Validation of Color Managed 3D Appearance Acquisition

1
Validation of Color Managed 3D Appearance
Acquisition

• Michael Goesele
• Max-Planck-Institut für Informatik(MPI
  Informatik)
• Vortrag im Rahmen des V3D2 Workshops
• 29.30. November 2004 in Berlin

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Acquired BRDF Model

• rendered BRDF model of a carafe
• Villeroy Boch (Mettlach, 19th century)

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Acquired BRDF Model

• Which model is (more) correct?

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Background BRDF

• bidirectional reflectance distribution function
  (BRDF)
• ratio of reflected to incident radiance at one
  pointfor any pair of directions

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Background BRDF Acquisition

• based on acquisition system for spatially varying
  BRDFsLensch et al. 2003
• determine local reflection properties for each
  surface point
• uses Lafortune BRDF model Lafortune 1997
• shown at previous V3D2 Workshops

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Background BRDF Acquisition
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Validation Questions

• How exact can this method capture describe the
  behavior of a real object?
• How exact can we reproduce an objects appearance?
• How can this be achieved?? will be discussed in
  this talk
• How good is the Lafortune model?? will not be
  discussed in this talk? see e.g. Ngan et al.,
  SIGGRAPH Sketch 2004

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Color and Computer Graphics

• yes, we have color
• it works (somehow)
• RGB is always the same (?)
• looks nice after some tuning
• more seriously
• not the main concern, other technologies more
  important
• eye can adjust to bad color reproduction
• comparison to ground truth often not
  possible/required
• an important issue for digitization!

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Background Color Management

• goal ensure correct color reproduction across
  devices
• used in graphical arts and printing industry
• defined profile connection space (PCS) with CIE
  XYZ or CIE Lab color space
• profiles describe conversion to PCS for all
  devices
• take limitations of devices into account

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ICC Profile Generation

• example input profile
• capture known test target with camera
• lighting conditions (spectrum) identical to
  finally used conditions
• profile generated by(commercial) software
• captures properties of
• camera
• lighting
• test target

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Key Idea

• introduce color management into BRDF acquisition
  and reproduction workflow
• convert input images into defined color
  space(during HDR image generation)
• convert rendered images into output device color
  space
• allows for objective assessment of quality of
  acquired models
• important for libraries, conservation,
• color management ensures best possible color
  reproduction
• takes limitations of devices into account

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BRDF Acquisition Workflow
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Accuracy of the Acquired Model

• compare spectrophotometer measurements with BRDF
  model evaluated under same conditions
• illumination at 45, observation at 0 (along
  surface normal)
• performs measurement in spectral domain
• can be converted to other color representations

BRDF
spectrophotometer
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Accuracy of the Acquired Model

• quality metric ?E
• distance in CIELab color space
• 1 ?E just noticable color difference under
  perfect conditions

BRDF(CIEXYZ) Spectrophotometer (CIEXYZ) ?E
Bird yellow 59.41, 61.23, 14.58 61.40, 61.51, 13.42 4.158
Bird orange 42.28, 33.17, 8.02 43.73, 32.73, 5.95 7.923
Bird blue 34.99, 40.09,46.65 32.74, 37.48, 48.16 4.283
Bird white 68.79, 73.35, 61.04 76.67, 78.75, 58.77 7.168
Carafe blue 17.63, 18.67, 33.26 17.72, 19.34, 36.325 3.398
Carafe white 57.16, 58.33, 52.11 54.00, 56.23, 43.27 7.003
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Accuracy of the Acquired Model

• compare renderings with photographs captured
  under identical conditions
• quality metric ?E

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Accuracy of the Acquired Model

• until now only comparison of acquired model to
  ground truth data
• important for conservation, long term storage
• further goal include output devices in
  validation
• can we (physically) reproduce the object
  correctly?

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Accuracy of Renderings

• visual comparison between rendering (on screen,
  printout) and real object under identical
  conditions
• all devices are calibrated
• no manual color adjustment was performed!

screen
real object
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Accuracy of Renderings

• visual comparison between rendering (on screen,
  printout) and real object under identical
  conditions

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Accuracy of Renderings

• visual comparison between rendering (on screen,
  printout) and real object under identical
  conditions

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Conclusion

• color management integrated into BRDF acquisition
  and rendering pipeline
• enables quantitative and visual validation of
  results
• correct color acquisition and rendering is
  important!
• approach
• acquire best possible model
• use best possible reproduction
• important for long term storage
• reproduction technology improves (displays,
  printers, )
• model should support these as far as possible

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Future Work

• improve quality of color management
• some colors are still quite problematic
• color management for HDR images?
• ongoing work in the community
• handling of high contrast, out-of-gamut colors
• tone mapping problem
• new display technologies (e.g., HDR display)

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• Thanks to
• Hendrik Lensch
• DFG Schwerpunktprogramm V3D2 Verteilte
  Vermittlung und Verarbeitung digitaler Dokumente
• More information
• Michael Goesele, Hendrik P. A. Lensch, Hans-Peter
  SeidelValidation of Color Managed 3D Appearance
  Acquisition.Proc. ISTs 12th Color Imaging
  Conference, pp. 265-270, 2004.
• Hendrik P. A. Lensch, Jan Kautz, Michael Goesele,
  Wolfgang Heidrich, Hans-Peter SeidelImage-Based
  Reconstruction of Spatial Appearance and
  Geometric Detail.ACM Transactions on Graphics,
  vol. 22, 2, pp. 234-257, 2003.