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Digital Media II: Light, Vision & Digital Images

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  1. Digital Media II:Light, Vision & Digital Images Glenn Bresnahan glenn@bu.edu

  2. Outline • What is light? • Properties of light • How do we see? • Digital representation of images • Computer display • Digital image formats BPC: Art and Computation – Fall 2006

  3. How Do We See? BPC: Art and Computation – Fall 2006

  4. How Do We Hear? • Sound waves move through the air • Waves interact (e.g. reflect) w/ environment • Sounds wave reach our ear BPC: Art and Computation – Fall 2006

  5. How Do We See? • Light is emitted from a source • Waves interact (e.g. reflect) w/ environment • Light reaches our eyes BPC: Art and Computation – Fall 2006

  6. What is Light • Light is a wave • Packets of light energy are called photons BPC: Art and Computation – Fall 2006

  7. Waves Revisited BPC: Art and Computation – Fall 2006

  8. Waves – Properties Wavelength (distance) Amplitude BPC: Art and Computation – Fall 2006

  9. Waves in Motion – Properties Period (time for one cycle) 2 Frequency cycles per time interval 1 Time BPC: Art and Computation – Fall 2006

  10. Cycles and Circles • Sine waves and circles are closely related Y axis (x,y) X axis angle BPC: Art and Computation – Fall 2006

  11. Cycles and Circles Y axis (x,y) X axis angle BPC: Art and Computation – Fall 2006

  12. Properties of Sound • Pitch is the perception of frequency • Human perception: 20 Hz – 20 KHz • Sound travels at approx. 1100 feet/second in air • Approx. 750 miles/hour or 1 mile every 4.8 sec. • Loudness perception of amplitude BPC: Art and Computation – Fall 2006

  13. Properties of Light • Color is the perception of frequency • Human perception: 430 – 750 THz (red – violet) • 1 THz = 1,000,000,000,000 Hz • Light travels at approx. 186,000 miles/second in air • Approx 1 foot every nanosecond • Brightness is perception of energy level (number of photons) BPC: Art and Computation – Fall 2006

  14. How Fast is Light? • 186,00 miles/sec or 300,000 meters/sec • 8 minutes to reach earth from sun BPC: Art and Computation – Fall 2006

  15. Wavelength • Wavelength = Speed / Freq • E.g. 1 ft/sec at 1 Hz = 1 ft wavelength • Higher frequencies == shorter wavelengths • Red = 300KM/Sec / 430 THz = 698 nm (nano (billionth) meters) • Violet = 300KM/Sec / 750 THz = 400 nm BPC: Art and Computation – Fall 2006

  16. Visible Spectrum Where is the white light? What happens at higher/lower frequencies? BPC: Art and Computation – Fall 2006

  17. Electromagnetic Spectrum • Visible light is electromagnetic force in a particular frequency range BPC: Art and Computation – Fall 2006

  18. Light Interaction with Materials • When light hits a surface, several things can happen. The light can be: • Absorbed by the surface • Converted to another form of energy • Reflected (bounced) off the surface • Transmitted (refracted) through the surface BPC: Art and Computation – Fall 2006

  19. Absorption and Reflection • Different materials will absorb different frequencies • The absorption vs. reflection determines the color of the material • Black materials absorbs all wavelengths • White material reflects all wavelengths • Blue material reflects blue and absorbs all other wavelengths • Combining pigments causes more wavelengths to be absorbed, fewer wavelengths to be reflected • Subtractive color BPC: Art and Computation – Fall 2006

  20. Reflection and Refraction BPC: Art and Computation – Fall 2006

  21. Reflection • Light reflects at an opposite and equal angle • Specular (mirror) reflection • Some light will be scattered in all directions BPC: Art and Computation – Fall 2006

  22. Refraction • Speed of a wave varies by material • Index of refraction is relative speed in the medium • Vacuum 1.0000 • Air 1.0003 • Ice 1.31 • Water 1.33 • Quartz 1.46 • Flint glass 1.57-1.75 • Diamond 2.417 BPC: Art and Computation – Fall 2006

  23. Refraction • When a wave chances speed it changes direction, i.e. bends • The angle depends of the change in refractive index BPC: Art and Computation – Fall 2006

  24. Refraction • Objects appear to bend in water BPC: Art and Computation – Fall 2006

  25. Refraction • Lens change size of objects BPC: Art and Computation – Fall 2006

  26. Combination of Light • White light? • Combination of multiple colors (freq) of light • What happens when we combine different frequencies of light, say red and green? • What happens when we combine different frequencies of sound, say an C and an E note? BPC: Art and Computation – Fall 2006

