noise-27apr10-150dpi-med (1)

noise-27apr10-150dpi-med (1) - Noise and ISO CS 178, Spring...

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Noise and ISO Marc Levoy Computer Science Department Stanford University CS 178, Spring 2010
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! Marc Levoy Outline ! examples of camera sensor noise don’t confuse it with JPEG compression artifacts ! probability, mean, variance, signal-to-noise ratio (SNR) ! laundry list of noise sources photon shot noise, dark current, hot pixels, Fxed pattern noise, read noise ! SNR (again), dynamic range (DR), bits per pixel ! ISO ! denoising by aligning and averaging multiple shots by image processing will be covered in a later lecture 2
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! Marc Levoy Nokia N95 cell phone at dusk 3 8 ! 8 blocks are JPEG compression unwanted sinusoidal patterns within each block are JPEG’s attempt to compress noisy pixels
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! Marc Levoy Canon 5D II at noon 4 ISO 200 f/13.0 1/320 sec RAW w/o denoising chromatic aberration!
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! Marc Levoy Canon 5D II at noon 5 ISO 200 f/13.0 1/320 sec RAW w/o denoising post-processed
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! Marc Levoy Canon 5D II at dusk 6 ISO 6400 f/4.0 1/13 sec RAW w/o denoising
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! Marc Levoy Canon 5D II at dusk 7 ISO 6400 f/4.0 1/13 sec
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! Marc Levoy Canon 5D II at dusk 8 ISO 6400 f/4.0 1/13 sec
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! Marc Levoy Photon shot noise ! the number of photons arriving during an exposure varies from exposure to exposure and from pixel to pixel ! this number is governed by the Poisson distribution 9
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! Marc Levoy Poisson distribution ! expresses the probability that a certain number of events will occur during an interval of time ! applicable to rare events that occur with a known average rate, and independently of the time since the last event ! if on average ! events occur in an interval of time, the probability p that k events occur instead is 10 probability density function p ( k ; ! ) = k e " k !
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! Marc Levoy Mean and variance ! the mean of probability density function p(x) is ! the variance of probability density function p(x) is ! the mean and variance of the Poisson distribution are ! the standard deviation is 11 μ= x p ( x ) dx ! ! 2 = ( x " μ ) 2 p ( x ) dx # 2 = " = Deviation grows slower than the average.
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! Marc Levoy Signal-to-noise ratio (SNR) ! example if SNR improves from 100:1 to 200:1, it improves 20 log 10 (200) - 20 log 10 (100) = +6 dB 12 SNR = mean pixel value standard deviation of pixel value = μ ! SNR (dB) = 20 log 10 " # $ % '
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! Marc Levoy Photon shot noise (again) ! photons arrive in a Poisson distribution
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noise-27apr10-150dpi-med (1) - Noise and ISO CS 178, Spring...

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