Multimedia Systems & Interfaces

1. Multimedia Systems &Interfaces Karrie G. KarahaliosSpring 2007

2. Perception • Review • Expectations • Perception • Homework

3. Color and Visual System • Color refers to how we perceive a narrow band of electromagnetic energy • source, object, observer • Visual system transforms light energy into sensory experience of sight

4. Human Visual System • Eyes, optic nerve, parts of the brain • Transforms electromagnetic energy

5. Human Visual System • Formation • cornea, sclera, pupil,iris, lens, retina, fovea • Transduction • retina, rods, and cones • Processing • optic nerve, brain

6. Image Formation • Cornea and sclera • Pupil • Iris • Lens • Retina • Fovea Sclera Retina Cornea Fovea Lens Pupil Iris

7. Part of sclera – hard white part of the eye Transparent part at front of eye Allows light to enter, refraction occurs Sclera Cornea The Cornea

8. Controls amount of light passing through diameter varies in response to light Iris controls the diameter of the pupil gives eye its color Pupil Iris The Pupil and Iris

9. Focuses light on the retina using refraction Changes shape to provide focus spherical forcloser objects flat for far objects accommodation Lens The Lens

10. Retina has photosensitive receptors at back of eye Fovea is small, dense region of receptors only cones (no rods) gives visual acuity Outside fovea fewer receptors overall larger proportion of rods Retina Fovea The Retina and Fovea

11. Transform light to neural impulses Receptors signal bipolar cells Bipolar cells signal ganglion cells Axons in the ganglion cells form optic nerve The Transduction Bipolar cells Rods Ganglion Cones Optic nerve

14. Contain photo-pigment Respond to low energy Enhance sensitivity Concentrated in retina, but outside of fovea One type, sensitive to grayscale changes Rods and Cones Cones Rods • Contain photo-pigment • Respond to high energy • Enhance perception • Concentrated in fovea, exist sparsely in retina • Three types, sensitive to different wavelengths

15. Rod and Cone Destiny 120 million rods 6-7 million cones From http://hyperphysics.phy-astr.gsu.edu/hbase/vision/rodcone.html

16. 3 types of cones (6 to 7 million of them) Red (64%), Green (32%), Blue (2%) Each type most responsive to a narrow band red and green absorb most energy, blue the least Light stimulates each set of cones differently, and the ratios produce sensation of color Tri-stimulus Theory

17. Tri-stimulus Theory

18. Opponent-Color Theory • Visual system contains two types of color-sensitive units • red / green; blue / yellow • Each component in a unit responds opposite the other component • e.g., if red-green responds to red, then green is inhibited • Explains concept of ‘after images’

19. Visual System Facts • Distinguish hundreds of thousands of colors • more sensitive to brightness • Can distinguish about 28 fully saturated hues • less sensitive to hue changes in less saturated colors • Can distinguish about 23 levels of saturation for fixed hue and lightness • 10 times less sensitive to blue than red or green • it absorbs less energy in the blue range

20. Physical Properties of Color • Dominant wavelength • electromagnetic waves • 400nm (violet) to 700nm (red) • Excitation purity • Luminance

21. Dominant wavelength spike in power (e2) white light is uniform energy distribution Excitation purity ratio between e2 / e1 Spectral Distribution e2 Energy distribution e1 Wavelength

22. Perceptual Properties of Color • Hue • distinguishes named colors, e.g., RGB • dominant wavelength of the light • Saturation • how far color is from a gray of equal intensity • Brightness (lightness) • perceived intensity

23. Color Perception • Hue • distinguishes named colors, e.g., RGB • dominant wavelength of the light • Saturation • how far color is from a gray of equal intensity • Brightness (lightness) • perceived intensity Tints Purecolors White Tones Grays Shades Black

24. Standard reference CIE

25. Color Models Additive (RGB) Subtractive (CMY) Applies to reflected light (printed images, paints, etc) Applies to light-emittingsources (TVs, monitors, etc)

