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Stanford EE 368 - Human Visual Perception

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1Bernd Girod: EE368b Image and Video Compression Human Visual Perception no. 1Human visual perception - topicsn Visual acuityn Weber-Fechner Lawn Lateral inhibition and excitationn Transfer functions of the color channelsn Spatial and temporal maskingn Eye movementsBernd Girod: EE368b Image and Video Compression Human Visual Perception no. 2Anatomy of the human eyecorneapupillensirisoptic axisvisual axisfoveaoptic nerveoptic diskretinascleraretinal imageobject2Bernd Girod: EE368b Image and Video Compression Human Visual Perception no. 3Optical properties of the human eyen Deviations from ideal perspective projection due tol Aperture of the eyel Focus errors (spherical aberration)l Chromatic aberrationl Dispersionn Effects can be summarized by a 2D convolution with the optical point-spread function (PSF).n Instead of a PSF, an optical line-spread function (LSF) is often given, which can be measured more easily.Bernd Girod: EE368b Image and Video Compression Human Visual Perception no. 4Optical LSF of the human eyeLSF measured fordifferent pupil diameters(Campbell+Gubisch)LSF calculated fromeye aperture(due to diffraction)3Bernd Girod: EE368b Image and Video Compression Human Visual Perception no. 5Optical modulation transfer function (MTF) of the human eyen MTF is measured directly with sinewave gratings.n The optical modulation transfer function (MTF) can be interpreted as Fourier transform of the optical LSF.contrast ratio image:objectspatial frequency [cpd]Bernd Girod: EE368b Image and Video Compression Human Visual Perception no. 6Sine wave gratingpositionintensitycontrast ratio=L2−L1L2+L14Bernd Girod: EE368b Image and Video Compression Human Visual Perception no. 7Human retina Ilightoptic nerve fibers ganglion cells inner synaptic layer amacrine cells bipolar cells horizontal cells outer synaptic layer receptor nuclei receptors pigmented layer (epithelium cells)Signal propagation in the retina:receptorsbipolar cellsganglion cells (=optic nerve)Bernd Girod: EE368b Image and Video Compression Human Visual Perception no. 8Human retina IIRodsConeshigh sensitivitylow sensitivitylow light visionday light vision > 1 cd/m2monochromecolor"scotopic vision""photopic vision" Video displaysreceptorsin 1000/mm2angle [degree]noseblind spot foveaconesrods5Bernd Girod: EE368b Image and Video Compression Human Visual Perception no. 9Visual acuityn Spatial resolution in lines/arcmin:n Minimum distance of adjacent cones in the central fovea limits spatial resolution. (2 - 2.3 µm 25 . . . 29 sec of arc) blind spot foveaangle [degree]conesrodsnoseBernd Girod: EE368b Image and Video Compression Human Visual Perception no. 10Weber-Fechner law, In Experiment: n Result:backgroundluminance LBsurroundluminance LSstimulusareaLB, background luminance (cd/m2)threshold ∆L in cd/m2CONST6Bernd Girod: EE368b Image and Video Compression Human Visual Perception no. 11Weber-Fechner law, IIn "Weber-Fechner Law”n Implies logarithmic relationship between physical luminance and subjectively perceived brightness.n Other proposed nonlinearities: square-root, cube-root, polynomialsn γ-characteristic of CRT displays is approximate inverse of nonlinearity of human brightness perception.∆L=cLB c=0.01 ... 0.02Bernd Girod: EE368b Image and Video Compression Human Visual Perception no. 12Inhibition and excitation in the retinan Receptive field of a ganglion cell (=fiber of the optic nerve) shows „center-surround response“ with bothl Lateral inhibitionl Lateral excitation receptive fieldcenteractivity of neuronaverage activitysurround7Bernd Girod: EE368b Image and Video Compression Human Visual Perception no. 13Spatial contrast sensitivityn Lateral inhibition and excitation together lead to a bandpasscharacteristic of the contrast sensitivity function of the human visual systemspatial frequency (cpd)contrast sensitivity (@ 500 cd/m2)Bernd Girod: EE368b Image and Video Compression Human Visual Perception no. 14Color vision: opponent color theoryn Retina carries out “matrix operation“ to represent colors in the opponent color system (Y, Y-B, R-G)Opponent color model: Opponent color space:B G RB G RYBRGChromatic channelsYLuminance signalblackwhitegreenredyellowblue8Bernd Girod: EE368b Image and Video Compression Human Visual Perception no. 15Color vision: contrast sensitivity in opponent color spacen Spatial frequency response of Y-B and R-G channel (Girod, 1988):n Bandwidth Y:RG:YB approximately 8:5:3.n Some researchers have observed bandpass characteristic also for chromaticity channels.spatial frequency (cpd)Bernd Girod: EE368b Image and Video Compression Human Visual Perception no. 16Spatial masking, IExperiment:visibility threshold(w-Modell, Girod, 1987)distance from edge [arcmin]80 → 230 80 → 180 80 → 130 6 dB0 10 20-20 -10 visibility threshold [log units]change in video signal amplitude [0 . . . 255]dynamic noise bardistance Dlightdark9Bernd Girod: EE368b Image and Video Compression Human Visual Perception no. 17Spatial masking, IIVisibility threshold for the γ-predistorted video signal (w-Modell, Girod, 1987):80 → 230 80 → 180 80 → 130 6 dB0 10 20-20 -10"γ-shift" visibility threshold [log units]distance from edge [arcmin]change of video amplitude [0 . . . 255]Bernd Girod: EE368b Image and Video Compression Human Visual Perception no. 18Spatiotemporal contrast sensitivity of luminance perceptionn Spatiotemporal contrast sensitivity of the luminance channel has bandpass characteristic.n Contrast sensitivity function separable for high spatial and temporal frequencies only.temporal frequencyspatial frequencyjust perceptable modulationn Plot of contrast sensitivity function (from Kelly):10Bernd Girod: EE368b Image and Video Compression Human Visual Perception no. 19Temporal maskingn Visibility thresholds for γ-predistorted video signal after luminance discontinuity (w-model, Girod, 1987):visibility threshold [arbitrary log units]time after discontinuity [ms]Bernd Girod: EE368b Image and Video Compression Human Visual Perception no. 20Eye movementscorrective saccadeslow driftsaccade; v up to 800 °/sSPEM ; v < 30 °/s saccadeSPEM: smooth pursuit eye movement11Bernd Girod: EE368b Image and Video Compression Human Visual Perception no. 21Temporal masking and SPEMsdarkbrighttrajectory of eye movementxtt0t0xtEye fixates screen Eye tracks moving edget=0t=t0t0< t < t1t1< tt1t1darkbrighttrajectory of eye movementTemporal maskingBernd Girod: EE368b Image and Video


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