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Digital Media II: Light, Vision & Digital Images

Digital Media II: Light, Vision & Digital Images. Glenn Bresnahan glenn@bu.edu. Outline. What is light? Properties of light How do we see? Digital representation of images Computer display Digital image formats. How Do We See?. How Do We Hear?. Sound waves move through the air

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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

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