4-Nov-10

Chapter 27Color

Chapter 26The Human Eye & Visual

Perception

Lecture 30

The Eye• As light enters the eye, it moves

through the transparent cover, the cornea, which does about 70% of the necessary focusing of the light before it passes through an opening in the iris (colored part of the eye).

• The light then reaches the crystalline lens, which fine-tunes the focusing of light that passes through a gelatinous fluid called vitreous humor.

• Light then passes to the retina,which covers the back two-thirds of the eye and has sensors that convert light to voltage signals.

Seeing Light – The Eye• For clear vision, light must focus

directly on the retina. • The retina is not uniform.

– In the middle is the macula, and a small depression.

– in the center is the fovea, the region of most distinct vision.

– Behind the retina is the optic nerve, which transmits signals from the photoreceptor cells to the brain.

– There is also a spot in the retina where optic nerves are connected; this is the blind spot.

Seeing Light – The EyeThe retina is composed of two types of tiny antennae that

resonate to the incoming light.

• Rods handle vision in low light.– They predominate toward the

periphery of the retina.• Cones handle color vision

and detail.– They are denser toward the

fovea.– There are three types of cones,

stimulated by low, intermediate and high frequencies of light (red, green, and blue).

Perception of DistanceVisually, we experience distance by• Occultation (objects hide what’s behind them)• Geometric Perspective (objects look smaller as

they get further away)• Atmospheric Perspective (distant objects are

hazy and bluish)• Lighting and shadows• Stereopsis (different view in each eye)• Relative motion (as you move, nearby objects

shift more than distant objects)

OcclusionThe simplest way that

we perceive distance is by the fact that closer objects occlude (hide) the objects behind them.

Even in this surreal painting we immediately see the boy as being closer to us than the woman because he partially blocks our view of her.

from The Madonna of Port Lligat, Salvador Dali, 1950

Distorted OcclusionThe image is disturbing

but the reason isn’t immediately apparent.

from Waterfall, M.C. Escher, 1961

This channel of water needs to be behind the lower part of the right tower.

When occlusion is incorrect, we are very cognizant of the distortion.

Pre-15th Century Paintings

Mongol Ruler and consort enthroned, 14th century Road to Calvary, Martini, 1315

Occlusion but no sense of distance

Renaissance PaintingsScenes in these paintings look realistic

The Annunciation, Botticelli, 1489

Marriage of the Virgin, Raphael, 1504

Perspective

The difference is the introduction of visual perspective by FilippoBrunelleschi of Florence.

Objects in the distance look smaller as determined by geometric rules.

Florence, Italy

Perspective Example

The gazelles in this photo appear to be roughly the same physical size.

Thanks to John Clapp for these slides

Move from here…to here

Perspective Example

Let’s move one using cut-and-paste. How big will it be?

Perspective Example

Surprised? Objects appear much smaller with distance!Your brain adjusts and “sees” the animals as equal size.

Distorted Perspective

Modern painters sometimes distort the perspective for dramatic effect.

Mystery and Melancholy of a Street, de Chirico, 1914

The two buildings converge to two different horizons. This feels weird and unnatural, which is what the artist intended (note the title of the painting).

4-Nov-10

Chapter 27Color & Color Perception

ColorColor we see depends on frequency of light.• Lowest frequency—perceived as red• In between lowest and highest frequency—perceived as

colors of the rainbow (red, orange, yellow, green, blue, indigo, violet)

• Highest frequency—perceived as violet• Beyond violet, invisible ultraviolet (UV)

Selective Reflection

Selective reflection• We see the color of a rose by the light it

reflects.

Selective ReflectionObjects reflect light of some frequencies and absorb the rest.• Rose petals absorb most of the light and reflect red.• Objects that absorb light and reflect none appear black.• Objects can reflect only those frequencies present in the

illuminating light.• Colors will look different under different lighting.

Selective TransmissionColor of transparent object depends on color of light it transmits.

