7Factors Affecting Visual Perception Perception of depthCues to determine relative positions of objectsObjects overlappingSize and height of objectProvides cue for distanceFamiliarityCertain size helps to judge the distance accordingly
8Perceiving Brightness subjective reaction to levels of lightaffected by luminance of objectLuminanceDepends onAmount of light falling on objectReflective properties of objectMeasured by photometerContrastFunction of the luminance of an object and the luminance of its background
9Perceiving Color 3 components 7 million colors can be perceived Hue Determined by the spectral wavelengthBlue (short)Green (medium)Red (long)150 hues determined by eyeIntensityBrightness of colorSaturationAmount of whiteness in the color7 million colors can be perceived
10Over Compensation Illusions Optical illusions sometimes occur due to over compensationOur perception of size is not reliableconcaveconvexthe Muller Lyer illusionthe Ponzo illusion
12In Today’s Lecture Vision Hearing Touch (Haptic Perception) Movement Color Theory3D VisionReadingHearingHuman EarProcessing SoundTouch (Haptic Perception)Skin PhysiologyTypes of haptic sensesMovementMovement Perception1
13Why Study Color?Color can be a powerful tool to improve user interfaces, but its inappropriate use can severely reduce the performance of the systems we build
14ColorSensory response to electromagnetic radiation in the spectrum between wavelengths micrometers
15Color Theory Primary Color Secondary Color Tertiary Color Color HarmonyColor Context2
16Color Theory Primary Color Secondary Color Tertiary Color Red,yellow and blue3 pigment colors that can not be mixedCan not be formed by any combination of other colorSecondary Colorformed by mixing the primary colorsTertiary Colorformed by mixing one primary and one secondary colorPRIMARY COLORS Red, yellow and blueSECONDARY COLORS Green, orange and purple1TERTIARY COLORS Yellow-orange, red-orange, red-purple, blue-purple, blue-green and yellow-green
17Color Harmonypleasing arrangement of parts, whether it be music, poetry, color, or even an ice cream sundaeWhen something is not harmonious, it's either boring or chaoticextreme unity leads to under-stimulationextreme complexity leads to over-stimulation.Harmony is a dynamic equilibrium.
18Color Harmony Some Formulas for Color Harmony analogous colors any three colors which are side by side on a 12 part color wheelyellow-green, yellow, and yellow-orangeUsually one of the three colors predominates.A color scheme based on analogous colors
19Color Harmony Some Formulas for Color Harmony complementary colors any two colors which are directly opposite each otherred and green and red-purple and yellow-greenA color scheme based on complementary colors
20Color HarmonyNature provides a perfect departure point for color harmonyred yellow and green create a harmonious design, regardless of whether this combination fits into a technical formula for color harmonyA color scheme based on nature
21Color ContextHow color behaves in relation to other colors and shapes is a complex area of color theory.Compare the contrast effects of different color backgrounds for the same red square
22Color Context Different readings of the same color Which is the lighter vertical bar?Figure bFigure a
23Color Perception via Cones Photopigments” used to sense color3 typesblue, green, “red” (really yellow)each sensitive to different band of spectrumratio of neural activity of the 3 colorother colors are perceived by combining stimulation
27Distribution of Photopigments Photopigments” used to sense colorNot distributed evenlymainly reds (64%) & very few blues (4%) ?insensitivity to short wavelengthscyan to deep-blueCenter of retina (high acuity) has no blue cones ?disappearance of small blue objects you fixate on
28Color Sensitivity & Image Detection Most sensitive to the center of the spectrumblues & reds must be brighter than greens & yellowsBrightness determined mainly by R+Gcombine brightness & colordifferences for sharpnessShapes detected by finding edgesImplications?hard to deal w/ blue edges& blue shapes
29Color Sensitivity & Image Detection As we agelens yellows & absorbs shorter wavelengths ?sensitivity to blue is even more reducedfluid between lens and retina absorbs more lightperceive a lower level of brightnessImplications?don’t rely on blue for text or small objects!older users need brighter colors
30FocusDifferent wavelengths of light focused at different distances behind eye’s lensneed for constant refocusing ?causes fatiguebe careful about color combinations
31FocusDifferent wavelengths of light focused at different distances behind eye’s lensneed for constant refocusing ?causes fatiguebe careful about color combinationsPure (saturated) colors require more focusing then less pure (desaturated)don’t use saturated colors in UIs unless you really need something to stand out (stop sign)
32Color Guidelines Opponent colors go well together (red & green) or (yellow & blue)Hering's opponent colors diagram Pick non-adjacent colors on the hue circle
33Color Guidelines Size of detectable changes in color varies hard to detect changes in reds, purples, & greenseasier to detect changes in yellows & blue-greensOlder users need higher brightness levels to distinguish colorsHard to focus on edges created by color alone ?use both brightness & color differencesAvoid red & green in the periphery - why?lack of RG cones there -- yellows & blues work in periphery
34Color Guidelines Avoid pure blue for text, lines, & small shapes blue makes a fine background coloravoid adjacent colors that differ only in blueAvoid single-color distinctionsmixtures of colors should differ in 2 or 3 colorse.g., 2 colors shouldn’t differ only by amount of redhelps color-deficient observers
35Color Guidelines Accurate color discrimination -+60 degree of straight head position.Not uniform across the whole field of visionLimit of color awareness-+90 degree of straight head positionCultural issueSociety classifies color differently
363D visiongive an immediate perception of depth on the basis of the difference in points of view of the two eyes.Also known as binocular vision and stereopsismost reliable clue for depthThis is possible only when the eyes of a creature look in the same direction, and have overlapping fields.Stereopsis gives a reliable distance clue as far away as 450 meters1
372D to 3D Strong Clues Weaker Clues Strongest Clue apparent sizes of objects of known sizeoverlapping and parallaxshadows and perspectiveWeaker Cluesatmospheric perspective (haze and scattering)speed of movementobserved detailStrongest Cluestereopsis1
382D to 3D free fusion stereo pair Keep two pictures side by side each eye sees its picture straight aheada third, fused, image appearsappears strikingly solidBrain regards this real onestereo pairThe two images are called a stereo pair
39ReadingStagesvisual pattern perceiveddecoded using internal representation of languageinterpreted using knowledge of syntax, semantics, pragmaticsReading involves saccades and fixations (perception here: 94%)Perception occurs during fixationsRegressions: eye move backwards and forwardsWord shape is important to recognition
40Reading Negative contrast improves reading from computer screen Average Reading Speed: 250 words per minuteWord shape important: Bat, BATReading speed is a measure of legibilityFont sizes ….Reading from computer screen slower due to …Legibility can be increased using negative contrast …
41Human Auditory Perception Second to sightWe under estimate amount of info received via earsLets hear a few soundsSound 1 of a Vehicle coming Towards youSound 2 of a Vehicle going away from you
42HearingProvides information about environment: distances, directions, objects etc.Vibration in air – Sound WavesHuman EarOuterMiddleInner Ear
43Human Ear Outer Ear Outer ears purpose Middle Ear Inner Ear Visible part divided into twoPinnaAuditory CannalOuter ears purposeProtect middle ear from damageMiddle ear temperature maintenancePinna and Cannal amplify some soundsMiddle EarTympanic membrane or ear drum connection to outer earCochlea to inner earOssicles (smallest bones in the body)Inner EarFilled with Cochlea fluidWithin are Cilia which bend with vibration and cause impulses to brain.
