Lecture 30 Chapter 26 The Human Eye & Visual Perception. Chapter 27 Color

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1 Lecture 30 Chapter 26 The Human Eye & Visual Perception Chapter 27 Color 4-Nov-10 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 finetunes the focusing of light that passes through a gelatinous fluid called vitreous humor. Light then passes to the retina, which covers the back twothirds of the eye and has sensors that convert light to voltage signals.

2 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 Eye The 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).

3 Perception of Distance Visually, 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) The simplest way that we perceive distance is by the fact that closer objects occlude (hide) the objects behind them. Occlusion 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

4 Distorted Occlusion The image is disturbing but the reason isn t immediately apparent. When occlusion is incorrect, we are very cognizant of the distortion. This channel of water needs to be behind the lower part of the right tower. from Waterfall, M.C. Escher, 1961 Pre-15 th Century Paintings Occlusion but no sense of distance Mongol Ruler and consort enthroned, 14 th century Road to Calvary, Martini, 1315

5 Renaissance Paintings Scenes 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 Filippo Brunelleschi of Florence. Objects in the distance look smaller as determined by geometric rules. Florence, Italy

6 Perspective Example The gazelles in this photo appear to be roughly the same physical size. Thanks to John Clapp for these slides Perspective Example Move from here to here Let s move one using cut-and-paste. How big will it be?

7 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. 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). Mystery and Melancholy of a Street, de Chirico, 1914

8 Chapter 27 Color & Color Perception 4-Nov-10 Color Color 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)

9 Selective Reflection Selective reflection We see the color of a rose by the light it reflects. Selective Reflection Objects 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.

10 Selective Transmission Color of transparent object depends on color of light it transmits. Colored glass is warmed due to the energy of absorbed light illuminating the glass. Color Vision in the Eye Three types of cones (color) One type of rod (B/W only) 4-Nov-10

11 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

12 Mixing Colored Light Additive primary colors: Red, green, and blue Produce any color in the spectrum Additive Color Synthesis

13 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) Mixing Colored Light CHECK YOURSELF Red, green, and blue light overlap to form A. red light. B. green light. C. blue light. D. white light E. magenta light

14 Mixing Colored Light CHECK YOUR ANSWER Red, green, and blue light overlap to form A. red light. B. green light. C. blue light. D. white light E. magenta light Mixing Colored Light CHECK YOURSELF 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)

15 Mixing Colored Light CHECK YOUR ANSWER A blue object will appear black when illuminated with A. blue light. B. cyan light. C. yellow light. D. magenta light. 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

16 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 Subtraction The shadows of the golf ball are subtractive. Magenta (- green) Cyan (- red) Yellow (- blue)

17 Absorbing filters or absorbing pigments remove colors from white light. Subtractive Color Check Yourself: Yellow = Red + Green Cyan = Blue + Green Yellow & Cyan filters (or pigments) combine to give 4-Nov-10 Overlapping color filters Subtractive & Additive Primaries Subtractive primaries are complementary to additive primaries. magenta + green = white = red + blue + green yellow + blue = white + red + green + blue Example: color printing

18 Mixing Colored Light CHECK YOURSELF 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 Light CHECK YOUR ANSWER A red rose will not appear red when illuminated only with A. red light. B. orange light. C. white light. D. cyan light.

19 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 30 The Human Eye Visual Perception Pure Colors (Colors of Rainbow) Additive Primary Colors Subtractive Primary Colors Color Mixing 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