  27. Color Experiment • If we combine red, green and blue light we get new colors in the region of overlap • Colors seem to “add” BPC: Art and Computation – Fall 2006

  28. How We See • Light is emitted from a source • Light interacts with surfaces in the environment • Light is reflected into our eyes BPC: Art and Computation – Fall 2006

  29. Human Vision • Light passes into the cornea, though a liquid filled chamber and out through the lens. These focus the light • The pupil acts as diaphragm, controlling the amount of light • The light is projected onto the retina at the back of the eye BPC: Art and Computation – Fall 2006

  30. Human Vision • The retina is covered with photosensitive receptor cells • Photoreceptor cells are attached to the optical nerve which feeds signals to the brain • Light (photons) enter the cell cause a chemical reaction which causes the cell to fire BPC: Art and Computation – Fall 2006

  31. Eat Your Carrots! • Photoreceptor cells contain opsin (a protein) + retinal = rhodopsin • Photo excitation causes the rhodopsin to twist and release the retinal • The released retinal causes a reaction which cause the attached nerve to fire • Retinal is destroyed in the process • Retinal is synthesized from vitamin A • Vitamin A is derived from beta-carotene BPC: Art and Computation – Fall 2006

  32. Digital Media II:Light, Vision & Digital ImagesPart 2 Glenn Bresnahan glenn@bu.edu

  33. Question? • When we combine light of two different frequencies we seem to get light of a different color. Why does this happen? Sound waves don’t combine this way. BPC: Art and Computation – Fall 2006

  34. Combining Waves • Sound waves do not combine to make new frequencies (pitch) • C + E not equal D C = 523.25 Hz / 65.9 cm (2.162 ft) D = 587.33 Hz / 58.7 cm (1.925 ft) E = 659.29 Hz / 52.3 cm (1.716 ft) BPC: Art and Computation – Fall 2006

  35. Length of Light Waves • Human hair ~ 1/500” • 0.005 cm • 50,000 nm • Cyan light = 500 nm • 100 wavelengths across a human hair BPC: Art and Computation – Fall 2006

  36. Human Vision • Light passes into the cornea, though a liquid filled chamber and out through the lens. These focus the image • The pupil acts as diaphragm, controlling the amount of light • The light is projected onto the retina at the back of the eye where a chemical reaction causes neurons to fire BPC: Art and Computation – Fall 2006

  37. Photoreceptors • The retina contains two types of receptor cells: rods and cones • Approx. 90 million rods; 4.5 million cones BPC: Art and Computation – Fall 2006

  38. Photoreceptors - Rods • Rods react to very low light levels • As few as several photons • Rods react to a broad spectrum of frequencies (max at 498 nm) • Rods react slowly (~100 milliseconds) BPC: Art and Computation – Fall 2006

  39. Photoreceptors - Cones • Cones require much more light to fire • Cones react much more quickly (10-15 ms) • Cones are much denser in the center (fovea) of the eye BPC: Art and Computation – Fall 2006

  40. Photoreceptors – Distribution BPC: Art and Computation – Fall 2006

  41. Photoreceptors - Cones • Three types of cones: S, M, L which react to different wavelengths of light • L Cones: peak at 564 nm • M Cones: peak at 533 nm • S Cones: peak at 437 nm BPC: Art and Computation – Fall 2006

  42. Photoreceptors – Response Spectrum • S = blue, M = green, L = red BPC: Art and Computation – Fall 2006

  43. Photoreceptors – Seeing Colors • Any response can be synthesized by combining red, green and blue light BPC: Art and Computation – Fall 2006

  44. Color Mixing • Adding red, green and blue light in various proportions can generate the perception of all colors BPC: Art and Computation – Fall 2006

  45. Cell Firings • Light reaching photoreceptors causes some number of cells to fire (after an interval) • Cells can not continually fire • Receptors can become saturated • Cell firings are discrete BPC: Art and Computation – Fall 2006

  46. Saturation – After Images BPC: Art and Computation – Fall 2006

  47. Saturation – After Images BPC: Art and Computation – Fall 2006

  48. Flicker Fusion • If light is flashed fast enough, it becomes indistinguishable from a steady light • The rate is called the flicker fusion or critical flicker frequency • Dependent on intensity, but about 45 Hz BPC: Art and Computation – Fall 2006

  49. Flicker Fusion & Animation • Flicker fusion makes animation possible • Each frame is displayed a fraction of a second BPC: Art and Computation – Fall 2006

  50. Flicker Fusion in Film and Video • Film uses 24 frames per second • Video uses 30 frames per second • Flicker fusion is >45 FPS • How does this work?? BPC: Art and Computation – Fall 2006