cs559-computer graphics copyright stephen chenney 2001 the human eye graphics is concerned with the...

CS559-Computer GraphicsCopyright 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)


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


Photographs

• First photograph due to Niepce,

• First on record shown - 1822


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


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


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


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


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


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 depth129 128 125


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


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


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


Light Spectra


Sunlight


More spectra


Absorption spectra: real pigmentscyan magenta yellow

brown


Fluorescence


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


Color receptors

• Output of cone is obtained by summing over wavelengths:

• Experimentally determined in a variety of ways

dEk

)()(


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


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


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


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


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:

C )()(+ B)()(A )()( dEcdEbdEa

cs559-computer graphics copyright stephen chenney 2001 the human eye graphics is concerned with the...

Documents

film light

film camera

time film

film digital images

film samples

discrete value discrete

discrete camera

continuous space