fps 15 fps 15 fps

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FlickerFlicker

Critical threshold of flicker rates is about Critical threshold of flicker rates is about 60 Hz in the fovea60 Hz in the fovea But gets higher for larger stimuliBut gets higher for larger stimuli

Recommended flicker rates between 66 Recommended flicker rates between 66 Hz and 120 Hz (Bridgeman, 1998)Hz and 120 Hz (Bridgeman, 1998)

Most monitors are adequate in this valueMost monitors are adequate in this value Notice difference between flicker and Notice difference between flicker and

frame update rate.frame update rate.

Stimulus DurationStimulus Duration

Pixels are not on continuously during a Pixels are not on continuously during a frameframe In part this is necessary for clean motionIn part this is necessary for clean motion Typical CRT phosphors last about 4 msec. Typical CRT phosphors last about 4 msec.

(Bridgeman, 1998)(Bridgeman, 1998) On LCD and other technologies, persistence On LCD and other technologies, persistence

is longeris longer Makes motion less clean but flicker less noticeableMakes motion less clean but flicker less noticeable

Differences in PersistenceDifferences in Persistence

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Determining Stimulus DurationDetermining Stimulus Duration

Possible errors:Possible errors: Not knowing which frame the stimulus is first Not knowing which frame the stimulus is first

presented onpresented on Assuming the stimulus is visible from the Assuming the stimulus is visible from the

beginning of the displaybeginning of the display Stimuli are not visible at beginning of frame but Stimuli are not visible at beginning of frame but

some time latersome time later Delay depends upon where on frame stimulus is Delay depends upon where on frame stimulus is

and frame rateand frame rate This is a constant errorThis is a constant error

Effects on TimingEffects on Timing

If no more than one frame delay:If no more than one frame delay: Additional error of up to 17msec added to all Additional error of up to 17msec added to all

conditionsconditions No longer msec timing but really 60 Hz timingNo longer msec timing but really 60 Hz timing

If frame delay is larger effects are worseIf frame delay is larger effects are worse

Example Results from Myors Example Results from Myors (1999)(1999)

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Luminance IssuesLuminance IssuesGammaGamma

GammaGamma

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Gamma CorrectionGamma Correction

Typically well described by: L=LTypically well described by: L=L00++aa(V-V(V-V00))

is typically near 2.3; L, Lis typically near 2.3; L, L00, , aa, , are affected are affected by contrast and brightness settingsby contrast and brightness settings

Can’t be done on webCan’t be done on web Make sure stimuli are not sensitive to Make sure stimuli are not sensitive to

distortions in gray scaledistortions in gray scale High contrastHigh contrast Photo’s may be distorted slightly and impact Photo’s may be distorted slightly and impact

results (Ruppertsberg et al., 2001)results (Ruppertsberg et al., 2001) Don’t require fine distinctionsDon’t require fine distinctions

Chromatic IssuesChromatic IssuesThe Trichromatic TheoryThe Trichromatic Theory

Color PickerColor PickerThe Color GamutThe Color Gamut

GlareGlareColor BlindnessColor Blindness

The CIE Color SystemThe CIE Color System

A set of Equations that A set of Equations that

allow predictions ofallow predictions of

matching.matching. Used in photo printing,Used in photo printing,

TV and film.TV and film. Wyszecki & StilesWyszecki & Stiles

(1967)(1967)

The Color GamutThe Color Gamut

The range of colors reproducible by any The range of colors reproducible by any systemsystem

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The Effect of Illuminance on The Effect of Illuminance on GamutGamut

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In DarkWith Lights onWith Lights on and 30% Luminance Level

IlluminanceIlluminance

Agostini & Bruno (1996)Agostini & Bruno (1996) Accuracy of the perceived stimulus is affected Accuracy of the perceived stimulus is affected

by the amount of illuminance.by the amount of illuminance. D. F. Neri (1990)D. F. Neri (1990)

Combination of light and gamut changed Combination of light and gamut changed chromaticitychromaticity

Request close windows and turn off Request close windows and turn off unnecessary light unnecessary light

Do not use subtle color differencesDo not use subtle color differences

Color BlindnessColor BlindnessDescription

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Color BlindnessColor Blindness

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Color BlindnessColor Blindness

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Color BlindnessColor Blindness

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How to handleHow to handle

Screening questions?Screening questions? Large sample and random assignmentLarge sample and random assignment Pre-test on color blindPre-test on color blind

Summary Summary

SpatialSpatial AliasingAliasing EdgesEdges

TemporalTemporal Frame per second in videoFrame per second in video

LuminanceLuminance GammaGamma

ColorColor GlareGlare Color deficienciesColor deficiencies