lec21_brdfs - CS6670: Computer Vision Noah Snavely Lecture...

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Lecture 21: Light, reflectance and photometric stereo CS6670: Computer Vision Noah Snavely
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Announcements Final projects Midterm reports due November 24 (next Tuesday) by 11:59pm (upload to CMS) State the problem Describe progress so far, any problems that have come up
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What is light? Electromagnetic radiation (EMR) moving along rays in space R( l ) is EMR, measured in units of power (watts) l is wavelength Perceiving light How do we convert radiation into ―color‖? What part of the spectrum do we see?
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Visible light We ―see‖ electromagnetic radiation in a range of wavelengths
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Light spectrum The appearance of light depends on its power spectrum How much power (or energy) at each wavelength daylight tungsten bulb Our visual system converts a light spectrum into ―color‖ This is a rather complex transformation fluorescent light
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The human visual system Color perception Light hits the retina, which contains photosensitive cells – rods and cones These cells convert the spectrum into a few discrete values
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Density of rods and cones Rods and cones are non-uniformly distributed on the retina Rods responsible for intensity, cones responsible for color Fovea - Small region (1 or 2°) at the center of the visual field containing the highest density of cones (and no rods). Less visual acuity in the periphery—many rods wired to the same neuron
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Demonstrations of visual acuity With one eye shut, at the right distance, all of these letters should appear equally legible (Glassner, 1.7).
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Demonstrations of visual acuity With left eye shut, look at the cross on the left. At the right distance, the circle on the right should disappear (Glassner, 1.8).
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Brightness contrast and constancy The apparent brightness depends on the surrounding region brightness contrast : a constant colored region seems lighter or darker depending on the surrounding intensity: http://www.sandlotscience.com/Contrast/Checker_Board_2.htm brightness constancy : a surface looks the same under widely varying lighting conditions. “Approximate brightness constancy, a similar effect, makes us tend to see objects in terms of
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lec21_brdfs - CS6670: Computer Vision Noah Snavely Lecture...

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