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Photographic Tone Reproduction for Digital Images

Brandon Lloyd

COMP238October 2002

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Problems with High Dynamic Range (HDR)

• The range of light in the real world spans 10 orders of magnitude!

• A single scene’s luminance values may have as much as 4 orders of magnitude difference

• A typical CRT can only display 2 orders of magnitude

• Tone-mapping is the process of producing a good image of HDR data

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Zone System

• Used by Ansel Adams. Utilizes measured luminance to produce a good final print

• Zone: an approximate luminance level. There are 11 print zones

• Middle-grey: Subjective middle brightness region of the scene, typically map to zone V

• Key: Subjective lightness or darkness of a scene

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Zone System

• Measure the luminance on a surface perceived as middle-gray - map to zone V

• Measure dynamic range from both light and dark areas.

• If dynamic range < 9 zones then full range can be captured in print

• Otherwise dodging-and-burning must be used to bring out details

• Dodging-and-burning: Witholding or adding light in development to lighten or darken the final print

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Algorithm

• First apply a scaling to the whole image. This similar to setting the exposure for mapping to middle-gray

• Apply automatic dodging-and-burning to compress dynamic range if necessary

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Luminance Scaling

• Use log-average luminance to approximate the key of the scene

• In a normal-key image middle-gray maps to a key value a = .18 suggesting the function:

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Luminance Scaling

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Luminance Scaling

• Control burn out of high luminances

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Automatic Dodging-and-Burning

• Think of this as local adaptation, choosing a key value for every pixel

• Need a properly chosen neighborhood• Dodging-and-burning is applied to regions

bounded by large contrasts• Use center-surround functions to measure

local contrast at different scales

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Automatic Dodging-and-Burning

• The effects of using different scales

Center

Surround

s1

s2

s3

s1 s2 s3

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Automatic Dodging-and-Burning

• Use difference of Gaussians for center-surround function

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Automatic Dodging-and-Burning

• Choose largest neighborhood around a pixel with fairly even luminances

• Take the largest scale that doesn’t exceed a contrast threshold:

• Final local operator

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Automatic Dodging-and-Burning

Details recovered by using dodging-and-burning

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Results

• Used FFT to compute the Gaussians• 8 discrete scales ranging from 1 pixel to 43

increasing by a factor of 1.6 = 0.05

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Results

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Results

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Results

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Comparison

Durand et al. Reinhard et al.

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Comparison

Durand et al. Reinhard et al.