The Human EyeWhat is an Image?PhotographsFilm CameraAn Image as a SampleDigital ImagesDigital CamerasDiscretization IssuesPerceptual IssuesDynamic RangeWhy Care Deeply About Color?Light and ColorLight SpectraSunlightMore spectraAbsorption spectra: real pigmentsFluorescenceSeeing in ColorColor receptorsColor PerceptionColor receptors and color deficiencyTrichromacyThe Math of TrichromacyColor matching functionsCS559-Computer GraphicsCopyright Stephen Chenney 2001The Human Eye•Graphics is concerned with the visual transmission of information•How do we see?–Light from the outside world excites nerves in our retina–The brain does the rest (not of concern in this class)CS559-Computer GraphicsCopyright Stephen Chenney 2001What is an Image?•Images represent what things look like, or would look like•Images of real scenes typically record the intensity, and maybe color, of light hitting a surface•Paintings, etchings, etc, produced by hand•Photographs, taken with a camera and film•Digital images, taken with a digital camera, scanned from film, or created from nothingCS559-Computer GraphicsCopyright Stephen Chenney 2001Photographs•First photograph due to Niepce, •First on record shown - 1822CS559-Computer GraphicsCopyright Stephen Chenney 2001Film Camera•The film samples the pattern of light that hits it•Lens lets more light in while maintaining focus•Aperture controls proportion of the light that gets to the film•Shutter controls how long light is allowed to get to the filmLight inLensApertureShutterFilmCS559-Computer GraphicsCopyright Stephen Chenney 2001An Image as a Sample•The film samples the amount of energy arriving at each point over a short period of time•Incoming light is mostly continuous in intensity, space and time•Film is effectively continuous in intensity, space and captures a discrete slice of time–Eventually grains can be seen if the film is enlarged–Movie cameras capture multiple discrete slicesCS559-Computer GraphicsCopyright Stephen Chenney 2001Digital Images•Computers work with discrete pieces of information•How do we digitize a continuous image?–Break the continuous space into small areas, pixels–Use a single value (no color) for each pixel–No longer continuous in space or intensityContinuousDiscretePixels: Picture ElementsCS559-Computer GraphicsCopyright Stephen Chenney 2001Digital Cameras•CCD stores a charge each time a photon hits it–“Bins” have discrete area, one per pixel–Spatially discrete•Camera “reads” the charges out of the bins at some frequency•Convert charges to discrete value–Discrete in intensity•Store values in memoryLight inLensCCDCS559-Computer GraphicsCopyright Stephen Chenney 2001Discretization Issues•Can only store a finite number of pixels–Resolution: Pixels per inch–Storage space goes up with square of resolution•Can only store a finite range of intensity values–Typically referred to as depth–Also concerned with the minimum and maximum intensity – dynamic range–Both film and digital cameras have highly limited dynamic rangeCS559-Computer GraphicsCopyright Stephen Chenney 2001Perceptual Issues•Humans can discriminate about ½ a minute of arc–At fovea, so only in center of view, 20/20 vision–At 1m, about 0.2mm (“Dot Pitch” of monitors)–Limits the required number of pixels•Humans can discriminate about 8 bits of intensity–“Just Noticeable Difference” experiments–Limits the required depth129 128 125CS559-Computer GraphicsCopyright Stephen Chenney 2001Dynamic Range•Real scenes have very high and very low intensities•Humans can see contrast at very low and very high light levels–Can’t see all levels all the time – use adaptation to adjust–Still, high range even at one adaptation level•Film has low dynamic range ~ 100:1•Monitors are even worse•Many ways to deal with the problem, but no great solutionCS559-Computer GraphicsCopyright Stephen Chenney 2001Why Care Deeply About Color?•Accurate color reproduction is commercially valuable - e.g. Kodak yellow, painting a house•Of the order of 10 color names are widely recognized by English speakers - other languages have fewer/more, but not much more•Color reproduction problems increased by prevalence of digital imaging - eg. digital libraries of art•Consistency in user interfaces, eg: monitor-printer consistencyCS559-Computer GraphicsCopyright Stephen Chenney 2001Light and Color•The frequency of light determines its “color”–Frequency, wavelength, energy all related•Describe incoming light by a spectrum–Intensity of light at each frequencyCS559-Computer GraphicsCopyright Stephen Chenney 2001Light SpectraCS559-Computer GraphicsCopyright Stephen Chenney 2001SunlightCS559-Computer GraphicsCopyright Stephen Chenney 2001More spectraCS559-Computer GraphicsCopyright Stephen Chenney 2001Absorption spectra: real pigmentscyanmagentayellowbrownCS559-Computer GraphicsCopyright Stephen Chenney 2001FluorescenceCS559-Computer GraphicsCopyright Stephen Chenney 2001Seeing in Color•The eye contains rods and cones–Rods work at low light levels and do not see color–Cones come in three types (experimentally and genetically proven), each responds in a different way to frequency distributionsCS559-Computer GraphicsCopyright Stephen Chenney 2001Color receptors•Output of cone is obtained by summing over wavelengths:•Experimentally determined in a variety of waysdEk)()(CS559-Computer GraphicsCopyright Stephen Chenney 2001Color Perception•Colors may be perceived differently:–Affected by other nearby colors–Affected by adaptation to previous views–Affected by “state of mind”•Experiment:–Subject views a colored surface through a hole in a sheet, so that the color looks like a film in space–Investigator controls for nearby colors, and state of mindCS559-Computer GraphicsCopyright Stephen Chenney 2001Color receptors and color deficiency•Some people are missing one type of receptor–Most common is red-green color blindness in men–Red and green receptor genes are carried on the X chromosome - most red-green color blind men have two red genes or two green genes•Other color deficiencies–Anomalous trichromacy, Achromatopsia, Macular degeneration–Deficiency can be caused by CNS, by optical problems in the eye, or by absent receptorsCS559-Computer GraphicsCopyright Stephen Chenney 2001Trichromacy•Experiment:–Show a target color beside a user
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