CS559-Computer Graphics Copyright Stephen Chenney 2001 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

CS559-Computer Graphics Copyright Stephen Chenney 2001 Film 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 film Light in Lens Aperture Shutter 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 Light in Lens CCD

CS559-Computer Graphics Copyright Stephen Chenney 2001 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 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

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 yellow brown

CS559-Computer Graphics Copyright Stephen Chenney 2001 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: 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 reproducing colors

CS559-Computer Graphics Copyright Stephen Chenney 2001 Color matching functions Choose primaries, say A, B, C Given energy function, E( ), what amounts of primaries will match it? For each wavelength, determine how much of A, of B, and of C is needed to match light of that wavelength alone. Gives a( ), b( ) and c( ) Match is: