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The nature of the beastThe nature of the beast Basic Perceptual IssuesBasic Perceptual Issues
SpatialSpatial TemporalTemporal ColorColor LuminanceLuminance
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
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 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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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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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