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Last TimeTodayIntensity PerceptionDynamic RangeMore Dynamic RangeDisplay on a MonitorGamma ControlSome Facts About ColorLight and ColorWhiteHelium Neon LaserNormal DaylightSlide 13Tungsten LightbulbEmission vs. AdsorptionAdsorption spectra: Red PaintRepresenting ColorSensorsA “Red” SensorThe “Red” Sensor ResponseChanging ResponseSeeing in ColorColor receptorsColor PerceptionThe Same Color?Slide 26Color DeficiencyNext Time9/9/04 © University of Wisconsin, CS559 Spring 2004Last Time•Course introduction•Digital Images–The difference between an image and a display–Ways to get them–Raster vs. Vector–Digital images as discrete representations of reality–Human perception in deciding resolution and image depth•Homework 1 – due Sept 149/9/04 © University of Wisconsin, CS559 Spring 2004Today•Intensity perception•Dynamic Range•Gamma mapping•Color•Start preparing for the projects: Programming Tutorial 19/9/04 © University of Wisconsin, CS559 Spring 2004Intensity 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 19/9/04 © University of Wisconsin, CS559 Spring 2004Dynamic 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 res9/9/04 © University of Wisconsin, CS559 Spring 2004More 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–Way beyond the scope of this course9/9/04 © University of Wisconsin, CS559 Spring 2004Display 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–Similar problem with other monitors, different causes•The outcome: A linear intensity scale in memory does not look linear on a monitor•Even worse, different monitors do different things9/9/04 © University of Wisconsin, CS559 Spring 2004Gamma 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 softwaremonitortodisplayII1imagemonitortoII 9/9/04 © University of Wisconsin, CS559 Spring 2004Some Facts About Color•So far we have only discussed intensities, so called achromatic light (shades of gray)•Accurate color reproduction is commercially valuable - e.g. painting a house, producing artwork•On the order of 10 color names are widely recognized by English speakers - other languages have fewer/more, but not much more•E-commerce has accentuated color reproduction issues, as has the creation of digital libraries•Color consistency is also important in user interfaces, eg: what you see on the monitor should match the printed version9/9/04 © University of Wisconsin, CS559 Spring 2004Light and Color•The frequency, , of light determines its “color”–Wavelength, , is related: – Energy also related•Describe incoming light by a spectrum–Intensity of light at each frequency–A graph of intensity vs. frequency•We care about wavelengths in the visible spectrum: between the infra-red (700nm) and the ultra-violet (400nm)19/9/04 © University of Wisconsin, CS559 Spring 2004White•Note that color and intensity are technically two different things•However, in common usage we use color to refer to both–White = grey = black in terms of color•You will have to use context to extract the meaning# PhotonsWavelength (nm)400 500 600 700WhiteLess Intense White (grey)9/9/04 © University of Wisconsin, CS559 Spring 2004Helium Neon Laser•Lasers emit light at a single wavelength, hence they appear colored in a very “pure” way# PhotonsWavelength (nm)400 500 600 7009/9/04 © University of Wisconsin, CS559 Spring 2004Normal Daylight# PhotonsWavelength (nm)400 500 600 700•The sky is blue, so what should this look like?9/9/04 © University of Wisconsin, CS559 Spring 2004Normal Daylight# PhotonsWavelength (nm)400 500 600 700•Note the hump at short wavelengths - the sky is blue•Other bumps came from solar emission spectra and atmospheric adsorption9/9/04 © University of Wisconsin, CS559 Spring 2004Tungsten Lightbulb•Most light sources are not anywhere near white•It is a major research effort to develop light sources with particular properties# PhotonsWavelength (nm)400 500 600 7009/9/04 © University of Wisconsin, CS559 Spring 2004Emission vs. Adsorption•Emission is what light sources do•Adsorption is what paints, inks, dyes etc. do•Emission produces light, adsorption removes light•We still talk about adsorption spectra, but now is it the proportion of light that is removed at each frequency–Note that adsorption depends on such things as the surface finish (glossy, matte) and the substrate (e.g. paper quality)–The following examples are qualitative at best9/9/04 © University of Wisconsin, CS559 Spring 2004Adsorption spectra: Red Paint•Red paint absorbs green and blue wavelengths, and reflects red wavelengths, resulting in you seeing a red


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