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					Computer Graphics                          Inf4/MSc




                Computer Graphics

                      Lecture Notes #16
                    Image-Based Lighting
Computer Graphics                                       Inf4/MSc

              Global Illumination and
              Image-Based Lighting
 • Traditional Computer Graphics involves:
    – Modelling with matter: geometry with reflectance
      properties.
 • Image-Based Lighting allows:
    – Combination of real and synthetic graphics with
      consistent illumination, using images as light sources.
    – Extension of Environment mapping




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Computer Graphics                           Inf4/MSc


             Environment Mapping
 • Computing the color according to the
   reflection vector
 • No self-shadowing, no inter-reflection
Computer Graphics                           Inf4/MSc


              Image Based Lighting
 • Add models or objects to scenes and allow
   them to be manipulated in the scenes.
 • Modelling with light allows added objects
   to be illuminated consistent with image
   existent lighting - photorealistic inclusions.
 • We start with the answer by finding the
   scene illumination.

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Computer Graphics                                  Inf4/MSc


              Image Based Lighting
                           (all pictures P. Debevec 98-99)‫‏‬
 • Real Scene




 • Goal: place synthetic objects on table
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Computer Graphics                         Inf4/MSc


              Extracting scene lighting
 • Capture illumination using illumination
   sphere




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Computer Graphics                    Inf4/MSc


              Image Based Lighting




         Real scene

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Computer Graphics                                 Inf4/MSc

              Image Based Lighting
                    captured illumination field




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Computer Graphics                                Inf4/MSc


              Image Based Lighting
                                   light based model




              synthetic objects
                                  local scene




        Real scene
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Computer Graphics                                        Inf4/MSc


              Image Based Lighting
• Use global illumination - compute effects of
  synthetic objects on local scene
                                         light based model




    synthetic objects (brdf known)‫‏‬   local scene (brdf estimated)‫‏‬
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Computer Graphics                    Inf4/MSc


              Image Based Lighting
 • Render into the scene




                    background



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Computer Graphics                    Inf4/MSc


              Image Based Lighting
 • Render synthetic objects




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Computer Graphics                        Inf4/MSc


              Image Based Lighting
 • Effect of local scene on real scene




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Computer Graphics                    Inf4/MSc


              Image Based Lighting
 • Add differences to image




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Computer Graphics   Some more results   Inf4/MSc
Computer Graphics                                         Inf4/MSc


                Light Probe Images

                    • Spherical Environment Maps
                    • In most digital images, pixel values
                      aren’t‫‏‬proportional‫‏‬to‫‏‬the‫‏‬light‫‏‬levels‫‏‬
                      in the scene.
                      – light levels are encoded nonlinearly so they
                        appear either more correctly or more pleasingly
                        on CRTs
Computer Graphics                         Inf4/MSc
            High-Dynamic Range
                Photography
 • Standard digital images typically represent
   only a small fraction of the dynamic
   range—the ratio between the dimmest and
   brightest regions accurately represented—
   present in most realworld lighting
   environments.
 • The bright light saturates the pixel colour
 • Need a high dynamic range image which
   can be produced by combining images of
   different shutter speed
Computer Graphics                  Inf4/MSc

              High-Dynamic Range
                  Photography
Computer Graphics                Inf4/MSc


                    HDR images
Computer Graphics   Inf4/MSc
Computer Graphics               Inf4/MSc
         Comparison: Radiance map
            versus single image
Computer Graphics                             Inf4/MSc


               Eyes For Relighting
 • Extracting environment maps from eyes for use in
   integrating and relighting scenes
 • Apply IBL to the 3D objects added into the image




 • http://www.youtube.com/watch?v=MllTSofXt8E
 Computer Graphics                                 Inf4/MSc


                Shape from Shading
  • Obtaining the 3D shape from an image
  • Assuming Lambertian reflection
  • Can recover the normal vector if we cast the
    light from 3 different directions      Example: sphere
                                           (lit from left)‫‏‬
                                              N
                                          
                                   L

                                         Object

               L
cos(L, n)        n  Lx n x  L y n y  Lz n z
               L
Computer Graphics             Inf4/MSc




 • Integrate the normals
   over the surface to
   compute the 3D surface
 • We can also estimate
   the colour information
   and use this for texture
     Surface inserted into new scenes
Computer Graphics                Inf4/MSc

• lit according to the global illumination of that scene.
• This illumination can of course come from eyes!
Computer Graphics                              Inf4/MSc


  Acquiring the Reflectance Field of a
             Human Face
 • Simulating reflectance on human faces
   requires a lot of effort and computation
     – Need to acquire the reflectance model
     – Subsurface scattering
 • Instead of simulating the global illumination,
   we can simply illuminate the face by real-
   lights from different directions
 • Creating images of arbitrary light conditions
   by combining the captured data
                    The Light Stage:
Computer Graphics      60-second
                         Inf4/MSc
                       exposure
Computer Graphics                                               Inf4/MSc
                                                                 Original
                                                               Resolution:
                    Light Stage Data                              6432




Lighting‫‏‬through‫‏‬image‫‏‬recombination:‫‏‬Haeberli‫‏,29‘‏‬Nimeroff‫‏,49‘‏‬Wong‫79‘‏‬
Computer Graphics                    Inf4/MSc

             Reflectance Functions
Computer Graphics                            Inf4/MSc


   Lighting Reflectance Functions


                                             1

  normalized        reflectance   lighting       rendered
   light map          function    product          pixel
Computer Graphics                    Inf4/MSc


                    References
 • Rendering Synthetic Objects into Real
   Scenes, SIGGRAPH 98
 • Acquiring the Reflectance Field of a
   Human Face, SIGGRAPH 2000
   http://www.debevec.org
 • Eyes For Relighting, Nishino SIGGRAPH
   2004

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