03/05/03 © 2003 University of Wisc onsin Last Time • Tone Reproduction • If you don’t use perceptual info, some people call it contrast reduction
Jan 03, 2016
03/05/03 © 2003 University of Wisconsin
Last Time
• Tone Reproduction
• If you don’t use perceptual info, some people call it contrast reduction
03/05/03 © 2003 University of Wisconsin
Today
• Perceptually driven tone-reproduction
03/05/03 © 2003 University of Wisconsin
Local Tone Reproduction
• Last time I introduced the idea of local tone reproduction methods– Filter to get a high-pass and low pass version of the image
– Reduce the contrast in the low pass
– Add the reduced low-pass to the high-pass to get a new image
• The filter you use should be edge preserving– A filter that smoothes out regions with low gradient, but does not
smooth across high gradients
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Edge Preserving Filters(Durand and Dorsey, SIGGRAPH 2002)
• The idea is to take a smoothing filter, say a Gaussian, and multiply it by a function that reduces the weight at sharp edges (and influence function)– It turns out, this is closely related to robust statistical
estimation – the problem of ignoring outliers in statistical data
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Robust Estimation
• Instead of least squares estimation, use something that reduces the weight of outliers from the mean– Least squares minimizes sum of squared differences
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Results
• Right, a full image• Below, still some artifacts at edges
– Fix by detecting unreliable pixels and smoothing
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Gradient Compression(Fattal, Lischinski, Werman, SIGGRAPH 2002)
• Instead of reducing the dynamic range of the data directly, reduce the size of the gradients– Ultimate effect is similar, but it’s easier to make it local
• Reduce large gradients more than small ones– Get most reduction in dynamic range– Small gradients are probably due to texture, large ones due to shadows,
occlusion, different surfaces
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Gradient Compression in 2D
• In 1D, you can reduce the gradients and then simply integrate to extract the new signal
• In 2D, the gradient field has to be conservative
• To work around this, seek a final image that has the closest legal gradient field to the desired one– Results in a Poison equation over the image
– Set appropriate boundary conditions and solve to get the image
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Details
• Work on luminance channel from CIE XYZ
• Do multi-resolution gradient attenuation– Generate a Gaussian image pyramid
– Use central differences to estimate gradient at each level
– Attenuate by multiplying by an attenuation function:
– Use linear filtering to push low-res results up to high-res
0.98.0 gradient, average times1.0
,
,,
yxH
yxHyx k
kk
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Dealing with Color
• Work in RGB space for final image
• For C in (R,G,B) compute:
– s between 0.4 and 0.6 worked well
• Not really the best thing to do
out
s
in
inout L
L
CC
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Results
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Local Filtering Methods
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Ward’s Method
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Attenuation Image
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Psychophysics
• The study of the perception of physical quantities– Does not try to explain anything, just observes what we observe
• Vital to many areas of graphics– Tone-reproduction: generating the same per perceptual sense with
different dynamic range– Rendering Control: Stopping rendering algorithms when the results
are no longer perceptible– Error metrics: Comparing images with a sense of what is visually
important
• Ferwerda, Pattanaik, Shirley and Goldberg, SIGGRAPH 1996, is a good reference– The next several slides were borrowed from the paper
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Dynamic Range (Again)
• Note the names for various ranges of perception– Related to which part of the vision system (rods or cones) is
functioning effectively
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Spectral Sensitivity
• Incoming radiance is integrated against these curves to get visual response– To match visual response with a different radiance, have to take these
curves into account• Note in particular the change in color sensitivity
– Rods are NOT color sensitive
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Detection Thresholds
• Experiment: Flash a light against a background. How much brighter does the light have to be in order to be noticed?– Sizes measured in degrees of arc
• Not so important in graphics– Most events we deal with
change illumination well beyond the detection threshold
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Contrast Thresholds
• Experiment: Show people a grating. What contrast must be present (difference in foreground/background luminance) for the grating to be distinguishable?
• Very important to graphics– Places a limit on the maximum useful display resolution
– Places a limit on the amount of detail that should be distinguishable in a tone-reproduction algorithm
– Useful in measuring the effect of various aliasing and noise effects
• We might expect the curve on the next slide to flatten out at high luminance. Why?
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Contrast Thresholds (Acuity)
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Adaptation
• Measures the effect of transition from light to dark, and vice versa
• This is light to dark
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Adaptation Dark-to-Light
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Perceptual Tone Reproduction
• Various tone reproduction algorithms can be modified to exploit these effects
• Deliberately reduce contrast if the light level is low
• Reduce color saturation at low light levels
• Model adaptation level of viewer over time by manipulating contrast and color
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Adjusting for Acuity and Color
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Adjusting for Adaptation
• Not so easy – you can’t make the monitor appear “painfully bright”
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Histograms and Perception
• Ward’s histogram method can be adapted to place limits on resolvable features– Another bound on the magnitude of histogram bins
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Other Effects
• To get color washout, reduce (x,y) by an amount that depends on the average luminance– Formulas in Ward’s and Ferwerda’s papers
• Veiling is due to light scattering inside the eye– Responsible for the halos around truly bright objects
– Ward’s method can handle this
– Glare is something different – an adaptation problem
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Histograms and Perception