• Colored glass is warmed due to the energy of absorbed light illuminating the glass.

4-Nov-10

Color Vision in the Eye

Three types of cones (color)One type of rod (B/W only)

Color and Color Perception• Color response of human eye

Primary Additive Colors (Transmitted Light)• Primary Additive Colors - RGB

– Red (R)– Green (G)– Blue (B)

• Human eye can perceive all other colors if given an RGB mixture– Yellow = Red + Green– Cyan = Blue + Green– Magenta (Purple) = Red + Blue– White = Red + Green + Blue

Mixing Colored Light

Additive primary colors:• Red, green, and blue• Produce any color in the spectrum

Additive Color Synthesis

White Light• White light is a mixture of all colors (or just of Red,

Green, and Blue).• There are different “whites”, depending on mixture of

primary colors used• White sunlight - Mixture is uneven.

– Most intense in yellow-green portion (where our eyes are most sensitive)

Red, green, and blue light overlap to form

A. red light.B. green light.C. blue light.D. white lightE. magenta light

Mixing Colored LightCHECK YOURSELF

Red, green, and blue light overlap to form

A. red light.B. green light.C. blue light.D. white lightE. magenta light

Mixing Colored LightCHECK YOUR ANSWER

A blue object will appear black when illuminated with

A. blue light.B. cyan light (B + G)C. yellow light (R + G)D. magenta light (R + B)

Mixing Colored LightCHECK YOURSELF

A blue object will appear black when illuminated with

A. blue light.B. cyan light.C. yellow light.D. magenta light.

Mixing Colored LightCHECK YOUR ANSWER

Subtractive Primary Colors (Reflected Light)• Paints, inks, dyes absorb (subtract) one or more

“spectral colors”– Yellow ink – absorbs everything except yellow;

if illuminated by white light, reflects yellow• Primary subtractive colors - subtract only R,G, or B

– Magenta (M): Subtracts Green– Yellow (Y): Subtracts Blue– Cyan (C): Subtracts Red

• Example –3 tank color inkjet printer: C,M,Y inks– or CMYK, where K stands for black

• Painter’s “Color Wheel” – RGY - Not primary set

Reflecting Colored Light

Subtractive primary colors• Combination of two of the three additive

primary colors:– red + blue = magenta ( = - Green)– red + green = yellow ( = - Blue)– blue + green = cyan (= - Red)

Color SubtractionThe shadows of the golf ball are subtractive.• Magenta (- green)• Cyan (- red)• Yellow (- blue)

4-Nov-10

Subtractive ColorAbsorbing filters or

absorbing pigments remove colors from white light.

Check Yourself:Yellow = Red + GreenCyan = Blue + GreenYellow & Cyan filters (or pigments)combine to give …

Overlapping color filters

Subtractive & Additive PrimariesSubtractive primaries are complementary to additive primaries.• magenta + green = white = red + blue + green• yellow + blue = white + red + green + blue

Example: color printing

A red rose will not appear red when illuminated only with

A. red light.B. orange light.C. white light.D. cyan (-R) light.

Mixing Colored LightCHECK YOURSELF

A red rose will not appear red when illuminated only with

A. red light.B. orange light.C. white light.D. cyan light.

Mixing Colored LightCHECK YOUR ANSWER

Mixing Colored Pigments

Only three colors of ink (plus black) are used to print color photographs—(a) magenta, (b) yellow, (c) cyan, which when combined produce the colors shown in (d). The addition of black (e) produces the finished result (f).

Key Points of Lecture 30Key Points of Lecture 30

Before Monday, read Hewitt Chap. 27.

Homework #20 due by 11:00 PM Friday Nov. 5.

Homework #21 due by 11:00 PM Sunday Nov. 7

Homework #22 due by 11:00 PM Tuesday Nov. 9

• The Human Eye• Visual Perception• Pure Colors (Colors of Rainbow)• Additive Primary Colors• Subtractive Primary Colors• Color Mixing