Prof. Greg Francis1PSY 310: Sensory and Perceptual ProcessesPurdue UniversityConstancyPSY 310Greg FrancisLecture 19It’s all an illusion!Purdue UniversityBrightness illusions Most people think of visual perception as ameasurement of light As it reflects off of objectsPurdue UniversityObject identification A basic task of the visual system is to figure out whether changesin intensity are due to changes in objects (color, paint) or tochanges in illumination (light, shadows, veils) One problem is that an edge from paint could be the same as anedge from a shadow Steps-movie.swfPurdue UniversityAmbiguity The process of identifying an pattern of light as a setof objects in inherently ambiguous There is no way that you can always be correct! The visual system must use some “tricks” to try tobe right as often as possible We can expose those tricks with illusionsPurdue UniversitySnakes illusion The diamonds have the exact same scale of grayPurdue UniversitySnakes illusion We see a dark transparent veil in front of some rows Which means the paint of the diamond behind it must belighter than the paint of the diamond without the veilProf. Greg Francis2PSY 310: Sensory and Perceptual ProcessesPurdue UniversityShadowsPurdue UniversityShadowsPurdue UniversityShadowsPurdue UniversityShadowsPurdue UniversityShadows We interpret the shadow as producing a change inintensity Which means the object surfaces (paint) must bedifferent There are lots of brightness illusions based on thisidea It’s a side effect of our effort to judge theproperties of objects in different kinds of lighting We want to be able to identify an object’s propertiesregardless of lighting Constancy (color, brightness) Let’s look at one morePurdue UniversityPerceiving lightProf. Greg Francis3PSY 310: Sensory and Perceptual ProcessesPurdue UniversityPerceiving lightPurdue UniversityPerceiving lightPurdue UniversityPerceiving lightPurdue UniversityPerceiving lightPurdue UniversityPerceiving lightPurdue UniversityDemonstration Pick up a copy of the image and randomly select a set ofinstructions. Follow the instructions and come back to the classroomwithin 10 minutesProf. Greg Francis4PSY 310: Sensory and Perceptual ProcessesPurdue UniversityLight levels People can see objects across an amazing range of lightlevels Some is due to pupil size, rod visionPurdue UniversityLight levels Even within indoor lighting ranges, we see things in a widevariety of levels of lightPurdue UniversityLight levels Consider a photograph of an office The light from outside is so bright you cannot see anything The light on the floor is so dark you cannot see anythingPurdue UniversityLight levels You can change exposure settings, but nothing really workswellPurdue UniversityLight levels You can change exposure settings, but nothing really workswell You have to go through a complicated process to take areally good picture Lots of light (like a photographer studio) Or process and combine different pieces of informationPurdue UniversityPhotoshop Take the over-exposed image and exaggerate the shadows The makes the details in the dark places of the floor visibleProf. Greg Francis5PSY 310: Sensory and Perceptual ProcessesPurdue UniversityPhotoshop Take the under-exposed image and exaggerate the brightregions The makes the details in the bright places of the windowvisiblePurdue UniversityPhotoshop Combine everything in the right way, and you get a betterpicture Your brain is doing something similar, but in a much moresophisticated wayPurdue UniversityConclusions Constancy We want to know the properties of objects Ambiguity It is not always possible to know if changes are due toobject properties of illumination properties Visual system Uses clever tricks to choose Usually correct Sometimes easy to foolPurdue UniversityNext time Depth perception Monocular
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