44Processing Sound Sound characteristics Pitch Loudness Timbre frequency of soundLoudnessproportional to amplitude of soundTimbrerelates to type of sound
45Processing Sound Audible range 20 Hz to 15 KHz Distinguish between changes less than 1.5 Hz but less accurate at higher frequenciesDifferent frequencies trigger neuron activity causing nerve impulsesAuditory system filters soundsCocktail Party Effect: can attend to sounds over background noiseSound rarely used in interface design
47Touch (Haptic Perception) Close your eyes and pick a cup of teaWhat can you tell about the cup?Can you identify its material ?Is it hot of coldWhat is the textureCan you find the handle?Speed and accuracy reduced without touch
48Touch (Haptic Perception) What has this got to do with Computers?Imagine a key board where you cannot feel the buttons pressed?Have you ever noticed the “marking” on “F” and “J” keys ?How your Mobile phone has an indicator to digit 5 ?Virtual Reality has no touchProvides important feedback about environment.May be key sense for someone who is visually impaired - braille
49How haptic interaction works? From MIT “TouchLab”
50Types of human haptic sensing Tactilesense stimulus though the skinheat, pain, pressure, texture (cutaneous)Kinesthesiasense of limb motionsaffects comfort and performanceDue to receptors in joints:Rapidly adapting: limb in particular directionSlowly adapting: limb movement and staticPositional receptors: limb staticProprioceptionawareness of limb locationsmovements and shapes, temperature, and pain
51Types of human haptic sensing active & passive touch“exploratory procedures”: directed manipulations to determine specific properties of touched objects
52Skin Physiology Some areas more sensitive than others e.g. fingers. 2-point threshold test2 pencils: place 12mm apart on thumb and move>distance => > sensitivityFingers have highest acuity (10x forearms)Spatial resolution of about 2.5 mmMultiple forces closer are sensed as oneStimulus received via sensory receptors in the skinthermoreceptors – heat and coldnociceptors – painMechanoreceptors: pressure (of concern in HCI)Rapidly adapting (immediate and increased pressure)Slowly adapting (continuously applied pressure)
53Movement (Motor Control) Time taken to respond to stimulus: reaction time + movement timeMovement time dependent on age, fitness etc.Reaction time - dependent on stimulus type:visual ~ 200msauditory ~ 150 mspain ~ 700msIncreasing reaction time decreases accuracy in the unskilled operator but not in the skilled operator.
54Movement Speed and accuracy Relationship between the two? Depends on skill: typists
55MovementFitts' Law describes the time taken to hit a screen target (function of size of target and distance that has to be moved):Mt = a + b log2(D/S + 1)where: a and b are empirically determined constantsMt is movement timeD is DistanceS is Size of targettargets as large as possible distances as small as possible
56Movement Perception Three modes of viewing a moving object hold our heads and eyes still as a moving object passes in front of usStare at birdPerceive Car moves from right to left.
57Movement Perceptionhold our heads still and let our eyes follow the objectimage remains on the fovea during the motion of the car and your eyes.
58Movement Perceptionhold our eyes steady and move only our head to follow an objectThis causes the image to project to exactly the same retinal location at each instant (assuming you move your head at precisely the correct angular velocity) as the car moves from right to left.
59Movement Perception Motion After Effect Explanation Phi phenomenon Known as adaptation effectwhen the stimulus is not movinga waterfall exampleExplanationmotion detection systemmotion detectorsundergo spontaneous activityPhi phenomenonmotion being seen, when there is no physical motiontwo illuminated spots of light about 6 to 8 inches apartFilms and CartoonsStill frames are rapidly projected.
60Movement Perception induced motion Auto kinetic movement Sit in a stationary trainWhen any other train passes next to youYou feel movingAuto kinetic movementview a small very dim light in an otherwise completely dark roomAfter sometime in the dark, the small light will appear to move somewhat randomlyCortical neuronspecialized for movementrespond best when the motion is in a specific direction
61Smell/olfaction Nose and Computing ??? Experiments are being done to transmit smells across the netBiometric devices to identify one person from another