Christian Riess Johannes Jordan Prof Elli Angelopoulou Pattern

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					A Common Framework for Ambient Illumination in the
Dichromatic Reflectance Model



                        Color and Reflectance in Imaging and
                            Computer Vision Workshop 2009

                                                   October 4, 2009



                  Christian Riess
                  Johannes Jordan
                  Prof. Elli Angelopoulou


                  Pattern Recognition Lab (CS 5)
                  University of Erlangen-Nuremberg
                                                                                                     Seite 2



 Natural Images: Shadows, Interreflections


  Typical assumption in Color Constancy and Illuminant Color
     Estimation: One single illuminant.

  Consider a challenging
     image:
     Multiple light sources,
     shadows, inter-
     reflections.



 Maybe we need a
     more complex model?

C. Riess
October 4, 2009   A Common Framework for Ambient Illumination in the Dichromatic Reflectance Model
                                                                                                      Seite 3



 Common Reflectance Models


  Lambertian Reflectance: Ideal Diffuse Reflectance



  Dichromatic Reflectance Model [1]: Specular and Diffuse



                                               Wave length
                                               Directions of incidence on surface
                                                   and exitance towards viewer
                                               Surface radiance
                                               Geometric terms
                                               Surface reflectance (albedo)
                                               Direct illuminant

[1] S. Shafer. Using Color to Separate Reflection Components. Color Research Application, pp. 210-218, 1985.
C. Riess
October 4, 2009   A Common Framework for Ambient Illumination in the Dichromatic Reflectance Model
                                                                                                        Seite 4



 Bi-Illuminant Dichromatic Reflection Model [1]


                      Direct light, specular refl.            Direct light, diffuse refl.




                    Ambient light, specular refl.                        Ambient light, diffuse refl.

                                               Wave length
                                               Directions of incidence on surface
                                                   and exitance towards viewer
                                               Surface radiance
                                               Geometric terms
                                               Surface reflectance (albedo)
                                               Direct illuminant
                                               Diffuse illuminant

[1] B. Maxwell, R. Friedhoff, and C. Smith. A Bi-Illuminant Dichromatic Reflection Model
     for Understanding Images. Computer Vision and Pattern Recognition, pp. 1-8, 2008.
C. Riess
October 4, 2009   A Common Framework for Ambient Illumination in the Dichromatic Reflectance Model
                                                                                                     Seite 5



 The Ambient Term


  The BIDR contains a very flexible “catch all” ambient term.




  For practical exploitation, further constraints have to be made


  These constraints can form a taxonomy of illumination-related
     techniques

C. Riess
October 4, 2009   A Common Framework for Ambient Illumination in the Dichromatic Reflectance Model
                                                                                                           Seite 6



 BIDR as a General Framework (I)


  Shadow Boundaries: Learning illuminant ratios [1]
           Channelwise relative attenuation of the direct illuminant
           Isotropic ambient light
           Weighting factor between direct and ambient light                           Split illuminant
                                                                                         into chromatic
                                                                                        and energy part



                              Exclude                                Make ambient
                            specularities                            light isotropic

                                              Attenuate
                                           direct illuminant


[1] K. Barnard and G. Finlayson. Shadow Identification using Colour Ratios. IS&T/SID 8 th Colour Imaging
     Conference: Colour Science, Systems and Applications, pp. 97-101, 2000.
C. Riess
October 4, 2009   A Common Framework for Ambient Illumination in the Dichromatic Reflectance Model
                                                                                                          Seite 7



 BIDR as a General Framework (II)


  Scene Illuminant Estimation with Flash/No-Flash Images [1]
           Consider normal illumination as ambient light, flash is direct light
           Knowledge of the flash light characteristics resolves the scene parameters




                              Exclude                                                       Assume Planckian
                            specularities                                                    illuminant, flash
                                                                                               light is known
           Flash image:
           Non-Flash image:
           Difference of log-chromaticities recovers average ambient light

[1] C. Lu and M. Drew. Practical Scene Illuminant Estimation via Flash/No-Flash Pairs. IS&T/SID 14th Colour
     Imaging Conference: Colour Science, Systems and Applications, 2006.
C. Riess
October 4, 2009   A Common Framework for Ambient Illumination in the Dichromatic Reflectance Model
                                                                                                             Seite 8



 Experiments: Setup


  Is the consideration of such a model justified at all?
  -> only if ambient illumination is complicated enough to
     disturb our analysis


  Chalk (sort-of Lambertian), direct light, ambient light and a
     combination of direct and ambient light.




         Direct illumination                   Ambient illumination                     Direct and ambient
C. Riess                                                                                    illumination
October 4, 2009   A Common Framework for Ambient Illumination in the Dichromatic Reflectance Model
                                                                                                     Seite 9



 Experiments: Expectations


  Dichromatic Reflectance under laboratory conditions



                                                                                 Specular triangle



                                                                              Diffuse Line

                                                                   RGB Cube

  With additional (more or less uniform) ambient lighting, we
     would expect the line to be rotated and/or shifted:



                                                                              Specular triangle


                                                                            Diffuse Line
C. Riess
                                                               RGB Cube
October 4, 2009   A Common Framework for Ambient Illumination in the Dichromatic Reflectance Model
                                                                                                     Seite 10



 Experiments: Results


  Diffuse and ambient pixels “almost” linearly distributed
     in RGB space:




C. Riess
October 4, 2009   A Common Framework for Ambient Illumination in the Dichromatic Reflectance Model
                                                                                                      Seite 11



  Experiments: Results



                                                                     Chalk: regions of interest
                                                                                    Per-pixel contribution
                                                                                      of each illuminant




 Blue




Green                 Red

 C. Riess
 October 4, 2009   A Common Framework for Ambient Illumination in the Dichromatic Reflectance Model
                                                                                                     Seite 12



 Conclusion and Outlook


  Dichromatic Reflection Model is limited to a single light source,
     making several real-world images difficult to analyze
           Outdoor scenes
           (Partially) shadowed areas

  Natural extension: Bi-Illuminant Dichromatic Reflectance Model
           A common framework for modelling shadows, interreflections or a
            second direct illuminant
           Experiments on real-world images demonstrate the need for more
            complex assumptions


  Outlook: Develop new illumination estimation algorithms based
     on less constraining assumptions

C. Riess
October 4, 2009   A Common Framework for Ambient Illumination in the Dichromatic Reflectance Model

				
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