26. HSV (aka HSB) • User-oriented, based on use of tints, shades, and tones

27. RGB • Based on the fact that the human visual system maintains three types of cones (RGB cones) • Different weightings produce different colors Green(0,1,0) Yellow(1,1, 0) Cyan(0,1,1) White(1,1,1) Black(0,0,0) Red(1,0,0) Blue(0,0,1) Magenta(1,0,1)

28. YIQ (aka YUV) • Used in US television broadcasting • Recoding of RGB for efficiency and compatibility with black-and-white TV • Y is luminance (not yellow!) • I and Q is chromaticity =

29. CMYK • Subtractive color model • used for printing, painting, etc. • CMY are the complements of RGB • two complementary colors gives a primary _ =

30. Gestalt

31. Auditory Perception

32. Waves • Periodic disturbance that travels through a medium (e.g. air or water) • Transport energy • Transverse or longitudinal • Electromechanical or mechanical

33. Sound • A longitudinal, mechanical wave • caused by a vibrating source • Pack molecules at different densities • cause small changes in pressure • Model pressure differences as sine waves

34. Volume and Pressure

35. Ears, parts of brain, and neural pathways Changes in pressure move hair-like fibers within the inner ear Movements result in electrical impulses sent to the brain Auditory System

36. Process of Hearing (Transduction)

37. Frequency (temporal) Theory • Periodic stimulation of membrane matches frequency of sound • one electrical impulse at every peak • maps time differences of pulses to pitch • Firing rate of neurons far below frequencies that a person can hear • Volley theory: groups of neurons fire in well-coordinated sequence

38. Place Theory • Waves move down basilar membrane • stimulation increases, peaks, and quickly tapers • location of peak depends on frequency of the sound, lower frequencies being further away

39. Amplitude height of a cycle relates to loudness Wavelength (w) distance between peaks Frequency (  ) cycles per second relates to pitch w = velocity Most sounds mix many frequencies & amplitudes Physical Dimensions Sound is repetitive changesin air pressure over time

40. Psychological Dimensions • Loudness • higher amplitude results in louder sounds • measured in decibels (db), 0 db represents hearing threshold • Pitch • higher frequencies perceived as higher pitch • hear sounds in 20 Hz to 20,000 Hz range

41. Timbre (tam-bre) complex patterns added to the lowest, or fundamental, frequency of a sound, referred to as spectra spectra enable us to distinguish musical instruments Multiples of fundamental frequency give music Multiples of unrelated frequencies give noise Psychological Dimensions

42. Sound Intensity • Intensity (I) of a wave is the rate at which sound energy flows through a unit area (A) perpendicular to the direction of travel P measured in watts (W), A measured in m2 • Threshold of hearing is at 10-12 W/m2 • Threshold of pain is at 1 W/m2

43. Decibel Scale • Describes intensity relative to threshold of hearing based on multiples of 10

44. Decibels of Everyday Sounds

45. Loudness from Multiple Sources • Use energy combination equation where L1, L2, …, Ln are in dB

46. Exercises • Show that the threshold of hearing is at 0 dB • Show that the threshold of pain is at 120 dB • Suppose an electric fan produces an intensity of 40 dB. How many times more intense is the sound of a conversation if it produces an intensity of 60 dB? • One guitar produces 45 dB while another produces 50 dB. What is the dB reading when both are played? • If you double the physical intensity of a sound, how many more decibels is the resulting sound?

47. Loudness and Pitch • More sensitive to loudness at mid frequencies than at other frequencies • intermediate frequencies at [500hz, 5000hz] • Perceived loudness of a sound changes based on the frequency of that sound • basilar membrane reacts more to intermediate frequencies than other frequencies

48. Fletcher-Munson Contours Each contour represents an equal perceived sound

49. Masking • Perception of one sound interferes with another • Frequency masking • Temporal masking

50. Frequency Masking • Louder, lower frequency sounds tend to mask weaker, higher frequency sounds From http://www.cs.sfu.ca/CourseCentral/365/