The 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 Graphics Copyright Stephen Chenney 2001 What 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 nothing CS559 Computer Graphics Copyright Stephen Chenney 2001 Photographs First photograph due to Niepce First on record shown 1822 CS559 Computer Graphics Copyright Stephen Chenney 2001 Film Camera Light in The film samples the pattern of light that hits it Lens Lens lets more light in while Aperture maintaining focus Aperture controls proportion Shutter of the light that gets to the film Shutter controls how long light Film is allowed to get to the film CS559 Computer Graphics Copyright Stephen Chenney 2001 An 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 slices CS559 Computer Graphics Copyright Stephen Chenney 2001 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 no color for each pixel No longer continuous in space or intensity Continuous Discrete Pixels Picture Elements CS559 Computer Graphics Copyright Stephen Chenney 2001 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 CS559 Computer Graphics Copyright Stephen Chenney 2001 Light in Lens CCD Discretization 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 range CS559 Computer Graphics Copyright Stephen Chenney 2001 Perceptual Issues 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 Limits the required number of pixels Humans can discriminate about 8 bits of intensity Just Noticeable Difference experiments Limits the required depth 129 128 125 CS559 Computer Graphics Copyright Stephen Chenney 2001 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 CS559 Computer Graphics Copyright Stephen Chenney 2001 Why 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 consistency CS559 Computer Graphics Copyright Stephen Chenney 2001 Light 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 frequency CS559 Computer Graphics Copyright Stephen Chenney 2001 Light Spectra CS559 Computer Graphics Copyright Stephen Chenney 2001 Sunlight CS559 Computer Graphics Copyright Stephen Chenney 2001 More spectra CS559 Computer Graphics Copyright Stephen Chenney 2001 Absorption spectra real pigments cyan magenta brown CS559 Computer Graphics Copyright Stephen Chenney 2001 yellow Fluorescence CS559 Computer Graphics Copyright Stephen Chenney 2001 Seeing 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 distributions CS559 Computer Graphics Copyright Stephen Chenney 2001 Color receptors Output of cone is obtained by summing over wavelengths E d k Experimentally determined in a variety of ways CS559 Computer Graphics Copyright Stephen Chenney 2001 Color 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 mind CS559 Computer Graphics Copyright Stephen Chenney 2001 Color 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 receptors CS559 Computer Graphics Copyright Stephen Chenney 2001 Trichromacy Experiment Show a target color beside a user controlled color User has knobs that add primary sources to their color Ask the user to match the colors By experience it is possible to match almost all colors using only three primary sources the principle of trichromacy Sometimes have to add light to the target CS559 Computer Graphics Copyright Stephen Chenney 2001 The Math of Trichromacy Write primaries as A B and C Many colors can be represented as a mixture of A B C M aA bB cC Additive matching Gives a color description system two people who agree on A B C need only supply a b c to describe a color Some colors can t be matched like this instead write M aA bB cC Subtractive matching Interpret this as a b c Problem for
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