Vision and colorReadingOpticsOptics, cont’dStructure of the eyeStructure of the eye, cont.RetinaThe human retinaThe human retina, cont’dNeuronal connectionsDemonstrations of visual acuityMach bandsMach bands, cont.Higher Level ReasoningThe radiant energy spectrumEmission spectraWhat is color?PhotopigmentsUnivarianceThe color matching experimentRods and “color matching”Cone photopigmentsCones and color matchingColor matchingColor matching, cont’dChoosing PrimariesColor as linear projectionEmission Spectrum is not ColorColored SurfacesSubtractive MetamersIllustration of Color AppearanceLighting designWhat’s wrong with RGB?Perceivable portion of CIE spaceThe CIE XYZ SystemCIE CoordinatesThe CIE Colour BlobThe CIE Chromaticity DiagramMore About ChromaticityGamutsNext class: Modern graphics HWUniversity of Texas at Austin CS384G - Computer Graphics Fall 2008 Don FussellVision and colorUniversity of Texas at Austin CS384G - Computer Graphics Fall 2008 Don Fussell 2ReadingGlassner, Principles of Digital Image Synthesis, pp. 5-32.Watt , Chapter 15.Brian Wandell. Foundations of Vision. Sinauer Associates, Sunderland, MA, pp. 45-50 and 69-97, 1995.University of Texas at Austin CS384G - Computer Graphics Fall 2008 Don Fussell 3OpticsThe human eye employs a lens to focus light.To quantify lens properties, we’ll need some terms from optics (the study of sight and the behavior of light):Focal point - the point where parallel rays converge when passing through a lens.Focal length - the distance from the lens to the focal point.Diopter - the reciprocal of the focal length, measured in meters.Example: A lens with a “power” of 10D has a focal length of 0.1m.University of Texas at Austin CS384G - Computer Graphics Fall 2008 Don Fussell 4Optics, cont’d1 1 1o id d f+ =By tracing rays through a lens, we can generally tell where an object point will be focused to an image point:This construction leads to the Gaussian lens formula:Q: Given these three parameters, how does the human eye keep the world in focus?University of Texas at Austin CS384G - Computer Graphics Fall 2008 Don Fussell 5Structure of the eyeThe most important structural elements of the eye are:Cornea - a clear coating over the front of the eye:Protects eye against physical damage.Provides initial focusing (40D).Iris - Colored annulus with radial muscles.Pupil - The hole whose size is controlled by the iris.Physiology of the human eye (Glassner, 1.1)University of Texas at Austin CS384G - Computer Graphics Fall 2008 Don Fussell 6Structure of the eye, cont.Crystalline lens - controls the focal distance:Power ranges from 10 to 30D in a child.Power and range reduces with age.Ciliary body - The muscles that compress the sides of the lens, controlling its power.Q: As an object moves closer, do the ciliary muscles contract or relax to keep the object in focus?Physiology of the human eye (Glassner, 1.1)University of Texas at Austin CS384G - Computer Graphics Fall 2008 Don Fussell 7RetinaRetina - a layer of photosensitive cells covering 200° on the back of the eye.Cones - responsible for color perception.Rods - Limited to intensity (but 10x more sensitive).Fovea - Small region (1 or 2°) at the center of the visual axis containing the highest density of cones (and no rods).Density of photoreceptors on the retina (Glassner, 1.4)University of Texas at Austin CS384G - Computer Graphics Fall 2008 Don Fussell 8The human retinaPhotomicrographs at increasing distances from the fovea. The large cells are cones; the small ones are rods.Photomicrographs at incresasing distances from the fovea. The large cells are cones; the small ones are rods. (Glassner , 1.5 and Wandell, 3.4).University of Texas at Austin CS384G - Computer Graphics Fall 2008 Don Fussell 9The human retina, cont’dLight gathering by rods and cones (Wandell, 3.2)Photomicrograph of a cross-section of the retina near the fovea (Wandell, 5.1).University of Texas at Austin CS384G - Computer Graphics Fall 2008 Don Fussell 10Neuronal connectionsEven though the retina is very densely covered with photoreceptors, we have much more acuity in the fovea than in the periphery.In the periphery, the outputs of the photoreceptors are averaged together before being sent to the brain, decreasing the spatial resolution. As many as 1000 rods may converge to a single neuron.University of Texas at Austin CS384G - Computer Graphics Fall 2008 Don Fussell 11With one eye shut, at the right distance, all of these letters should appear equally legible (Glassner, 1.7).Demonstrations of visual acuityBlind spot demonstration (Glassner, 1.8)University of Texas at Austin CS384G - Computer Graphics Fall 2008 Don Fussell 12Mach bands were first dicussed by Ernst Mach, an Austrian physicist.Appear when there are rapid variations in intensity, especially at C0 intensity discontinuities:And at C1 intensity discontinuities:Mach bandsUniversity of Texas at Austin CS384G - Computer Graphics Fall 2008 Don Fussell 13Mach bands, cont.Possible cause: lateral inhibition of nearby cells.Q: What image processing filter does this remind you of?Lateral inhibition effect (Glassner, 1.25)University of Texas at Austin CS384G - Computer Graphics Fall 2008 Don Fussell 14Higher Level ReasoningMany perceptual phenomena occur at a higher level in the brainChecker Shadow Effect (Edward Adelson, 1995)University of Texas at Austin CS384G - Computer Graphics Fall 2008 Don Fussell 15The radiant energy spectrumWe can think of light as waves, instead of rays. Wave theory allows a nice arrangement of electromagnetic radiation (EMR) according to wavelength:University of Texas at Austin CS384G - Computer Graphics Fall 2008 Don Fussell 16Emission spectraA light source can be characterized by an emission spectrum:The spectrum describes the energy at each wavelength.Emission spectra for daylight and a tungsten lightbulb (Wandell, 4.4)University of Texas at Austin CS384G - Computer Graphics Fall 2008 Don Fussell 17What is color?The eyes and brain turn an incoming emission spectrum into a discrete set of values.The signal sent to our brain is somehow interpreted as color.Color science asks some basic questions:• When are two colors alike?• How many pigments or primaries does it take to match another
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