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Computer Vision – Fundamentals of Human Vision

Computer Vision – Fundamentals of Human Vision. Hanyang University Jong-Il Park. Introduction. Understanding of Mechanism of Human Vision To construct the measures of image fidelity & intelligibility To design and evaluate image processing algorithms and imaging systems .

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Computer Vision – Fundamentals of Human Vision

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  1. Computer Vision – Fundamentals of Human Vision Hanyang University Jong-Il Park

  2. Introduction • Understanding of Mechanism of Human Vision • To construct the measures of image fidelity & intelligibility • To design and evaluate image processing algorithms and imaging systems

  3. Brightness (Perceived Luminance) • Light ~ radiant energy which, by its action on theorgans of visions, enables them to perform their function of sight • Spectral energy distribution of the light source L( ), = 350nm ~ 780nm • Light received from an object : reflectivity or transmissivity of the object

  4. Human Eye • visible range : 350 nm < wavelength < 780 nm • photoreceptors of the retina • rods : about 75-150 millions • cones : about 6.5 millions (Color Vision) • scotopic vision : rods (dark environment) • mesopic vision : rods + cones (middle range) • photopic vision : cones (bright environment) • rods are more sensitive to light than the cones

  5. Distribution of Photoreceptors

  6. The Human Visual System 공막 홍채 (눈알의) 맥락막 각막 (안구의)수양액 Shape of lens, rather than the distance between lens and screen, is changed

  7. The Human Visual System (cont.)

  8. Eye Physiology • Luminance (or Intensity) : independent of luminance of the surrounding object • Luminosity Function (Relative Luminous Efficiency Function) Relative luminous efficiency function Light distribution

  9. Brightness • Contrast • Brightness : dependent upon the surroundings

  10. Brightness adaptation Dynamic range ~ 1010

  11. Intensity Discrimination Experiment

  12. Weber’s Law

  13. (100,101) (100,102) (100,105) (150,151) (150,153) (150,162)

  14. rod cone

  15. Brightness(cont.) • Mach Bands

  16. MTF of the Visual System • Measurement of visual system in frequency domain MTF: Modulation Transfer function

  17. Isopreference k: # of bits/pel  gray-level resolution N: # of pels  spatial resolution Preference depends on image!

  18. Optical illusions Optical illusions

  19. Moire pattern Moire pattern

  20. Image Fidelity Criteria • Goal : • Image quality measurement • Performance evaluation of image processing techniques or systems • Quantitative Criteria • Mean square criterion : • SNR(signal-to-noise ratio) : • PSNR(peak-to-peak SNR) :

  21. Image Fidelity Criteria (cont.) • Subjective Criteria

  22. Perception of Intermittent Light • Perception depends on its frequency ( N cycles/sec) • small N : Flashes appear separated in time • increase N : unsteady flicker, unpleasant • increase N further : Continuous light perception • Fusion frequency : Frequency at which an observer begins perceiving light flashes as continuous light • Critical Fusion frequency (CFF) : about 50 ~ 60 Hz. Consider a light that flashes on for a brief duration N times/sec

  23. Perception of Intermittent Light (cont.) • Higher fusion frequency for larger size and larger intensity of the flickering object • very dim, small light : A few cycles/sec • very bright, large light : Over 100 cycles/sec • Examples of intermittent light • fluorescent light : Over 100 times/sec • motion picture : 24 frames/sec with 1 frame shown twice • TV monitor : 30 frames/sec, 2fields/frame 60 fields/sec (NTSC system)

  24. Empirical Observation • Sharper images look better • Same noise in uniform background region is more visible than noise in edge areas (spatial masking) • Same noise in dark areas is more visible than noise in bright areas • Same amount of artificial noise appears worse than natural looking noise

  25. Colorimetry • The perceptual attributes of color (HIS system) • Intensity : the amount of light, perceived luminance ex) distinction between dark grey and light grey • hue : the color as described by wave length ex) distinction between red and yellow • saturation : the amount of color that is present ex) distinction between red and pink the vividness of color • Three primaries : Red, Green, Blue (RGB)

  26. Color representation • Hue varies along the circumference • Saturation varies along the radial distance

  27. Eg. Color representation

  28. Color absorption spectra

  29. Color Vision Model Details to be covered later

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