Last Time Course introduction Image basics 1 22 04 University of Wisconsin CS559 Spring 2004 Today More on digital images Introduction to color Homework 1 Programming pre project 1 1 22 04 University of Wisconsin CS559 Spring 2004 Ideal Images The information stored in images is often continuous in nature For example consider the ideal photograph It captures the intensity of light at a particular set of points coming from a particular set of directions it s called irradiance The intensity of light captured by a photograph can be any positive real number and it mostly varies smoothly over space Where do you see spatial discontinuities in a photograph Film Focal point 1 22 04 University of Wisconsin CS559 Spring 2004 Digital 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 see Continuous Discrete Pixels Picture Elements 1 22 04 University of Wisconsin CS559 Spring 2004 Digital 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 1 22 04 University of Wisconsin CS559 Spring 2004 Light in Lens CCD Photography Can you make an arbitrarily large print of a digital image Hence does it record continuous information accurately Resolution determines how much information is recorded Can you take a photograph of a really bright thing Can you take a photograph of a really dark thing Can you take a photograph with light and dark things at the same time The ratio of the brightest thing to the darkest thing you can capture is called dynamic range 1 22 04 University of Wisconsin CS559 Spring 2004 Discretization Issues Can only store a finite number of pixels Choose your target physical image size choose your resolution pixels per inch or dots per inch dpi determine width height necessary 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 and typically little choice Most common depth is 8 but can also get 16 for grey Also concerned with the minimum and maximum intensity dynamic range The big question is What is enough resolution and enough depth 1 22 04 University of Wisconsin CS559 Spring 2004 Perceptual Issues Spatially humans can discriminate about a minute of arc At fovea so only in center of view 20 20 vision At 1m about 0 2mm Dot Pitch of monitors Sometimes 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 required 1 22 04 University of Wisconsin CS559 Spring 2004 Intensity 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 1 1 22 04 University of Wisconsin CS559 Spring 2004 High range low res 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 resolution Low range high res Dynamic Range All possible intensities 1 22 04 University of Wisconsin CS559 Spring 2004 More 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 course 1 22 04 University of Wisconsin CS559 Spring 2004 Display 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 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 things 1 22 04 University of Wisconsin CS559 Spring 2004 Gamma Control The mapping from voltage to display is usually an exponential function I display I to monitor To correct the problem we pass the pixel values through a gamma function before converting them to the monitor I to monitor I 1 image 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 1 22 04 University of Wisconsin CS559 Spring 2004 Some 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 painting a house producing artwork Of the order of 10 color names are widely recognized by English speakers other languages have fewer more but not much more Online 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 version 1 22 04 University of Wisconsin CS559 Spring 2004 Light and Color The frequency of light determines its color Frequency wavelength energy all related Describe incoming light by a spectrum
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