Last TimeTodayDigital ImagesDigital CamerasAlternative Imaging MethodsDiscretization IssuesPerceptual IssuesIntensity PerceptionDynamic RangeMore Dynamic RangeDisplay on a MonitorGamma ControlSome Facts About ColorLight and ColorWhiteHelium Neon LaserNormal DaylightSlide 18Tungsten LightbulbRed PaintFrequency ResponseA “Red” SensorThe “Red” Sensor ResponseChanging ResponseSeeing in ColorColor receptorsColor PerceptionThe Same Color?Slide 29Color DeficiencyTrichromacyThe Math of TrichromacyColor matching functions1/24/02 © University of Wisconsin, CS559 Spring 2002Last Time•Course introduction•Assignment 1 (not graded, but necessary)–View is part of Project 1•Image and film basics1/24/02 © University of Wisconsin, CS559 Spring 2002Today•More on Digital Images•Introduction to color•Homework 11/24/02 © University of Wisconsin, CS559 Spring 2002Digital 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 for each pixel - the pixel value (no color, yet)–No longer continuous in space or intensity•This process is fraught with danger, as we shall seeContinuousDiscretePixels: Picture Elements1/24/02 © University of Wisconsin, CS559 Spring 2002Digital 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 memory - the image•Still have issues of motion blur, depth of field, dynamic range, etcLight inLensCCD1/24/02 © University of Wisconsin, CS559 Spring 2002Alternative Imaging Methods•We obviously don’t have to use a digital camera to generate a digital image•You can write the pixels directly, or use a paint program, or describe a 3D scene and have a computer render it into an image, or …•This course is all about these other methods•However, it still helps to think of any digital image as a sample of some ideal image1/24/02 © University of Wisconsin, CS559 Spring 2002Discretization Issues•Can only store a finite number of pixels–Resolution: Pixels per inch, or dpi (dots per inch from printers)–Storage space goes up with square of resolution•600dpi has 4× more pixels than 300dpi•Can only store a finite range of intensity values–Typically referred to as depth - number of bits per pixel•Directly related to the number of colors available–Also concerned with the minimum and maximum intensity – dynamic range–Both film and digital cameras have highly limited dynamic range•The big question is: What is enough resolution and enough depth?1/24/02 © University of Wisconsin, CS559 Spring 2002Perceptual 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 depth for typical dynamic ranges–Actually, it’s 9 bits, but 8 is far more convenient•BUT, while perception can guide resolution requirements for display, when manipulating images much higher resolution may be required129 128 1251/24/02 © University of Wisconsin, CS559 Spring 2002Intensity Perception•Humans are actually tuned to the ratio of intensities, not their absolute difference–So going from a 50 to 100 Watt light bulb looks the same as going from 100 to 200–So, if we only have 4 intensities, between 0 and 1, we should choose to use 0, 0.25, 0.5 and 1•Most computer graphics ignores this, giving poorer perceptible intensity resolution at low light levels, and better resolution at high light levels–It would use 0, 0.33, 0.66, and 11/24/02 © University of Wisconsin, CS559 Spring 2002Dynamic Range•Image depth refers to the number of bits available, but not how those bits map onto intensities•We can use those bits to represent a large range at low resolution, or a small range at high resolution•Common display devices can only show a limited dynamic range, so typically we fix the range at that of the display device and choose high resolutionAll possibleintensitiesLow range, high resHigh range, low res1/24/02 © University of Wisconsin, CS559 Spring 2002More Dynamic 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 solution–Way beyond the scope of this course1/24/02 © University of Wisconsin, CS559 Spring 2002Display on a Monitor•When images are created, a linear mapping between pixels and intensity is assumed–For example, if you double the pixel value, the displayed intensity should double•Monitors, however, do not work that way–For analog monitors, the pixel value is converted to a voltage–The voltage is used to control the intensity of the monitor pixels–But the voltage to display intensity is not linear–Same problem with digital monitors, they just do the pixel to intensity conversion differently•The outcome: A linear intensity scale in memory does not look linear on a monitor!1/24/02 © University of Wisconsin, CS559 Spring 2002Gamma Control•The mapping from voltage to display is usually an exponential function:•To correct the problem, we pass the pixel values through a gamma function before converting them to the monitor•This process is called gamma correction•The parameter, , is controlled by the user–It should be matched to a particular monitor–Typical values are between 2.2 and 2.5•The mapping can be done in hardware or softwaremonitortodisplayII1imagemonitortoII 1/24/02 © University of Wisconsin, CS559 Spring 2002Some Facts About Color•So far we have only discussed intensities, so called achromatic light (black and white)•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 have been increased by the
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